Bioactive Compounds in Medicinal Plants: A Condensed Reviewscientificexhalters.com/PharmaCognosy/SEJ Cognosy -18-RW -1001.pdf · importance in the area of Nutraceuticals, which have

SEJ Pharmacognosy and Natural Medicine

Cite this Article: Swabha Takshak. Bioactive Compounds in Medicinal Plants: A Condensed Review. SEJ Pharmacognosy. 2018 (1)1.

Volume 1 Issue 1- Review Article

Bioactive Compounds in Medicinal Plants: A Condensed Review

Swabha Takshak*

Department of Botany, Banaras Hindu University, India

*Corresponding author: Swabha Takshak, Laboratory of Air

Pollution and Global Climate Change, Department of Botany,

Banaras Hindu University, Varanasi-221005, India; Email:

[email protected]

Submitted: 26 March 2018

Accepted: 16 April 2018

Published: 18April 2018

Keywords:

Alkaloids

Bioactive compounds

Medicinal plants

Pharmacological significance

Phenolics

Terpenoids

1. Abstract

Herbal drugs form the basis of all ancient medicines and majority of modern-day medicines. They are derived, the world over, from

medicinal plants. The pharmacological properties of herbal drugs derived from these plants are rooted in the plants‟ secondary

metabolites. The plant secondary metabolites with known pharmacological significance are more appropriately called as bioactive

compounds. The present review summarizes their emerging importance, classification, and pharmacological significance. The

classification terminology presented here is based on that given by M. Daniel in his book, Medicinal Plants- Chemistry and Properties,

since it is one of the most comprehensive classifications available in published literature. As the need and demand for plant-based

medicines increase, so will the exploitation of their resources. Hence, the present review also explores the plausible futuristic

perspectives pertaining to their efficient utilization and conservation.

2. Medicinal plants and their emerging

importance

For centuries, Phyto-medicines have dominated the health

scenario in India. Plants have been a source of natural

medicines and drugs in traditional systems such as Unani and

Ayurveda from ancient times. Since plant-based medicines

have very few side-effects compared to the synthetically

produced ones, people are more inclined towards the usage of

the former. Also, Phyto-medicines are more affordable,

especially in poor and developing countries [84,87]. The

research in the area of secondary metabolite production by

plants has gained importance in view of their being re-

recognized as natural sources of herbal drugs, pharmaceuticals

and Phyto-medicines. These compounds have also gained

importance in the area of Nutraceuticals, which have chronic

positive health effects and may prove to be advantageous in

the treatment of diseases like cancer, cardiovascular diseases

and diabetes [40]. Plant-based medicinal compounds serve as

the prototype for the preparation of their synthetic analogues

[157]. According to the National Medicinal Plant Board, the

revenue from the Indian herbal drug industry is likely to

increase up to Rs. 80 to 90 billion [154].

3. Bioactive compounds in Medicinal plants Bioactive compounds in plants have been defined as

“secondary plant metabolites eliciting pharmacological or

toxicological effects in man and animals” [15]. These are

present in comparatively higher concentrations in medicinal

plants and are produced from primary metabolites through

methylation, hydroxylation, and glycosylation biochemical

pathways [97]. During severe and/or chronic stress, the plant

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may shift its metabolism more towards the secondary, making

more resources available for defense, hence compromising on

growth [168].Bioactive compounds have a wide range of

biological activities like serving as antibiotics and

counteracting oxidative stress. When herbal medicines are

ingested, these compounds are expected to play the same role

in the human body [75,81]. However, their functions cannot

be established with conviction. For instance, a compound may

not have the same function in the plant as when it is extracted,

purified, and used ex-planta. Azadirachtin, is a commercially

important pesticide, but the neem tree, its source, is attacked

by insects probably because the latter overcome the feeding

barrier induced by this compound and also because this toxin

is concentrated in the neem seeds and not the leaves which are

under insect attack [72]. Also, some bioactives have multiple

functions. For example, salicylic acid serves as a signal

molecule during the plant pathogen attack, pollination (e.g.

