Studies in Natural Products Chemistry Volume 27 Bioactive Natural Products (Part H)

Studies in Natural Products Chemistry edited by Atta-ur-Rahman

Vol. 1 Stereoselective Synthesis (Part A) Vol. 2 Structure Elucidation (Part A) Vol. 3 Stereoselective Synthesis (Part B) Vol. 4 Stereoselective Synthesis (Part C) Vol. 5 Structure Elucidation (Part B) Vol. 6 Stereoselective Synthesis (Part D) Vol. 7 Structure and Chemistry (Part A) Vol. 8 Stereoselective Synthesis (Part E) Vol. 9 Structure and Chemistry (Part B) Vol.10 Stereoselective Synthesis (Part F) Vo1.l IStereoselective Synthesis (Part G) Vo1.12 Stereoselective Synthesis (Part H) Vol. 13 Bioactive Natural Products (Part A) Vo1.14 Stereoselective Synthesis (Part I) Vo1.15 Structure and Chemistry (Part C) Vol. 16 Stereoselective Synthesis (Part J) Vo1.17 Structure and Chemistry (Part D) Vo1.18 Stereoselective Synthesis (Part K) Vo1.19 Structure and Chemistry (Part E) Vo1.20 Structure and Chemistry (Part F) Vo1.21 Bioactive Natural Products (Part B) Vo1.22 Bioactive Natural Products (Part C) Vo1.23 Bioactive Natural Products (Part D) Vo1.24 Bioactive Natural Products (Part E) Vo1.25 Bioactive Natural Products (Part F) Vo1.26 Bioactive Natural Products (Part G ) Vo1.27 Bioactive Natural Products (Part H)

Studies in Natural Products Chemistry

Volume 27Bioactive Natural Products (FWt H)

Edited by

Atta-ur-Rahman

H.E.J. Research lnstitute of Chemistry, University of Karachi, Karachi 75270, Pakistan

2002

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O 2002 Elsevier Science B.V. All rights reserved.This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all form of document delivery. Special rates are available for educational institutionsthat wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science.via their homepage (http://www.elsevier.corn) by selecting 'Customer support' and then 'Permissions'. Alternatively you can send an e-mail to: permissions@elsevier.com,or fax to: ( 4 ) 1865 853333. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+I) (978) 7508400, fax: (+I) (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90Tottenham Court Road, London WIP OLP, UK; phone: ( 4 ) 207 631 5555; fax: ( 4 ) 207 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work. including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a mrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury andfor damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

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FOREWORDThe present volume, the 27*^ of this series, is devoted to the chemistry of several exciting classes of natural products. The article by Kinghom and Kim reviews the current state of knowledge of sweet-tasting and sweetness-modifying constituents of plants. Some of these highly sweet natural products are already marketed as sweetners or flavoring agents in some countries. Spiteller has presented an overview of chemical responses to plant injury and plant aging. Interest in sialic acid chemistry is growing, particulariy due to its role in the regulation of a number of important biological processes. The article by Bianco and Melchioni on the structure, chemistry and biological activity of neuraminic acid covers this area comprehensively. Quaternary benzo[c]phenanthridine alkaloids (QBA) are bright coloured compounds which are interesting both for their chemistry and their biological activities. The article by Dostal and Slavfk covers some aspects of the chemistry of these compounds. Sicker and Schuiz have reviewed the field of acetai glycosides of the 2hydroxy-2H-1, 4-benzoxazine-3(4H)-one skeleton which occur naturally in a number of plant families and which can impart resistance in plants towards insects, microbes as well as pathogenic fungi. Structural identification and bioactivity aspects of simple indolizidine and quinolizidine alkaloids isolated from amphibians, ants, fungi, plants and marine sources is covered comprehensively in an article by Lourenco and co-workers. These substances are responsible for the toxic and teratogenic effects observed in livestock and their action can be associated with their affinity for nicotinic and muscarinic receptors. The metabolism of stevioside has been described by Geuns. Monoterpenoids occur widely in various plant and are also important constituents in essential oils. Enantioselective chromatographic analysis and bioactivity of chiral monoterpenoids has been reviewed by Asztemborska and Ochocka. Abscisic acid is the primary hormone which induces adaptive reactions in plants to various environmental stresses. More than 100 abscisic acid analogs of this compound have been reported so far. The scope of using abscisic acid analogs for probing the mechanism of abscisic acid reception and inactivation is presented by Todoroki and Hirai. Kimura and Okuda have reviewed the biochemical and phamriacological studies of natural products isolated from various medicinal plants and foodstuffs including their effects on lipolysis and iipogenesis in fat cells anti-obesity action, lipid and arachidonate metabolism and reduction of side effects of cancer chemotherapy. Astragalus L. is the largest genus in the family Leguminosae and it is widely distributed throughout the temperate regions of the worid, particulariy in Europe, Asia and North America. The structures and biological activity of secondary metabolites of this genus is presented by Pistelli. Plants of the genus Tanacetum (Compositae) have been used for their medicinal properties for over 2000 years.

VI

Chemical characterization and biologicai activities of compounds found in this genus are reviewed by Goren and co-workers. Contreras and co-workers have described bioactive components of Bupleurum rigidum L. subscp. rigidum, some of which have shown interesting anti-inflammatory activity. Olive oil is widely consumed in Mediterranean countries and is considered to be of benefit in reducing risks of coronary heart disease and cancer. DeH'agli and Bosisio have presented the chemistry and bioactivity of minor polar compounds of olive oil. Class III plant peroxidases are a polymorphic group of heme-containing enzymes located in the vacuole and the plant cell wall. The article by Barcel6 and Pomar covers the chemistry and bioactivity of plant peroxidases. The review of Llakopoulou-Kyriakides covers the field of naturally occurring oligopeptides and presents a variety of biological activities possessed by such compounds. The process of phosphorylation and dephosphorylation of proteins in a variety of physiological events is of growing interest. Manez and Recio have described the modulation of protein phosphorylation reactions by different natural products. There has been considerable interest in the chemistry and biological activity of flavonoids. The cytotoxic effects of flavonoids on cancer cell lines is discussed by Ayuso and co-workers. It is hoped that this volume will be another substantial addition to this series which has benefited from contributions by most of the leading natural product chemists of the worid in the past. I would like to express my thanks to Dr. Shakeel Ahmad and Miss Farzana Siddique for their assistance in the preparation of the index. I am also grateful to Mr. Waseem Ahmad for typing and to Mr. Mahmood Alam for secretarial assistance.

Atta-ur-RahmanPh.D. (Cantab), Sc.D. (Cantab) Minister for Science & Technology Government of Pakistan

April, 2002

Vll

PREFACENatural Product Chemistry continues to present exciting opportunities for medicinal chemists to discover new bioactive compounds against various diseases. Even in cases where the structures are too complex to be obtained on a large scale by synthesis, the interest is often focussed on those portions of the structures which are responsible for the biological activity and which can serve as simpler phamnacophores for synthesis and study of structure-activity relationships. The enomrious structural diversity offered by natural products of terrestrial and marine origin therefore offers a vast resource to medicinal chemists in search for new bioactivity profiles or mechanisms of actions. The present volume of "Studies in Natural Product Chemistry", the 2 / ^ of this prestigious series, presents the current frontiers in several important fields of natural product chemistry. All-in-all, the volume presents a very interesting collection of comprehensive reviews by leading experts in a number of important fields. It should be though of as a valuable new edition to this important encyclopedic series on natural product chemistry. Prof. Atta-urRahman, the Editor of this series, is now the Federal Minister for Science and Technology of the Government of Pakistan and it is noteworthy that he manages to take out time to continue his interests in chemistry as Director H.E.J. Research Institute of Chemistry at Karachi University, as Editor of this Series, as well as Editor of a number of intemational chemistry journals including "Current Organic Chemistry", "Current Medicinal Chemistry" and Co-Editor of "Natural Product Letters". He deserves congratulations for maintaining the high standard of this excellent series in the field of Natural Product Chemistry, which should be of interest to a large number of natural product and medicinal chemists who wish to keep breast with developments in biologically active natural products.