Arum lily), herbivory(e.g. Salix spp.), and allelopathy (e.g.

Quercus falcate) [134]. Thirdly, some compounds function as

both primary and secondary metabolites. For example,

canavanine protects Canavalia ensiformis from bruchid beetle

attack; it also metabolizes during seed germination, serving as

a carbon source [53]. Lastly, secondary metabolites are

complex molecules, with a plethora of structures. Synthesized

via innumerable intertwined pathways, the end product of

interest is synthesized along with a multitude of other

secondary compounds. Consequently, it becomes difficult to

assign a unique function to any particular compound [72].

Bioactive compounds can be divided broadly into alkaloids,

terpenoids/terpenes, and phenolics. A detailed classification

[46], is outlined here. Though the compounds have been

classified, there is no watertight compartmentalized

classification. For instance, many of the alkaloids have

phenolic groups attached to them and can be categorized as

both alkaloids and phenolics. Also, alkaloids can be

categorised into monoterpenoid, diterpenoid, sesquiterpenoid,

steroidal, etc.

3.1. Alkaloids

Alkaloids are plant secondary metabolites containing carbon,

hydrogen, nitrogen, and occasionally oxygen. Their alkaline

nature is due to nitrogen. Their biosynthesis pathways are

complex, involving 20 or more enzymatic steps. A majority of

alkaloids are derived from amino acids (scuh as ornithine,

leucine, phenylalanine, tyrosine, and tryptophan, amongst

others), while others have their structural basis in compounds

like purine, nicotine, acetate, and isoprenoids [54,78]. They

may be distributed throughout the plant or restricted to storage

tissues, like fruits, roots, and seeds. They protect the plants

against herbivory and microbial attack, attract insects for

pollination, and are important from nutritional as well as

pharmacological perspective [46].

As per [46], alkaloids have been categorized into 17

categories. A brief description of these is given here. Their

medicinally active components and

their pharmacological importance are summarized in Table

(1).

Table 1: Classification, plant sources, medicinal/pharmacological compounds, and medicinal properties of some major categories of

alkaloids.

Alkaloids Some Source(s) Active Ingredients Medicinal Properties References

Alkaloidal

amines

Sida cordifolia,

Ephedra sinica, Catha

edulis, Urtica dioica

Ephedrine,

pseudoephedrine,

cathinone

Cardiac stimulants, anti-

inflammatory, sympathomimetic,

adaptogenic, immune-stimulant,

treatment of paralysis, arthritis,

haemorrhages, as hypoglycemic,

diuretic, expectorant, blood

purifier

Daniel 2006,

Mallikarjuna et al.,

2013

Diterpenoid

alkaloids

Aconitum napellus, A.

ferox, A. heterophyllum

Aconitine, artisine,

ajacine

Analgesic, diaphoretic, diuretic,

anti-diabetic, antimicrobial, anti-

diarrhoeal, hypotensive, sedative,

febrifuge, tonic, useful in

Daniel 2006,

Srivastava et al.,

2010



neuralgia, muscular rheumatism,

dyspepsia, acute inflammation,

loss of memory, hysteria, piles,

inflammatory joint infections

Imidazole

alkaloids

Lepidium meyenii,

Pilocarpus jaborandi,

P. microphyllus, P.

pinnatifolius

Pilocarpine, pilosine,

lepidiline A and B

Contraction of eye-pupil,

stimulate saliva secretion

Daniel 2006, Santos

and Moreno 2013

Indole alkaloids

Catharanthus roseus,

Uncaria guianensis, U.

tomentosa, Strychnos

nux-vomica,

Physostigma

venenosum, Ipomoea

nil, I. mauritiana,

Rauwolfia serpentina,

Passiflora incarnate,

Alstonia scholaris

Ergolines, vincristine,

brucine, vinblastine,

leurosidine,

leurosine, vincolidine

reserpine,

ergometrine,

ergotamine,

physostigmine,

serpentine, strychnine

Treat arthritis, inflammation,

tumours, diabetes, gastric ulcers,

chronic dysentery, cholera,

rheumatism, delirium, impotence,

leucorrhoea, insomnia, skin

diseases, used as anthelmintic,

aphrodisiac, sedative, diuretic,

anti-depressant, respiratory

stimulant, brain stimulant, anti-

hypertensive, tranquilizer,

improve mental function, reduce

stress, effective in fevers,

treatment of insanity, epilepsy,

nervous delibility, gastric and

duodenal ulcers, leukoderma,

conjunctivitis, paralysis, high

blood pressure, insanity,

schizophrenia, snake bites,

chronic diarrhoea, bowel

complaints, as blood purifier,

anthelmintic, cardiac depressant,

diuretic, laxative, nervine tonic,

for insomnia, liver congestion,

dropsy, ulcers, sores, cardiac

disorders, ulcers, nausea,

biliousness, dyspepsia

Szantay 1990,

Daniel 2006



Isoquinoline

alkaloids

Papaver somniferum,

Lophophora williamsii,

Tiliacora racemosa,

Argemone mexicana,

Berberis aristata.,

Chelidonium majus,

Alangium salvifolium,

Mahonia aquifolium,

Sanguinaria

Canadensis

Ipecac, berberine,

sanguinarine,

papaverine,

sanguinarine,

dihydrosanguinarine,

coptisine,

protoverine,

protopine,

chelidonine

chelerythrine

allocryptopine

cissampeline,

pareirubines A and B

Antimicrobial, antimalarial,

astringent, anthelmintic,

purgative, emetic,

hypoglycaemic, antispasmodic,

expectorant, cholagogue,

laxative, diaphoretic,

immunosuppressive, anti-

inflammatory, analgesic, anti-

hepatotoxic, diuretic, anti-

pyretic, antileukemic,

emmanagogue, treatment of

pain, cancer, bacterial infections,

amoebic dysentery, skin diseases,

eye infections, ulcers,

haemorrhoids, haemorrhages,

jaundice, piles, conjunctivitis,

rheumatism, diabetes, fever,

dysentery, piles, dropsy, urino-

genital troubles, tuberculosis,

asthma, HIV, stimulation of

bone-marrow leucocytes and

myocardial contractility, exhibit

anticholinesterase activity, as

rejuvenating agent, and for

improving intellect

Cui et al., 2006;

Daniel 2006

Monoterpenoid

alkaloids

Valeriana officinalis,

Enicostemma

hyssopifolium, Alstonia

rostrata

Valerian, gentianine,

actinidine, β-

skytanthine,

boschniakine

Carminative, antispasmodic,

sedative, restorative, anti-

diabetic, in fevers and snakebites

Bao et al., 2012,

Ishikura et al., 2013

Phenanthro-

indolizidine

alkaloids

Tylophora indica, T.

ovata, T.

atrofolliculata, Ficus

septica

Tylophorine,

tylophorinine,

dehydrotylophorine,

tylophorine,

ficuseptine-A, S-(+)-

Anticancerous, antiamoebic,

antiasthmatic, antifungal,

antibacterial, anti-inflammatory,

antitumour, antianaphylactic,

immunomodulatory, antiarthritis

Yang et al., 2006,

Liu et al., 2011



deoxytylophorinidine

, 3-O-demethyl

tylophorinidine,

tylophorinidine,

tyloindicines A–J,

tylocrebrine, O-

methyl

tylophorinidine

Purine alkaloids

Camellia assamica, C.