Steven V. Ley, FRS, C.B.E. Professor of Organic Chemistry and Novartis Research Felllow Department of Chemistry Cambridge University Lensfield Road Cambridge CB2 1EW U.K. April, 2002

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ix

CONTENTSForeword Preface ContributorsSweet-tasting and sweetness modifying constituents of plants NAM-CHEOL KIM AND A. DOUGLAS KINGHORN Chemical responses to plant injury and plant aging GERHARD SPITELLER Neuraminic acid and its Structures, Chemistry, Biological Activity ARMANDODORIANO BIANCO AND CRISTIANA MELCHIONIV

vii

xi

3 59 103155

JIE DOSTAL AND JIM SLAV~K

Some aspects of the chemistry of quaternary benzo[c]phenanthridine alkaloids

Benzoxazinones in plants: occurrence, synthetic access, and biological activity DIETER SICKER AND MARGOT SCHULZ Indolizidine and quinolizidine alkaloids structure and bioactivity A.M. LOURENCO, P. MAXIMO, L.M. FERREIRA AND M.M.A. PEREIRA Safety evaluation of Stevia and stevioside JAN M.C. GEUNS Abscisic acid analogs for probing the mechanism of abscisic acid reception and inactivation YASUSHI TODOROKI AND NOBUHIRO HIRAI Chiral monoterpenoids in plants-enantioselective chromatographic analysis, and their bioactivity MONIKA ASZTEMBORSKA AND J. RENATA OCHOCKA Biochemical and pharmacological studies of natural products isolated from various medicinal plants and foodstuffs YOSHIYUKI KIMURA AND HIROMICHI OKUDA Secondary metabolites of Genus Astragalus: structure and biological activity L. PISTELLI

185 233 299

32 1

361

393 443

X

Chemical characterization and biological activities of the genus Tunaceturn (Compositae) NEZHUN GOREN, NAZLI ARDA AND ZERRIN CALISKAN Bioactive components of Bupleurum rigidum L. Sub sp. rigidurn S. S h C H E Z CONTRERAS, ANA M. DiAZ LANZA, M. BERNABE PAJARES, C. BARTOLOME ESTEBAN, L. v. CASTILLO, M.R.A. MART~NEZ,P.B. BENITO AND L.F. MATELLANO Minor polar compounds of olive oil: Composition, factors of variability and bioactivity MARIO DELLAGLI AND ENRICA BOSISIO Plant peroxidases: versatile catalysts in the synthesis of bioactive natural products A. ROS BARCELO AND F. POMAR Naturally occurring oligopeptides with more than one biological activities M. LIAKOPOULOU-KYRIAKIDES Modulation of protien phosphorylation by natural products S. MZ AND M. DEL CARMEN RECIO Cytotoxicity of flavonoids of cancer cell lines. Strucutre-activity relationship C. M. LOPEZ-LAZARO, M. GALVEZ, MARTIN-CORDER0 AND M.J. AYUSO Subject Index

547

659

697 735 793 819

89 1 933

XI

CONTMBUTORS Nazli Arda Monika Asztemborska M.J. Ayuso A. Ros Barcelo Paulina Bermejo Benito Araiandodoriano Bianco University of Istanbul, Faculty of Science, Department of Biology, Vezneciler 34459, Istanbul, Turkey Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Departamento de Farmacologia, Facultad de Farmacia, Universidad de Sevilla, Spain Department of Plant Biology (Plant Physiology), University of Murcia, E-30100 Murcia, Spain Departamento de Farmacologia, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain Scuola di Specializzazione in Chimica e Technologia delle Sostanze Organiche Naturali-Facolta di Scienze Matematiche, Fisiche e Naturali Universita La Sapienza Roma, Italy Institute of Pharmacological Sciences, Faculty of Pharmacy, University of Milan, Via Balzaretti 9, 20133 Milan, Italy Yildiz Technical University, Faculty of Science and Arts, Department of Biology, Main Campus, Cukursaray, 80750 Besiktas-Istanbul, Turkey Laboratorio de Farmacognosia, Departamento de Farmacologia, Facultad de Farmacia, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain Laboratorio de Farmacognosia, Departamento de Farmacologia, Facultad de Farmacia, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain Institute of Pharmacological Sciences, Faculty of Pharmacy, University of Milan, Via Balzaretti 9, 20133 Milan, Italy Department of Biochemistry, Faculty of Medicine, Masaryk University, Komenskeho nam. 2, CZ-662 43 Bmo, Czech Republic

Enrica Bosisio

Zerrin ^aliskan

Lucinda Villaescusa Castillo Sandra Sanchez Contreras Mario Dell'Agli

Jifi Dostal

Xll

Carmen Bartolome Esteban

Departamento de Biologia Vegetal, Facultad de Ciencias, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain Departamento de Quimica, Centre de Quimica Fina e Biotechnologia, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Caparica, Portugal Departamento de Farmacologia, Facultad de Farmacia, Universidad de Sevilla, Spain Laboratory of Plant Physiology, KULeuven, Mercierlaan 92, B 3001, Leuven, Belgium Kard.

L.M. Ferreira

M. Galvez

Jan M.C. Geuns

Nezhun Goren

Yildiz Technical University, Faculty of Science and Arts, Department of Biology, Main Campus, Cukursaray, 80750 Besiktas-Istanbul, Turkey Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan Chemistry and Life Sciences, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709, USA Second Department of Medical Biochemistry, School of Medicine, Ehime University, Shigenobu-cho, Onsen-gun, Ehime 791-0295, Japan Program for Collaborative Research in the Pharmaceutical Sciences and Department of Medicinal Chemistry and Pharmacognosy (M/C 781), College of Pharmacy, University of Illinois at Chicago, 833 South Wood Street, Chicago, Illinois 60612, USA Department of Chemical Engineering, Section of Chemistry, Aristotle, University of Thessaloniki, Thessaloniki 54006, Greece Laboratorio de Farmacognosia, Departamento de Farmacologia, Facultad de Farmacia, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain Departamento de Farmacologia, Facultad de Farmacia, Universidad de Sevilla, Spain

Nobuhiro Hirai

Nam-Cheol Kim

Yoshiyuki Kimura

A. Douglas Kinghom

M. LiakopoulouKyriakides

Ana M. Diaz Lanza

M. Lopez-Lazaro

Xlll

A.M. Loviren9o

Departamento de Quimica, Centro de Quimica Fina e Biotechnologia, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Caparica, Portugal Departament de Farmacologia, Facultat de Farmacia, Universitat de Valencia, Av. V.A. Estelles s/n, 46100 Buijassot, Spain Departamento de Farmacologia, Facultad de Farmacia, Universidad de Sevilla, Spain Departamento de Farmacologia, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain Laboratorio de Farmacognosia, Departamento de Farmacologia, Facultad de Farmacia, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain Departamento de Quimica, Centro de Quimica Fina e Biotechnologia, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Caparica, Portugal Scuola di Specializzazione in Chimica e Technologia delle Sostanze Organiche Naturali-Facolta di Scienze Matematiche, Fisiche e Naturali Universita La Sapienza Roma, Italy Department of Biology and Pharmaceutical Botany, Medical University of Gdansk, Hallera 107, 80-416 Gdansk, Poland Second Department of Medical Biochemistry, School of Medicine, Ehime University, Shigenobu-cho, Onsen-gun, Ehime 791-0295, Japan Departamento de Quimica Organica Biologica, Instituto de Quimica Organica General, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain Departamento de Quimica, Centro de Quimica Fina e Biotechnologia, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Caparica, Portugal Dipartimento di Chimica Bioorganica e Biofarmacia, Universita di Pisa, Via Bonanno, 33-56126 Pisa, Italy

Salvador Maiiez

C. Martin-Cordero

Maria J. Abad Martinez

Lidia Fernandez Matellano

P. Maximo

Cristiana Melchioni

J. Renata Ochocka

Hiromichi Okuda

Manuel Bemabe Pajares

M.M.A. Pereira

L. Pistelli

XIV

F. Pomar Maria Del Carmen Recio

Department of Plant Biology (Plant Physiology), University of Murcia, E-30100 Murcia, Spain Departament de Farmacologia, Facultat de Farmacia, Universitat de Valencia, Av. V.A. Estelles s/n, 46100 Burjassot, Spain Universitat Bonn, Institut fiir Landwirtschaftliche Botanik, Karlrobert-Kreiten-Str. 13, 53115 Bonn, Germany Universitat Leipzig, Institut ftir Organische Chemie, Johannisallee 29, 04103 Leipzig, Germany Department of Biochemistry, Faculty of Medicine, Masaryk University, Komenskeho nam. 2, CZ-662 43 Bmo, Czech Republic Organische Chemie I, Universitat Bayreuth, Universitatsstrape 30, 95440 Bayreuth, Germany Department of Applied Biological Chemistry, Faculty of Agriculture, Shizuoka University, Shizuoka 422-8529, Japan

Margot Schxilz

Dieter Sicker

Jifi Slavik

Gerhard Spiteller

Yasushi Todoroki

Bioactive Natural Products

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Atta-ur-Rahman (Ed.) Studies in Natural Products Chemistry, Vol 27 2002 Elsevier Science B.V. All rights reserved.

SWEET-TASTING AND SWEETNESS-MODIFYING CONSTITUENTS OF PLANTS NAM-CHEOL KIM^ and A. DOUGLAS KINGHORN* Program for Collaborative Research in the Pharmaceutical Sciences and Department of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, University of Illinois at Chicago, Chicago, Illinois 60612, U.SA,ABSTRACT: The demand for new alternative sweeteners has increased due to certain health problems associated with the use of sucrose. Although the currently developed and conunercially used sucrose substitutes are mostly synthetic compounds, the search for such compounds from natural sources is continuing and about 85 plant-derived sweet compounds of 19 major structural types are known, which have been obtained from 25 different families of green plants. Some of these highly sweet natural products are marketed as sweeteners or flavoring agents in several countries either as pure compounds or refined extracts. Several naturally occurring sweeteners have been chemically and enzymatically modified in order to increase their sweetness potency and/or improve their sweetness quality. In addition to natural sweet-tasting compounds, a number of naturally occurring sweetness-modifying compounds which induce or inhibit the sweet taste have also been isolated from plant sources. Several proteins and triterpenoids have sweetness-inducing properties. Over 60 triterpenoid glycosides have been reported from five plant species of the families Asclepidaceae and Rhamnaceae as sweetness-inhibitory (antisweet) principles.