sinensis, Ilex

paraguariensis, Coffea

arabica,

Caffeine,

theobromine,

theacrine,

triacanthine,

theophylline, 7-

methylxanthosine

Stimulants, muscle relaxant

Ashihara et al.,

2008, 2013, Li et

al., 2013

Pyridine-

piperidine

alkaloids

Areca catechu, Piper

nigrum, P. longum,

Punica granatum,

Conium maculatum,

Lobelia inflata

Piperine, lobeline,

arecoline,

pelletierine, coniine,

nicotine, anabasine,

arecaidine, guvacine,

guvacoline,

lobelidine,

lobelanine,

isolobelanine,

lobelanidine

CNS-depressant, anti-pyretic,

analgesic, anti-inflammatory,

anthelmintic, antioxidant, hepato-

protective, antispasmodic,

sedative, anodyne, treatment of

chronic bronchitis, epilepsy,

acute mania, whooping cough,

obstructions in lymphatic system,

cataract, asthma, eczema,

diabetes, cardiac diseases, night

blindness

Daniel 2006

Quinazoline

alkaloids

Adhatoda zeylanica,

A.vasica, Aconitum

pseudo-laeve var

erectum, Nitraria

schoberi, Dichora

febrifuga, Polygonum

tinctorium, Sida acuta

Vasicine, vasicinone ,

anisotine

adhavasinone,

vasicoline,

vasicolinone,

adhatodine,

Antispasmodic, anthelmintic,

emmanagogue, antibacterial,

anti-tubercular, antifungal,

antihyperglycemic, anti-

inflammatory, cholinesterase

inhibitor, antifolate, expectorant,

sedative, hypotensive, cardiac

depressant, respiratory stimulant,

treatment of cold, cough, chronic

D'yakonov and

Telezhenetskaya

1997, Daniel 2006,

Eguchi 2006



bronchitis, diarrhoea, dysentery,

glandular tumours

Quinoline

alkaloids

Cinchona spp.,

Toddalia asiatica,

Glycosmis pentaphylla,

Camptotheca

acuminate, Ruta

chapelensis,

Zanthoxylum heitzii,

Pilocarpus grandiflorus

Camptothecin,

skimmianine,

kokusaginine,

evoxine, quinine,

choisyine,

cinchonine,

quinidine,

cinchonidine,

sonminine,

dictamnine,

isodictamnine,

haplopine, robustine,

isopteleine, 5-

methoxydictamnine

Diaphoretic, stomachic, anti-

pyretic, carminative, mutagenic,

possess stimulatory ganglionic-

blocking, curare-like,

spasmolytic activities, treatment

of malaria, diarrhoea, cough,

gonorrhoea, rheumatism,

anaemia, jaundice, fever, liver

complaints, eczema, skin

diseases

Daniel 2006, Pi et

al., 2010

Quinolizidines/

Lupinane

alkaloids

Cytisus scoparius,

Boerhavia diffusa,

Lupinus varius

orientalis, L. albus

albus, L. hartwegii, L.

densiflorus

Sparteine, lupanine,

cytisine, lupanine,

laburnine, 13-

hydroxylupanine

Diuretic, anti-inflammatory,

cardioprotective, anticancer,

block ganglionic transmission,

decrease cardiac contractility,

and contract uterine smooth

muscles

Daniel 2006,

Bunsupa et al., 2012

Pyrrolizidine

alkaloids

Onosma erecta,

Symphytum officinale,

Tussilago farfara,

Cynoglossum australe,

C. creticum, C.

officinale, Heliotropium

acutiflorum, H.

indicum, Lindelofia

angustifolia, Rindera

oblongifolia

Lycopsamine,

parsonsine, spiraline,

intermedine, bisline,

amabiline,

heliosupine, isoline,

latifoline,

viridiflorine,

jacobine, anadoline,

heliotrine,

lasiocarpine,

coromandaline,

indicine, rinderine,

Hepatotoxic, carcinogenic,

genotoxic, teratogenic,

pnemotoxic

Prakash et al., 1999,

Damianakos et al.,

2013



otosenine,

platyphylline,

senecionine,

integerrimine,

retrorsine, usaramine,

supinine,

Sesquiterpene

alkaloids

Celastrus paniculatus,

Euonymus japonica

Dendrobine, evonine,

maytoline, mayteine,

hippocrateine,

euonymine,

euojaponines D F J

and K, wilfornine,

wilfordinine,

rupestine

Sedative, emetic, diaphoretic,

emmenagogue, anti-dysentric,

anti-pyretic, analgesic,

tranquilizer, anti-tumour,

treatment of hysteria, pneumonia,

blood pressure, cancer,

rheumatism, lupus, depression,

sharpen memory

Han et al., 1990,

Daniel 2006

Steroidal

alkaloids

Veratrum viride,

Holarrhena pubescens,

Solanum americanum,

S. surattense, Wrightia

arborea, Fritillaria

unibracteata, Buxus

papilosa

Solanine, protoverine

A and B, jervine,

cyclopamine,

vaganins, cyclopsine,

conessine, holamide,

kuroyurinidine,

pimpifolidine,

taipaienine,

pubescinine

Anti-cancerous, hepatoprotective,

hypoglycaemic, expectorant,

anti-pyretic, antiamoebic, anti-

inflammatory, antithrombotic,

spasmolytic, hypotensive,

sedative, cardiac depressant,

treatment of dropsy, dysentery,

stomach disorders, abdominal

and glandular tumours, bronchial

asthma and other respiratory

diseases, reduction of plasma

cholesterol levels

Kui'kova et al.,

1999, Daniel 2006,

Zhang et al., 2011

Tropane

alkaloids

Atropa belladonna,

Hyoscyamus niger,

Datura metel, Scopolia

carniolica,

Mandragora

officinarum, Convlvulus

arvensis

Hyoscyamine,

scopolamine, (+)-

Pseudococaine,

atropine, hyoscine,

cocaine

Anticholinergic, CNS-

stimulatory, decreases the action

of salivary glands, pancreas and

lachrymal glands, reduce smooth

muscle spasms, hyper-secretion,

used in ophthalmic practices,

treatment of rheumatic pains,

ulcers, skin diseases, pain

Daniel 2006,

Grynkiewicz and

Gadzikowska 2008



Tropolone

alkaloids

Colchicum autumnale,

C. speciosum, Gloriosa

superba, Premna

corymbosa, Crataeva

nurvala

Colchicine,

colchiceine,

demecolcine,

demecolceine, iso-

colchicine,

isocolchiceine, N-

deacetylcolchicine,

N-deacetyl-

colchiceine, N-

deacetyI-N-

formylcolchicine, N-

deacetyl-N-

formylcolchiceine,

colchamine,

lumicolchicine

Antitumour, anti-inflammatory,

treatment of neoplastic diseases,

gout, rheumatic fever, leprosy,

colic, skin diseases, snake bites,

fevers, heart diseases,

neurological diseases, leukemia

Basova and

Rozengart 2005,

Daniel 2006

3.1.1. Alkaloidal Amines: This is a group of about 50

members also called protoalkaloids. Derived from the

aromatic amino acids (phenylalanine, tyrosine, and

tryptophan), these compounds are not heterocyclic, and are

also referred to as pseudo-alkaloids. They are found in

families like Cactaceae, Chenopodiaceae, Ephedraceae,

Fabaceae, Celastraceae, Malvaceae, and Urticaceae [31,46].

3.1.2. Monoterpenoid alkaloids: The alkaloids are about 20

and present in families like Valerianaceae, and Gentianaceae.

Most of them are derived from iridoid monoterpenes. Iridoids

are a class of bicyclic monoterpenes and often serve as

intermediates in the synthesis of alkaloids. They are typically

found in plants as glycosides, most often bound to glucose.