INTRODUCTION The consumption of sucrose as a sweetener has been associated with several nutritional and medical problems, with dental caries being the most well-documented [1]. Sucrose intake may also be a factor in cardiovascular disease, diabetes mellitus, obesity, and micronutrient deficiency [2]. Therefore, there has been a continual demand for novel* Address correspondence to this author at Program for Collaborative Research in the Pharmaceutical Sciences and Department of Medicinal Chemistry and Pharmacognosy (M/C 781), College of Pharmacy, University of Illinois at Chicago, 833 South Wood Street, Chicago, Illinois 60612, U.S.A. E-mail: kinghom@uic.edu. ^ Current address: Chemistry and Life Sciences, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709, U.S.A.

highly sweet, non-caloric and non-cariogenic sucrose substitutes for the diabetic and dietetic market. Synthetic or natural sucrose substitutes are required to exhibit a sucrose-like taste quality with properties such as demonstrated non-toxicity, non-cariogenicity, lack of any offensive odor, and should exhibit satisfactory water solubility and hydrolytic and thermal stability. The so-called "high potency" sweeteners are at least 50100 times more highly sweet than sucrose [3]. Such compounds are also referred to as "intense sweeteners" and may be placed in a separate sweetener category than the less sweet caloric or "bulk" sweeteners represented by certain monosaccharides, disaccharides, and polyols, which are approximately equal to sucrose in sweetness potency [4,5]. To date, most of the currently available potently sweet sucrose substitutes in the world market are synthetic compounds: these include acesulfame-K, alitame, aspartame, cyclamate, saccharin, and sucralose [5,6]. These synthetic sweeteners are used as sucrose substitutes in most western countries but the regulations for each sweetener vary from country to country [7-12]. At present, in the United States, four are permitted as food additives, namely, acesulfame-K, aspartame, saccharin, and sucralose [5]. In the United States, food substitutes inclusive of the artificial sweeteners account for an approximately $1.2 billion market [6]. However, several problems with these compounds have long been apparent. For example, the general-purpose sweetener aspartame may not be consumed by persons with phenylketonuria because of the formation of a major metabolite, phenylalanine [9], Saccharin has been used as a sweetener for many years, but is now permitted only on an interim basis, owing to an association with bladder cancer in laboratory animals [11]. Therefore, currently, containers of products that include saccharin must have a cancer warning and state the amount of this sweetener [5]. Cyclamate was used in the United States until the Food and Drug Administration (FDA) banned this sweetener in 1969 because a saccharin and cyclamate mixture was found to cause cancer in laboratory animals [13]. However, cyclamate is still used as a sucrose substitute in about 50 countries [7]. A major metabolite of cyclamate is cyclohexylamine, which is somewhat toxic in causing testicular atrophy and untoward cardiovascular effects at high doses [7]. The search for improved sucrose substitutes is continuing, and one of the most potent sweeteners synthesized so far is the Ncyclononylguanidine derivative, sucrononic acid. This is actually the sweetest compound reported in the literature to date with a sweetness

potency of some 200,000 times that of sucrose [14]. Another synthetic compound of interest is superaspartame, which is produced by the combination of the urea derivative, suosan, with aspartame, and is about 14,000 times sweeter than sucrose [15]. Neotame, a A^-alkylated aspartame derivative, has been developed as a non-caloric sweetening agent, and has a sweetness potency of 10,000 times that of sucrose [15]. Besides the naturally occurring saccharides and polyols, there are a number of plant-derived highly sweet compounds, which are mostly terpenoids, flavonoids, and proteins [16-18]. Several of these sweet substances are used commercially as sucrose substitutes, as will be described in the next section. In addition, a number of plant substituents are known to mediate the sweet-taste response, either by inducing or inhibiting the perception of sweetness [19]. Thus far, all of the known natural product sweet-tasting substances and sweetness modifiers have been obtained from green plants [16-19]. In the remaining sections of this chapter, plant-derived sweet compounds with commercial use will be described, followed by a section on recent theories on the sweet taste phenomenon, and then individual descriptions of potent sweeteners, sweetness inducers, and sweetness inhibitors from plants will be presented in tum. The literature has been surveyed for this chapter until the end of 1999. Commercially Used Highly Sweet Natural Products While many isolated natural compounds have a sweet taste [20], only a few of these have been developed for commercial use. Natural product highly sweet compounds with some commercial use include glycyrrhizin (1), mogroside V (2), phyllodulcin (3), rebaudioside A (4), stevioside (5), and thaumatin, which are used as sucrose substitutes in one or more countries [16,21]. Some of these compounds have been modified chemically or biochemically to produce analogs that are more desirable as sweeteners, in being more highly sweet and/or more pleasant tasting. Although a number of commercially available "bulk" sweeteners with approximately the same sweetness potency as sucrose are naturally occurring, these compounds will not be considered further in this chapter. Examples include the monosaccharide, fructose; the monosaccharide polyols, erythritol, mannitol, sorbitol, and xylitol; and the disaccharide polyols, lactitol, and maltitol [5].

Glycyrrhizin (1), also known as glycyrrhizic acid, is an oleanane-type triterpenoid diglycoside isolated from the roots of Glycyrrhiza glabra L. (Leguminosae) and other species in the genus Glycyrrhiza [21]. Glycyrrhizin (1) is 93-170 times sweeter than sucrose, depending on concentration. In Japan, root extracts of G, glabra (which contain >90% w/w pure glycyrrhizin) are used to sweeten foods and other products, such as cosmetics and medicines. The ammonium salt of glycyrrhizin has Generally Recognized As Safe (GRAS) status in the United States and is used primarily as a flavor enhancer [22]. There have been several attempts using various glycosylation methods to increase the sweetness potency of glycyrrhizin (1). The Tanaka group at Hiroshima University in Japan glycosylated glycyrrhetinic acid to afford various glycyrrhizin monoglycoside analogs using a chemical and enzymatic glycosylation procedure [23]. A coupling reaction using mercury(II) cyanide [Hg(CN)2] for chemical glycosylation was effected, resulting in a significant enhancement of sweetness in the analogs obtained, especially the 3-0-pD-xylopyranoside (6) and 3-O-P-D-glucuronide (MGGR, 7). The sweetness intensities of compounds 6 and 7 were rated as 544 and 941 times sweeter than sucrose, respectively. Such chemically modified products of glycyrrhizin also showed improved taste qualities [24]. MGGR (7), in being more than five times sweeter than glycyrrhizin (1), as well as being readily soluble in water, is now used commercially as a sweetening agent in Japan [25].COOH

1 6 7

R=p-glcA2-P-glcA R=|3-xyl R = P-glcA

Mogroside V (2) is a cucurbitane-type triterpenoid glycoside isolated from the fruits of Siraitia grosvenorii (Swingle) C. Jeffrey

(Cucurbitaceae) [26]. An extract of the dried fruits of S, grosvenorii, containing mogroside V (2) as the major sweet principle, is used in Japan as a sweetener in certain foods and beverages. The sweetness intensity of mogroside V (2) has been rated as 250-425 times sweeter than sucrose, depending on concentration [22]. Recently, a major corporation in the United States has filed a patent concerning the use of extract of S. grosvenorii and other Siraitia species as a sweet juice [27].

P-glc^p-glc

p-glc^-P-glc

p-glc

A dihydroisocoumarin-type sweetener, phyllodulcin (3) occurs in glycosidic form in the leaves of Hydrangea macrophylla Seringe var. thunbergii (Siebold) Makino (Saxifragaceae) ("Amacha") and other species in this genus. After the fermentation of the leaves or by crushing, the native glycosides are enzymatically hydrolyzed, and the sweet phyllodulcin (3, x 400 sweeter than 2% sucrose) is produced. The fermented leaves of K macrophylla are used to prepare a sweet ceremonial tea in Japan, especially at "Hamatsuri", a Buddhist religious festival [22].

Rebaudioside A (4) and stevioside (5) are en^kau^ene-type diterpene glycosides isolated from the leaves of the Paraguayan plant, Stevia rebaudiana (Bertoni) Bertoni (Compositae) [28,29], with stevioside (5) being the more abundant compound in this plant part. The sweetness intensity of stevioside (5) has been rated as 210 times sweeter than sucrose, although this value varies with concentration [30]. However, rebaudioside A (4) (the second most abundant 5. rebaudiana e/-kaurene glycoside with a sweetness intensity rated as about 240 times sweeter than sucrose) is considerably more pleasant-tasting and more highly water-soluble than stevioside (5), and thus better suited for use in food and beverages [30]. Extracts of S. rebaudiana containing stevioside and/or purified stevioside are permitted as food additives in Japan, South Korea, Brazil, Argentina, and Paraguay, and have been used as herbal dietary supplements elsewhere, such as in several European countries, the People's Republic of China, and the United States [30]. In Japan, the largest market for the S. rebaudiana sweeteners to date, three different forms of stevia sweetener products are commercially available, namely, "stevia extract", "sugar-transferred stevia extract", and "rebaudioside A-enriched stevia extract". "Stevia extract" is a powder or granule made by several industrial steps and standardized so as to contain more than 80% of steviol glycosides, inclusive of dulcoside A (3-5%), rebaudioside A (20-25%), rebaudioside C (5-10%), and stevioside (5055%) [31]. "Sugar-transferred stevia extract" is made by transglycosylation of steviol glycosides present in commercially available "stevia extract" with a cyclomaltodextringlucanotransferase (CGTase)starch system prepared from Bacillus macerans [24]. There have been many attempts to improve the taste qualities of the major S. rebaudiana sweet steviol glycoside, stevioside (5), because of its sensory limitations [24,32-36]. Several systematic studies on the structure-sweetness relationship of steviol glycosides have been conducted [35,37]. For example, the sweetness-pleasantness of stevioside (5) may be increased by treating stevioside-galactosyl ester (Sgal), prepared by removal of the 19-0-glucosyl group of stevioside, and replacing it with a p-galactosyl group. Transglucosylation of the intermediate with soluble starch using CGTase prepared from 5. macerans then affords a mixture of mono-, di-, tri-, and tetra-a-glycosylated compounds. The product with four glucosyl units attached at the C-13 position showed an enhanced sweetness (8, Sgal-2) [35]. A rebaudioside A analog (9) with a (sodiosulfo)propyl

group at C-19 in place of a p-glucosyl moiety showed improved sweetness qualities [33]. Stevioside (5) has been converted synthetically to rebaudioside A (4), by removing a glucose unit from stevioside (5) at the C-13 position using amylase and then reintroducing synthetically two glucose units of different linkage to the remaining glucose unit at the C13 position [38]. "Rebaudioside A-enriched extract" is made from improved varieties of S. rebaudiana, which produce more rebaudioside A (4) than the native Paraguayan species [39].

COOR1 19 Ri R2 P-glc^-P-glc p-glc

4

p-glc

5 8 9

p-glc P-gal (CH2)3S03Na

P-glc'-P-glc P-glc^-P-glc^-p-glc^-a-glc p-glc^-p-glc p-glc

Thaumatin is a protein sweetener isolated from the fruits of Thaumatococcus daniellii (Bennett) Benth. (Marantaceae) [40]. Five different thaiunatin analogs are now known (thaumatins I, II, III, a, and b), and thaumatins I and II are the major forms with both having 207 amino acid residues [41]. The molecular weights of thaumatins I and II are 22,209 daltons and 22,293 daltons, respectively [42]. The threedimensional structure of thaumatin I, based on X-ray analysis has been reported [43,44]. The sweetness of thaumatin I is rated between 1,600 and 3,000 times in comparison to sucrose on a weight basis [22]. Talin protein, the trade name of the commercial form of thaumatin protein as an aluminum ion adduct, is approved as a sweetener in Australia, the United Kingdom, and some other countries, and was first permitted for use as a

10

food additive in Japan in 1979 [22]. Talin protein has GRAS status as a flavor enhancer for use in chewing gum in the United States [22]. Two natural product derived semisynthetic compounds are utilized as a limited basis, namely, perillartine [28] and neohesperidin dihydrochalcone [45]. Perillartine is an a-syn-oximQ and synthesized from perillaldehyde, a constituent of the volatile oil of Perilla frutescens (L.) Britton (Labiatae), and used in Japan as a sweetener for tobacco [28]. Neohesperidin dihydrochalcone is a dihydrochalcone glycoside prepared from a flavanone constituent of Citrus aurantium L. (Rutaceae), which is permitted for use in chewing gum and certain beverages in Belgium and elsewhere [45]. Discovery of Natural Sweeteners Searching for novel high-potency sweeteners from plants requires an initial dereplication stage for the presence of saccharides and polyols, which, as indicated earlier, exhibit sweetness potencies close to that of sucrose. If the combined amount of those saccharides and polyols exceeds 5% w/w in a given plant part, the resultant sweetness can be considered as being due to the presence of these "bulk" sweeteners. A suitable dereplication procedure using gas chromatography/mass spectrometry (GC/MS) has been developed for this purpose to rule out the sweetness contribution from saccharides and polyols in candidate sweet-tasting plants [46,47]. The general approach to the discovery of new sweetening agents of natural origin used at the University of Illinois at Chicago has been described previously [17,20,22]. RECENT REPORTS ON THE THEORY OF SWEET TASTE Many studies have been conducted to elucidate the functional groups present in sweet-tasting molecules that mediate the sweet taste. Initially, Shallenberger and Acree [48] suggested that sweet molecules contain both a hydrogen donor group (AH) and a hydrogen accepting group (B), and that these groups interact at a receptor by hydrogen bonding to exhibit a sweet taste. The existence of a third binding site (X) was proposed to explain the difference in sweetness between D- and L-amino acids, and was termed the AH-B-X model [49]. The hypothesis of an e-n (electrophile-nucleophile) group rather than a AH-B group was put

11

forward because of the lack of hydrogen bonding exhibited by some sweeteners [50]. Nofre and Tinti proposed a multipoint attachment (MPA) model in which the existence of eight optional cooperative recognition sites in the sweet receptor to interact with sweet molecules was suggested [51,52]. The binding of sweet molecules to the receptor site(s) can be achieved by ionic and hydrogen bonding, as well as by hydrophobic interactions in receptor sites which are designated as AH, B, and XH. There are also other sites that are designated as Gl, G2, 03, and 04 which fit sterically with the sweet molecules involved. Another recognition site, designated as D, was suggested as the hydrogen donor group. Even though all of the known sweet-tasting molecules may not bind to all of these binding sites, the most potent sweet taste intensities may result from the cumulative presence of such binding sites [51,52]. Recent work has been conducted to examine sweetener recognition by identifying the receptor molecules in the sweet taste receptor cells biochemically and physiologically. It is thought that "intense" sweeteners react with membrane receptor proteins connected to a O protein system [53]. In more recent studies, the concept of a multiple binding mechanism has been proposed [54]. In this system, calcium ion, inositol triphosphate (IP3), and cyclic AMP (cAMP) are involved as secondary messengers. The reaction of sugar sweeteners with the receptor can lead to an accumulation of cAMP, while interaction with the receptor of non-sugar sweeteners, such as saccharin, may result in the accumulation of IP3, suggesting that the reactions between sugar sweeteners and receptors, and between non-sugar sweeteners and receptors, have different mechanisms [54]. The receptor involved in these two different mechanisms appears on the same sensory cell. After the contact of a sweet molecule with the receptor, three different kinds of subunits of O protein, a, P, and y, may be activated by the receptor protein. In the sugar sweetener mechanism, stimulation by a sugar triggers a cAMP mediated cascade in which cAMP depolarizes the taste cell by reducing K^ conductance in protein kinase A " (PKA). This depolarization initiates the entry of Ca^"^ influx from the extracellular medium through voltage-dependent Ca^"^ channels, which stimulates the release of neurotransmitters in the synapse with sensory nerve fibers that carry the signal to the brain. The a-subunit of the O protein acting on phosphodiesterase (PDE) keeps the cAMP level low before and after sugar sweetener stimulation [54]. In the non-sugar sweetener system, stimulation mediates the a-subunit of the O protein

12

and triggers the phosphoinositide mechanism by transforming phosphatidyl inositol (PIP2) to IP3 by phospholipase C (PLC). This IP3 then stimulates the release of intracellular Ca^^ [54]. The types of G proteins which are involved in these two different mechanisms may be different, such as the Gs-type for sugar sweeteners and the Gq-type for non-sugar sweeteners. A taste-specific G protein, called gustducin, has been cloned, and possesses a, p, and y subunits, and the involvement of this protein in sweet taste transduction was also suggested [55,56]. Gustducin is related to transducin which is involved in vision, and both can activate PDE [57,58]. a-Gustducin may involve the cAMP pathway [59] while the py-gustducin complex stimulates PLC [60,61]. aGustducin maintains cAMP at a low level after the increase of cAMP by sugar sweetener stimulation. In a-gustducin knockout mice, the level of cAMP is high and the sweet taste stimulation is impared [62]. An interesting hypothesis has been postulated in terms of the sweet taste being activated by a receptor-independent mechanism [63]. It has been proposed that some amphipathic molecules, which have both hydrophobic and hydrophilic groups in the same molecule, can activate G proteins, and thus the enzymes responsible for the transduction pathway or channels, directly. Some non-sugar sweeteners are amphipathic molecules. These molecules can by-pass the taste receptors and permeate the plasma membrane, and activate GTPase directly in a concentrationdependent fashion, according to an in vitro experiment [63]. This stimulation penetration through the plasma membrane can also explain the slow taste onset and the lingering aftertaste which sometimes characterizes the taste of non-sugar sweeteners [64,65]. Certain non-sugar sweeteners even show a sweet taste nerve response when they are injected intravenously or intralingually which is independent of taste cell receptors in the tongue [66]. HIGHLY SWEET NATURAL PRODUCTS In this section, the presently known highly sweet substances of natural origin are described. Sweet-tasting compounds are listed in Table 1, with information published subsequent to an earlier chapter [16] then discussed in more detail. The structures of the compounds mentioned will be interspersed in the text, with the following abbreviations used to designate the sugar units of glycosides: api = D-apiofiiranosyl; ara = L-

13 Table 1. Highly Sweet Compounds from PlantsCompound type/name* Plant name Sweetness potency** Reference

MONOTERPENE Perillartine (10)'= Perillafrutescens (Labiatae) (L.) Britten

370

28

SESQUITERPENES Bisabolanes (+)-Hemandulcin (11) Lippa dulcis Trev. (Verbenaceae) L. dulcis 1,500

2867

4P-Hyclroxyhemandulcin (12) Acyclic glycoside Mukurozioside lib (13)

N.S.''

Sapindus rarak DC. (Sapindaceae)

ca. 1

47,68

DITERPENES Diterpene acid 4P, 1 Oa-Dimethyl-1,2,3,4,5,10hexahydrofluorene-4a,6adicarboxylic acid (14)' eitr-Kaurene glycosides DulcosideA(15) Stevia rebaudiana (Bertoni) Bertoni (Compositae) S. rebaudiana S. rebaudiana S. rebaudiana S. rebaudiana S. rebaudiana Rubus suavissimus S. Lee (Rosaceae) S. rebaudiana R. suavissimus Pine tree*^ 1,300-1,8008

28

30

28

Rebaudioside A (4) Rebaudioside B (16) Rebaudioside C (17) Rebaudioside D (18) Rebaudioside E (19) Rubusoside (20)

242 150 30 221 174 115 90N-S.**

28 28 28 28 28 68 2869,70

Steviolbioside (21) Steviol 13-0-P-D-glucoside (22) Stevioside (5)

S. rebaudiana

210

28

14 Table 1. Highly Sweet Compounds from Plants (continued)Compound type/name' Plant name Sweetness potency'' Reference

eif/-Kaurene glycosides (continued) Suavioside A (23) Suavioside B (24) Suavioside G (25) Suavioside H (26) Suavioside I (27) Suavioside J (28) Labdane glycosides Baiyunoside (29) Phlomis betonicoides Diels (Labiatae) P. betonicoides Baccharis gaudichaudiana DC. (Compositae) R. suavissimus R. suavissimus R. suavissimus R. suavissimus R. suavissimus R. suavissimus N.S.^ N.S.** N.S.** N.S.** N.S.** N.S.^

69 69 69 69 69 69

500N.S.**

28 20 71

Phlomisoside I (30) Gaudichaudioside A (31)

55

TRITERPENES Cucurbitane glycosides Bryodulcoside'' Bryonia dioica Jacq. (Cucurbitaceae) B. dioica B. dioica Hemsleya camosiflora C.Y. Wu et Z.L. Chen (Cucurbitaceae) H. camosiflora Siraitia grosvenorit (Swingle) C. Jeffrey (Cucurbitaceae) S. grosvenorii Siraitia siamensis Craib (Cucurbitaceae) N-S.**

28 72 72 73

Bryoside (32) Bryonoside (33) Camosifloside V (34)

N.S." N.S."

51

Camosifloside VI (35) Mogroside IV (36)

77233-392*

20 74

Mogroside V (2) 1 l-Oxomogroside V (37)

250-425 N.S.^

28 75

15 Table 1. Highly Sweet Compounds from Plants (continued)Compound type/name' Plant name Sweetness potency** Reference

Cucurbitane glycosides (continued) Scandenoside R6 (38) Hemsleya panacis-scandens C . Y . W u e t Z.L.Chen (Cucurbitaceae) H. panacis-scandens Siraitia grosvenorii; S. siamensis

54

73,74

Scandenoside Rl 1 (39) Siamcnoside I (40) Cycloartane glycosides Abrusoside A (41)

N.S.**

7674,75

563

Abrus precatorius L.; A.fruticulosus Wall et W. & A. (Leguminosae) A.precatorius\ A.fruticulosus A. precatorius; A.fruticulosus A. precatorius; A.fruticulosus A. precatorius

30

77-79

Abrusoside B (42)

100 50 75N.S.''

78,79

Abrusoside C (43)

78,79

Abrusoside D (44)

78,79

Abrusoside E (45) Dammarane glycosides Cyclocarioside A (46)

80

Cyclocarya paliurus (Batal.) Iljinsk (Juglandaceae) C. paliurus Gynostemma pentaphyllum Makino (Cucurbitaceae)

200

81

Cyclocarioside I (47) GypenosideXXJ(48)

250N.S.^

82 83

Oleanane glycosides Apioglycyrrhizin (49) Glycyrrhiza inflata Batal. (Leguminosae) G. inflata Glycyrrhiza glabra L. Periandra dulcis Mart.; P. mediterranea (Veil.) Taub. (Leguminosae)

300 15093-170

84 84 28 28

Araboglycyrrhizin (50) Glycyrrhizin (1) Pcriandrin I (51)

90

16 Table 1. Highly Sweet Compounds from Plants (continued)Compound type/name* Plant name Sweetness potency** Reference

Oleanane glycosides (continued) Periandrin II (52) Periandrin III (53) Periandrin IV (54) Periandrin V (55) Secodammarane glycosides Pterocaryoside A (56) Pterocarya paliurus Batal. (Juglandaceae) P. paliurus P. dulcis; P. mediterranea P. dulcis; P. mediterranea P. dulciSy P. mediterranea P. dulcis

95 92 85 220

28 28 28 85

50 100

86 86

Pterocaryoside B (57) STEROIDAL SAPONINS Osladin (58)

Polypodium vulgare L. (Polypodiaceae) Polypodium glycyrrhiza DC. Eaton P. glycyrrhiza

500 600N-S.**

8788-90

Polypodoside A (59)

Polypodoside B (60) PHENYLPROPANOIDS trans-Anetholc (61 f

89,91

Foeniculum vulgare Mill. (Umbelliferae) Illicium verum Hook f. (Illiciaceae) Myrrhis odorata Scop. (Umbelliferae) Osmorhiza longistylis DC. (Umbelliferae) Piper marginatum Jacq. (Piperaceae) Tagetes filicifolia Lag. (Compositae)

13

92

/raw5-Cinnamaldehyde (62)

Cinnamomum osmopholeum Kanehira (Lauraceae)

50

20

17

Table 1. Highly Svi^eet Compounds from Plants (continued)Compound type/name' DIHYDROISOCOUMARIN Phyllodulcin' (3) Hydrangea macrophylla Seringe var. thunbergii (Siebold) Makino (Saxifragaceae) Plant name Sweetness potency** Reference

400

28

FLAVONOIDS Dihydrochalcone glycosides Glycyphyllin (63) Smilax glycyphylla Sm. (Liliaceae) Citrus paradisi Macfad. (Rutaceae) Citrus aurantium L. N.S.*"

28 28 28 28

Naringin dihydrochalcone"^ (64) Neohesperidin dihydrochalcone'^ (65) Phlorizin (66)

3001,000

Symplocos lancifolia Sieb. et Zucc. (Symplocaceae) Symplocos microcalyx Hayata

N.S.**

Trilobatin (67) Dihydroflavonols and Dihydroflavonol glycosides 3-Acetoxy-5,7-dihydroxy-4'methoxyflavanone (68) 2/?,3/?-(+)-3-Acetoxy-5,7,4'trihydroxyflavanone (69) Dihydroquercetin 3-O-acetate 4'-methyl ether' (70)

N.S.**

28

Aframomum hanburyi K. Schum. (Zingiberaceae) A. hanburyi

N.S.^

93 93

N.S.'*

Tessaria dodoneifolia (Hook. & Am.) Cabrera (Compositae) T. dodoneifolia', Hymenoxys turneri K. Parker (Compositae) H. turneri

400

20

(2/?,3/?)-Dihydroquercetin 30-acetate (71)

80

20

(2/?,3^)-2,3-Dihydro-5,7,3',4'tetrahydroxy-6-methoxy-3-0acetylflavonol (72) (2/?,3/?)-2,3-Dihydro.5,7,3',4'tetrahydroxy-6methoxyflavonol (73)

25

94

H. turneri

15

94

18 Table 1. Highly Sweet Compounds from Plants (continued)Compound type/name* Plant name Sweetness potency** Reference

Dihydrochalcone glycosides (continued) (27?,3/?)-2,3-Dihydro-5,7,4'trihydroxy-6-methoxy-3-0acetylflavonol (74) Huangqioside E (75) H. tumeri

20

94

Engelhardtia chrysolepis Hance (Juglandaceae) E. chrysolepis

N.S.**

95 96

Neoastilbin (76) PROANTHOCYANIDINS Cinnamtannin B-1 (77)

N.S.**

Cinnamomum sieboldii Meisner (Lauraceae) C. sieboldii Selliguea feci Bory (Polypodiaceae) Arachniodes sporadosora Nakaike; A. exilis Ching (Aspidiaceae) A. sporadosora; A. exilis

RS.** N.S.^ 35N.S.**

97 97 98 99

Cinnamtannin D-1 (78) Selligueain A (79)

Unnamed (80)

Unnamed (81) BENZOl*]INDENO[i,2-2)-p-Dglucopyranosyl derivative] of abrusoside E exhibited about 150 times the sweetness potency of 2% sucrose, making it the sweetest compound in this series. When the aglycone carboxylic acid group was methylated, as in abrusoside E dimethyl ester, no sweetness was apparent [18,110].

COOH41 42 43 44 45 R = p-glc R=p-glcA-6-CH3^-P-glc R=p-glc^-glc R=p-glcA^-glc R=p-glc^-glcA

Cyclocarioside A (46) is a dammarane-type triterpenoid glycoside sweet principle from the leaves of Cyclocarya paliurus (Batal.) Iljinsk (Juglandaceae), a plant used in the People's Republic of China in the treatment for diabetes [81]. Recently, another sweet-tasting principle, cyclocarioside I (47), was isolated from the same plant along with two other compounds with the same dammarane-type triterpenoid aglycone structure [82]. Cyclocarioside I was rated as about 250 times sweeter than sucrose [82]. From the crude extract of the vine of Gynostemma pentaphyllum Makino (Cucurbitaceae), which is used to make a sweet tea ("Amachazuru") in Japan, gypenoside XX (48) was isolated [83]. Although the sweetness of this compound was not reported when it was

29

Ri

R2

46 47

a-ara-5-Ac a-ara

a-rha P-qui

first characterized, it was later stated to be sweet [18]. The relative sweetness intensity of gypenoside XX (48) to sucrose has not been determined, but this represents the first sweet dammarane-triterpenoid documented from a plant source.CH2OH

R2

48

P-glc^-P-glc

p-glc'-p-glc

a-rha

1;

As mentioned ealier, glycyrrhizin (1) and its ammonixun salts are available commercially for sweetening and flavoring purposes, and glycyrrhizin 3-0-D-glucuronide (MGGR, 7) is a promising new intense sweetener [21-24]. Apioglycyrrhizin (49) and araboglycyrrhizin (50) have been isolated from the roots of Glycyrrhiza inflata Batal. (Leguminosae) [84]. While glycyrrhizin has a C-3-affixed diglucuronate unit,

This Page Intentionally Left Blank

30

apioglycyrrhizin (49) has an P-D-apiofuranosyl-(l->2)-P-Dglucuronopyranosyl group and araboglycyrrhizin (50) an a-Larabinopyranosyl-(l->2)-p-D-glucuronopyranosyl group at the C-3 position of the aglycone, glyc)nThetinic acid. The sweetness intensities of apioglycyrrhizin (49) and araboglycyrrhizin (50) were rated as 300 and 150 times sweeter than sucrose, respectively.COOH

49 50

R=P-glcA^-P-api R = p-glcA^-a-ara

Periandrins I-IV (51-54) were characterized in the 1980's as oleananetype triterpenoid glycoside sweeteners from Periandra dulcis Mart. (Leguminosae) (Brazilian licorice) by the Hashimoto group at Kobe Pharmaceutical University in Japan [28], and the sweetness potency was determined as about 90 times sweeter than sucrose for each compound. Periandrins I-IV (51-54) were also found in another species, P. mediterranea (Veil.) Taub. [28]. A fifth compound in this series, periandrin V (55), was isolated from the roots of P. dulcis and found to be based on the same aglycone as periandrin I (51) [85]. The terminal Dglucuronic acid residue of periandrin I (51) was substituted by a D-xylose moiety in periandrin V (55). Periandrin V (55) exhibited 220 times the sweetness of 2% sucrose and was accordingly ranked as the sweetest substance obtained so far in the periandrin series [85]. Two novel sweet secodammarane glycosides, pterocaryosides A (56) and B (57), were isolated and structurally determined from the leaves and stems of Pterocarya paliurus Batal. (Juglandaceae) [86]. Pterocarya paliurus Batal. is a preferred taxonomic name for Cyclocarya paliurus (Batal.) Iljinsk (see above). The leaves of P. paliurus are used by local populations in Hubei Province of the People's Republic of China to

31

Ri

R2

51 53 55

P-glcA^-P-glcA p-glcA^-p-glcA p-glcA^-P-xyl

CHOCH2OH

CHO

HOOC

Ri p-glcA^-p.glc p-glcA^-P-glcA

R2

CHOCH2OH

sweeten cooked foods. While pterocaryoside A (56), which has a Pquinovose unit attached to the C-12 position, is 50 times sweeter than sucrose, pterocaryoside B (57), with an a-arabinose unit at C-12, was rated as 100 times sweeter than sucrose [86]. These are the first highly sweet secodammarane glycosides to have been isolated and structurally characterized, and represent interesting lead compounds for synthetic optimization. The steroidal saponin osladin (58) was isolated as a sweet principle from the fern Polypodium vulgare L. (Polypodiaceae) nearly 40 years ago [20,28]. However, the original structure proposed was later revised because the synthetic compound produced was not sweet at all [87]. The correct structure of osladin (58) was characterized by single crystal X-ray

32

HOOCx,^^

56 57

R = P-qui R = a-ara

crystallography and the stereochemistry of osladin was reassigned as 22/?, 25iS', and 26R [87]. The actual sweetness potency of osladin was revised as being as 500, rather than 3,000 times sweeter than sucrose [87]. Polypodosides A (59) and B (60) were isolated from the rhizomes of North American fern Polypodium glycyrrhiza DC. Eaton (Polypodiaceae) as additional highly sweet steroidal glycosides [88,89,91]. Their aglycone, polypodogenin, is the A'^'^-derivative of the aglycone of osladin. The structure of polypodoside (59) was also revised as 22/?, 255, 26/?, by a chemical interconversion procedure [17,89,90]. Polypodoside A (59) shows a high sweetness potency and was rated as 600 times sweeter than sucrose [88,89].

'

jCX^0Ri R: a-rha a-rha a-rha Other 7,8-dihydro p-glc^-a-rha p-glc^-a-rha P-glc

58 59 60

-

33

Phenylpropanoids The phenylpropanoids /ra5-anethole (61) and /ra5-cinnamaldehyde (62) are used as flavoring agents in foods in the United States and some other countries [20]. ^ra5-Cinnamaldehyde (62) was isolated from Cinnamomum osmophloeum Kanehira (Lauraceae) as a sweet principle, while ^ra5-anethole (61) was isolated as the volatile oil constituent responsible for the sweet taste of several plant species, as listed in Table 1 [92]. These two compounds occur widely in the plant kingdom. Therefore, it is necessary to rule out their presence in any candidate sweet plant by a dereplication procedure in a natural product sweetener discovery program using gas chromatography-mass spectrometry (GC/MS) [46,47].

Ri 61 62 CH3 CHO

R2 OCH3 H

Dihydroisocoumarin The dihydroisocoumarin, 3i?-phyllodulcin (3, obtained from the leaves of Hydrangea macrophylla var. thunbergii via enzymatic hydrolysis), was mentioned earlier in the chapter as having commercial use. Recently, it has been demonstrated that this sweet substance occurs naturally in unprocessed leaves of its plant of origin as a 5:1 enantiomer with the previously undescribed compound, SS-phyllodulcin [111]. Also reported in this study were the novel 3i?- and SS'-phyllodulcin 3'-0-glycosides, although the presence or absence of a sweet taste in these three new phyllodulcin analogs was not disclosed [111]. Flavonoids Glycyphyllin (63), phlorizin (66), and trilobatin (67) are sweet dihydrochalcone glycosides and were isolated from Smilax glycyphylla

34

Sm. (Liliaceae), Symplocos lancifolia Sieb. et Zucc, and Symplocos microcalyx Hayata (Symplocaceae), respectively [28]. Naringin dihydrochalcone (64) and neohesperidin dihydrochalcone (65) are semisynthetic dihydrochalcone glycosides and can be obtained as byproducts of the citrus industry. Neohesperidin dihydrochalcone (65) is regarded as the more promising sweetener of these two compoimds, because it is sweeter and has acceptable hedonic properties, and its longlasting sweetness is useful in chewing gum, candies, and oral hygiene products [16]. There have been a large number of attempts to synthesize improved dihydrochalcones, with such compounds requiring 3-hydroxy4-alkoxy substitution in ring B [16]. No additional sweet-tasting dihydrochalcones appear to have been isolated and characterized from plant sources in recent years.

Ri 63 64 65 66 67 H P-glc^-a-rha p-glc^-a-rha

R2 H CH3 CH3 H H

R3 H H OH H H

R4 a-rha H H H H

R5

H H Hp-glc H

Hp-glc

The seeds of Aframomum hanburyi K. Schum. (Zingiberaceae) are used as an antidote and ingredient in certain medicinal preparations in Cameroon [93]. From an acetone extract of the seeds of this plant, two sweet dihydroflavonols, 3-acetoxy-5,7-dihydroxy-4'-methoxyflavanone (68) and 2i?,3i?-(+)-3-acetoxy-5,7,4'-trihydroxyflavanone (69), were isolated [93]. 3-Acetoxy-5,7-dihydroxy-4'-methoxyflavanone (68) was previously isolated from a different species, Aframomum pruinosum Gagnepain [112]. However, the sweetness intensities of these compounds were not indicated [93,112]. The previously known (2i?,3i?)dihydroquercetin 3-0-acetate (71) which was rated as 80 times sweeter than sucrose, was isolated from Tessaria dodoneifolia (Hook. & Am.)

35

Cabrera and Hymenoxys turneri K. Parker (Compositae) [20]. The sweetness of this compound was increased to 400 times that of sucrose by methylation at the 4'-0H position (70) [20]. Two dihydroflavonols, huangqioside E (75) and neoastilbin (76), were isolated from Engelhardtia chrysolepis Hance (Juglandaceae) [95]. However, their sweetness was not evaluated. A series of three sweet additional dihydroflavonols (72-74) was isolated from K turneri [94].R3

HO^1 7

^0>1 1

l ir ^ sii

4'

R2^

OHRi

0R2 R3 R4

Other 1R,ZR IR.'hR

68 69 70 71 72 73 74 75 76

Ac Ac Ac Ac Ac H Ac a-rha^-p-glc a-rha

H H H H CH3O CH3O CH3O H H

H H OH OH OH OH H OH OH

CH3 H CH3 H H H H H H

2R,3R 2R,3R 2R,3R 2R,3R 2/2,3/? 25,35

Proanthocyanidins Several doubly linked ring-A proanthocyanidins are known to be sweettasting [97,99]. For example, two proanthocyanidins, cinnamtannin B-1 (77) and cinnamtannin D-1 (78), isolated from the roots of Cinnamomum sieboldii Meisner (Lauraceae) showed sweet properties [97]. Other sweettasting proanthocyanidins with carboxylic acid (80) and lactone (81) ftmctionalities, were isolated from the ferns Arachniodes sporadosora Nakaike and A, exilis Ching (Aspidiaceae) [99]. However, none of these proanthocyanidins was ever quantitatively rated for its sweetness intensity relative to sucrose. A sweet-tasting proanthocyanidin, selligueain A (79) was isolated from the rhizomes of the fern Selliguea feci Bory

36upper unit

middle unit ll terminal unit

OHRi 77 78 79 OH OH H R2 p-OH a-OH P-OH

80

37

81

(Polypodiaceae), collected in Indonesia [98]. Selligueain A may be distinguished from the previously known sweet-tasting proanthocyanidins since it has an afzelechin residue rather than an epicatechin moiety as the lower terminal unit of the molecule. When evaluated by a small human taste panel, selligueain A (79) showed 35 times the sweetness of a 2% sucrose solution and was not perceived as astringent when in solution [98]. A further doubly linked ring-A proanthocyanidin, selligueain B, was also isolated from the rhizomes of S.feei, but was not perceived as sweettasting [113]. As a result of the investigation of selligueain A (79) and related compounds, stringent structural requirements seem to be necessary for proanthocyanidins of this type to exhibit a sweet taste. In this connection, it is notable that an epimer of selliguaein A [epiafzelechin-(4p->8,2p^O->7)-epiafzelechin-(4p->8)-epiafzelechin] was astringent without any hint of sweetness [98]. Benzo [b] indeno [l,2'd\ py ran From the extract of the heartwood of Haematoxylon campechianum L. (Leguminosae), a sweet principle was isolated, namely, (+)-hematoxylin (82) [100]. This compound has been used for a long time as a microscopic staining reagent, but the sweetness of this compound was not recognized previously. Also, in the same study, brazilin, the 4-deoxy derivative of (+)-hematoxylin, and a constituent of Caesalpinia echinata Lam. (Leguminosae), was found not to be sweet [100]. In a follow-up study.

38

(+)-hematoxylin (82) was rated as 120 times sweeter than 3% sucrose, while its synthetic (-)-enantiomer was only 50 times sweeter [114].

Amino acid A highly sweet amino acid, monatin (83), was isolated from an African plant, Schlerochiton ilicifolius A. Meeuse (Acanthaceae) [101]. Monatin (83) was rated as being comparable to the synthetic amino acid, 6-chloroD-tryptophan, which showed a sweetness intensity of 1,300 times that of sucrose. Monatin (83) appears to be the only native plant amino acid with a highly sweet taste to have been discovered.

83

Proteins Several plant-derived proteins have been reported previously as sweeteners, inclusive of curculin [103], mabinlin [104,105], monellin [28,106], pentadin [107], and thaumatin, with the latter compound already mentioned as having commercial use as a sweetener and flavor enhancer [22]. Recently, a sixth sweet protein of plant origin, brazzein, was isolated from the fruits of an African climbing vine, Pentadiplandra

39

brazzeana Baillon (Pentadiplandraceae), which grows in Gabon, Zaire, and Cameroon [102]. Pentadin was also isolated from this same plant [107]. Brazzein has 54 amino acid residues and a molecular weight of 6,473 daltons making it a relatively small protein compared to other sweet proteins such as curculin (12,491 daltons), mabinlin (12,441 daltons), monellin (11,086 daltons), and thaumatin (22,206 daltons) [102]. Brazzein has four disulfide bridges and promising thermostability, since its sweetness was not destroyed at 80 C for 4 hours exposure [115]. Most of the other protein sweeteners are unstable to heat and inappropriate for use at high temperature. The sweetness of brazzein was rated as 2,000 times sweeter than 2% sucrose [102]. Brazzein has considerable potential as a new naturally occurring sweetening agent, because of its favorable taste profile and thermostability.

NATURALLY OCCURRING SWEETNESS INDUCERSA number of compoimds have been known for some time as sweetness inducers, including the caffeic acid conjugates cynarin and chlorogenic acid [116]. Arabinogalactin is also known to enhance the sweetness potencies of saccharin, cyclamate, and protein sweeteners such as thaumatin and monellin [22]. Miraculin, a protein isolated from the fruits of Richardella dulcifica (Schum. et Thonn.) Baehni (Sapotaceae) (miracle fruit) [117,118], and curculin, a protein isolated from the fruits of Curculigo latifolia Dryand. (Hypoxidaceae) [103] (see previous section), have sweetness-inducing activity [41]. While miraculin has no sweet taste per se, curculin has a sweet taste but this dissipates before the sweetnessinducing effect on water becomes evident [41]. Miraculin is a glycoprotein with a molecular weight of about 24,000 daltons and has the property of making sour or acidic materials taste sweet. Miracle fruit concentrate was formerly on the market in the United States, but was removed because prior FDA approval for the scientific claims made had not been realized [22]. Recently, five oleanane-type triterpenoid glycosides, strogins 1-5, were isolated from the leaves of Staurogyne merguensis Wall. (Acanthaceae). Strogins 1, 2, and 4 (84-86) show sweetness-inducing activity [119]. In Malaysia, S, merguensis grows wild and the local people use the leaves of this plant to sweeten rice during cooking [119]. The sweetness-inducing activities of strogins 1-5 were measured by a

40

psychometric method [119-121]. Briefly, using four subjects, 2 mL of a solution of each compound (1 mM) were held in the mouth for three minutes and then expectorated. Next, the subject tasted 5 mL of water. The induced sweetness activity was measured by comparing with a 0.050.4 M standard sucrose solution. A concentration level of strogins 1, 2, and 4 (84-86) of 1 mM exhibited the same perceived sweetness as 0.3 M sucrose solution [119]. Strogins 1, 2, and 4 (84-86) also showed a sweet taste. The sweetness of strogin 1 (84) at a 1 mM concentration was comparable to a 0.15 M sucrose solution. Strogins 2 (85) and 4 (86) tasted sweet, but the sweetness intensities were less than that of strogin 1 (84) [119].CH2OH ORha-2',3',4'-Ac

CH2OH 84 85 86 R=P-glcA2-P-xyl R=P-glcA R=P-glcA'-p-glc

NATURALLY OCCURRING SWEETNESS INHIBITORS It has long been known that a number of synthetic compounds and certain enzymes suppress the sweet taste in humans and animals [22,122-128]. Three plant species, Gymnema sylvestre (Retz.) R. Br. ex Schult. (Asclepiadaceae), Hovenia dulcis Thunb. (Rhamnaceae), and Ziziphus jujuba P. Miller (Rhamnaceae), have been studied extensively for their sweetness inhibitory (antisweet) constituents [19]. In recent years, additional sweetness-inhibiting agents have been isolated from G. sylvestre and K dulcis, as well as two other plant species, Gymnema altemifolium and Stephanotis lutchuensis Koidz. var. japonica (Asclepiadaceae). The presently known triterpenoid sweetness inhibitory agents from these species are reported in Table 2. A 35-amino acid

41 Table 2. Sweetness Inhibitors from PlantsPlant name Compound name* Sweetnessinhibitory potency** 0.125 0.125 0.125 1 1 0.5 0.25 0.5 0.5 0.5 N.S.'^ N.S.'^ 0.5 1 1 0.5 0.5 1 1 1 1 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.125 0.125 0.125 0.125 0.25 0.25 0.25 N.S." 0.125 0.125 0.125 0.0625 0.25 0.25 0.25 0.25 0.5 0.25 0.25 0.25 0.25 Reference 131 131 131 132 132 132 133 132 134 134 135 135 136 136 136 136 136 137 137 137 137 138 138 138 138 138 139 139 139 139 139 139 140 140 140 140 139 139 139 139 141 141 141 141 141 142 142 142 142

Gymnema sylvestre R. Br. ex Gymnemasaponin III (87) Gymnemasaponin IV (88) Schult. (Asclepiadaceae) Gymnemasaponin V (89) Gymnemic acid I (90) Gymnemic acid II (91) Gymnemic acid III (92) Gymnemic acid IV (93) Gymnemic acid V (94) Gymnemic acid VI (95) Gymnemic acid VIII (96) Gymnemic acid IX (97) Gymnemic acid X (98) Gymnemic acid XI (99) Gymnemic acid XII (100) Gymnemic acid XIII (101) Gymnemic acid XIV (102) Gymnemic acid XV (103) Gymnemic acid XVI (104) Gymnemic acid XVII (105) Gymnemic acid XVIII (106) Gymnema alternifolium^ (Asclepiadaceae) Altemoside I (107) Altemoside II (108) Altemoside III (109) Altemoside IV (110) Altemoside V (111) JujubosideB(112) HodulosideI(113) HodulosideII(114) HodulosideIII(115) HodulosideIV(116) HodulosideV(117) HodulosideVII(118) Hoduloside VIII (119) Hoduloside IX (120) Hoduloside X (121) Hovenoside I (122) Saponin C2 (123) Saponin E (124) Saponin H (125) Sitakisoside I (126) Sitakisoside II (127) Sitakisoside III (128) Sitakisoside IV (129) Sitakisoside V (130) Sitakisoside VI (131) Sitakisoside VII (132) Sitakisoside VIII (133) Sitakisoside IX (134)

Hovenia dulcis Thunb. var. tomentella Makino (Rhamnaceae)

Stephanotis lutchuensis Koidz. wax.Japonica (Asclepiadaceae)

42 Table 2. Sweetness Inhibitors from Plants (continued)Plant name Compound name* Sweetnessinhibitory potency** 0.25 0.25 0.25 0.25 0.25 0.5 0.5 0.25 0.25 0.25 0.25 0.5 0.125 0.125 0.25 Reference

Stephanotis lutchuensis Koidz. ydn.japonica (Asclepiadaceae) (continued) Ziziphus jujuba P. Miller (Rhamnaceae)

Sitakisoside XI (135) SitakisosideXII(136) Sitakisoside XIII (137) Sitakisoside XVI (138) Sitakisoside XVIII (139) Jujubasaponin II (140) Jujubasaponin III (141) Jujubasaponin IV (142) Jujubasaponin V (143) Jujubasaponin VI (144) JujubosideB(112) Ziziphin (145) Zizyphus saponin I (146) Zizyphus saponin II (147) Zizyphus saponin III (148)

143 143 143 143 143 144 144 144 144 144 144 144,145 144 144 144

" Structures of compounds 87-148 are shown in the text. ^ Potency compared with gymnemic acid I (90) (x 1). * N.S. = Sweetness-inhibitory potency not given. ^ * Plant taxonomic authority not given in the original article. *

peptide called gxmnarin has been isolated from the leaves of G. sylvestre^ and has also been found to exhibit a sweetness-inhibitory effect [129,130]. The sweetness inhibitory activity of plant terpenoids is evaluated by placing 5 mL of 0.5 or 1 mM solution of the compound in the mouth for 2-3 minutes. On expectorating, the mouth is then washed with distilled water. Then, different concentrations of sucrose (0.1-1 mM) are tasted. The maximum concentration of sucrose at which complete supression of sweetness is perceived may then be recorded for each tastant [133]. In practice, antisweet compounds of plant origin have tended to be ranked in terms of sweetness inhibitory potency by comparison with gymnemic acid I (90) [19]. Since the initial reports of sweetness-inhibitory oleanane-type gymnemic acids from the leaves of Gymnema sylvestre, plant species of the family Asclepiadaceae have served as bountiful sources of sweetnessinhibitory compounds. The initial isolation and structural characterization of these compounds was very challenging, and these early investigations have been reviewed [19]. In 1989, gymnemic acids I-VI (90-95) were isolated with a common gymnemagenin (149) aglycone structure and a

43

glucuronic acid moiety [132-134]. A different series of antisweet compounds were then isolated, namely, gymnemasaponins III-V (87-89) [131]. These non-acylated compounds show slightly less potent sweetness-inhibitory activities compared with the previously isolated gymnemic acids. Subsequently, the additional sweetness-inhibitory gymnemic acids VIII-XVIII (96-106), have been isolated from G. sylvestre [135-137]. Gymnemic acids XIII (101) and XIV (102) were previously named gymnemic acids VIII and IX when they were isolated by Yoshikawa et al [136]. However, Liu et al independently isolated different compounds designated as gymnemic acids VIII (96) and IX (97) from the same plant species [135]. Therefore, for clarification purposes, gymnemic acids VIII and IX were renamed as gymnemic acids XIII (101) and XIV (102), respectively [137]. The antisweet potencies of gymnemic acids XIII (101) and XIV (102) were rated as about half the potency of gymnemic acid I. For gymnemic acids XV-XVIII (103-106), their sweetness-inhibitory potencies were judged to be as about the same as that of gymnemic acid I (90) [137].

CH20R'

Ri

R2

87 88 89

P-glc P-glc*-P-glc P-glc'-P-glc

P-glc'^-p-glc p-glc P-glc^-P-glc

Gymnema altemifolium is an evergreen tree growing in the forests of Taiwan and the southem part of mainland China. The roots of this plant have been used for detoxification purposes, and for the treatment of edema and fever [138]. No phytochemical studies had been performed on G. altemifolium until a Chinese group isolated several common compounds including P-amyrin and cycloartenol from the fruits of this

44

^0R2

r^^^V'^i^^C 1 I 1 H r*CH20R4iCH2C)H 'OR5

^

OR3

tg^:

0 II C-

-c=c.

? " ^ /CH3H

nt)a:

c- - C H CH2CH3CH I3 R4 R5

0 II

Ri

R2

R3

90 91 92 93 94 95 96 97 98 99

p-glcA P-glcA p-glcA P-glcA p-glcA P-glcA^-P-glc P-glcA^-p-OG P-glcA^-P-OG p-glcA P-glcA P-glcA^-P-glc P-glcA P-glcA P-glcA P-glcA P-glcA P-glcA

tga mba mba tga tga tga mba tgaH

H H H H

Ac AcH H H H H H

H H H H H H H H H H H H H H

tgaH H H H H H H H

Ac

tga tgaH H

tgaAc

100 101 102 103 104 105 106 149

mba tgaH H H

mbaH

tga tgaH H H

tgaH H H

BzH H

BzH

H

plant [146]. More recently, several oleanane-type triterpenoid glycosides, altemosides I-V (107-111), have been isolated as sweetness inhibitors from the roots of G. alternifolium [138]. Complete hydrolysis of altemosides I-V (107-111) yielded a known oleanane-type triterpenoid, chichipegenin (150) [147]. There is no functional group at the C-21 and C-23 positions of the altemosides, as commonly present in the gymnemic acids. The antisweet effects of altemosides I-V have been evaluated using a 1 mM solution of each compound, and found to completely suppress the sensation of sweetness induced by 0.2 M sucrose solution in all cases.

45

The sweetness-inhibitory potencies of altemosides I-V (107-111) were rated as about half those of gymnemic acids XIII (101) and XIV (102) [136].

111" rj^ 1

y 0R2 ^CHaORa 'OR4 tga: II cR4

RiC23 Ri R2 R3

0

?"3 /CH3 H

107 108 109 110 111 150

p-glcA'-p-glc p-glcA^-p-glc p-glcA'-p-glc p-glcA p-glcA H

Ac H tga Ac H H

a-rha a-rha a-rha a-rha a-rha H

H Ac H H Ac H

Subsequent to the isolation of the dammarane-type triterpenoid glycosides jujuboside B (112), hodulosides I-V (113-117), hovenoside I, and saponins C2, E, and H (122-125) as sweetness inhibitors from the leaves of Hovenia dulcis Thunb. var. tomentella Makino [139], hodulosides VII-X (118-121) were isolated as sweetness-inhibitory agents [140]. Hodulosides I (113) and II (114) have hovenolactone (151) as their aglycone which is the same compound as in saponins E (124) and H (125). Hodulosides III-V and VII-X (115-121) are based on two different dammarane-type aglycone structures, however [139,140]. The sweetnessinhibitory potencies of hodulosides are shown in Table 2. The sweetnessinhibitory potency of hoduloside X (121) was not determined [140]. Recently, from the stems of Stephanotis lutchuensis var. japonica, an evergreen woody climber growing in forests near the warm coastal areas of Japan, several oleanane-type sweetness-inhibitory triterpenoid glycosides have been isolated, namely, sitakisosides I-IX, XI-XIII, XVI, and XVIII (126-139) [141-143]. Some sitakisosides have a Nsitakisosides VI (131), VII (132), XI, XII, and XIII (135-137) afforded sitakisogenin (152) [142,143], while hydrolysis of sitakisosides II (127)

46

112

R = a-ara^-P-glc^-P-xyl a-rha

115

R = a-ara^-P-qui p-glc

116

R = a-ara^-P-glc P-glc

l'

117

R = P-glc^-a-rha p-glc

122

R = a-ara^-xylp-glc

123

R = a-ara^-a-rha P-glc

CH2OR2

Ri

R2

118 119 120 121

a-ara^-a-rha a-ara a-ara^-a-rha a-ara^-a-rha.3

P-glc P-glc'-P-xyl P-glc^-P-xyl P-glc

P-glc

47

Ri

R2 p-glc

113 114

P-glc^-a-rha p-glc^-a-rha p-glc

H

124 125 151

P-glc^-a-rha p-glc

H H H

H

CH20H155

and XVIII (139) yielded marsglobiferin (153) [141,143]. In turn, hydrolysis of sitakisoside VIII (132) afforded 3p,16p,2ip,28tetrahydroxyoleanan-12-en-22-one (154) as the aglycone [142]. Sitakisoside IX (134) has a gymnestrogenin-type aglycone structure (155) [142]. The sweetness-inhibitory potencies of the sitakisosides are about 25% of that of gymnemic acid I, except for the most potent analog, sitakisoside V [130, 0.5 of the activity of gymnemic acid I (90)] (Table 2). In the late 1980s, ziziphin (145) was isolated from the Chinese jujube tree, Ziziphus jujuba P. Miller as the first recognized antsweet principle of this plant [146]. Ziziphin (145) has the same dammarane-type aglycone structure as in hodulosides III-V (115-117). Yoshikawa et aL isolated

48

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