Iridoids are found in many medicinal plants and may be

responsible for some of their pharmaceutical activities. They

are produced by plants as defence against herbivores and/or

microorganisms [46,153].

3.1.3. Sesquiterpene alkaloids: Families in which these

alkaloids are found include Celastraceae, Orchidaceae,

Ranunculaceae, Garryaceae, Ericaceae, Asteraceae, Rosaceae,

and Fabaceae. Majority of these compounds are confined to

two major groups: the dendrobines and guaipyridines, the

latter being relatively structurally simpler than the former [28].

The dendrobine sesquiterpenoid alkaloids possess a highly

condensed tricyclic skeleton with a nitrogen atom, a bridged

lactone, and a cyclohexane ring, the latter being surrounded by

six stereogenic centers. The guaipyridine alkaloids are bi- or

tri-cyclic compounds with a 7-membered carboxylic ring and

only two stereogenic centers [28,153].

3.1.4. Diterpenoid alkaloids: Consisting of about 90

compounds, a majority of them are isolated from

Ranunculaceae (Aconitum and Delphinium). Garryaceae,

Asteraceae, and Rosaceae are the other families in which these

compounds are present [46]. Majority of these compounds are

derived from complex polyhydric alcohols possessing a

diterpenoid skeleton. These are further categorized into C18-,

C19-, and C-20 diterpenoid alkaloids [176].

3.1.5. Imidazole alkaloids: These alkaloids are found mainly

in the families of Rutaceae, Euphorbaceae, and Fabaceae [46].

These are biosynthesized from the precursors derived from

histidine. Pilocarpine is the only alkaloid which is

economically and pharmaceutically important [46]. Other

alkaloids with an imidazole nucleus are histamine, histidine,

and pilosine [5].

3.1.6. Indole alkaloids: This is the largest class of alkaloids

with about 600 members. They are practically confined to

families Apocyanaceae, Asclepiadaceae, Rubiaceae,

Loganiaceae, Convolvulaceae, Clavicipitaceae, Passifloraceae,

Fabaceae. Zygophyllaceae, and Rutaceae [46]. A majority are

derived from tryptophan and in many cases contain a part

derived from mavalonate via geraniol. They provide

protection against UV-B radiation [17].

3.1.7. Quinoline alkaloids: Comprising of about 100

members, these alkaloids are primarily reported from



Rubiaceae and Rutaceae [46]. 4-methylquinoline nucleus

combines with quinuclidine nucleus forming ruban, which, in

its alcoholic form of 9‟-rubanol is the basic skeleton for

various quinolone alkaloids such as quinine and quinidine [5].

3.1.8. Isoquinoline alkaloids: These are about 400 in number

and possess an isoquinoline nucleus. These are found in the

families of Cactaceae, Papaveraceae, Menispermaceae,

Ranunculaceae, Rubiaceae, Berberidaceae, Alangiaceae,

Aristolochiaceae, Araliaceae, and Nymphaceae. These can be

further sub-divided into simple isoquinolines, benzyl-

isoquinolines, and phenanthrenes [46]. These alkaloids are

derived from tyrosine [153].

3.1.9. Quinolizidines/ Lupinane alkaloids: These are

distributed in Ranunculaceae, Magnoliaceae, Lythraceae,

Asteraceae, Leguminoseae, and Nyctaginaceae. These are

generally toxic. The only medicinally important alkaloid is

sparteine [46].These alkaloids are derivedfrom lysine and

atructurally consist of two fused 6-membered rings sharing a

nitrogen atom. Higher concentrations of these alkaloids are

found in young photosynthetic tissues. Lupine alkaloids (from

the genus Lupinus) are the simplest alkaloids of this group

[5,153].

3.1.10. Quinazoline alkaloids: These are found in families

like Acanthaceae, Polygonaceae, Cruciferae, Malvaceae,

Fabaceae Araliaceae, Rubiaceae Scrophulariaceae, Asteraceae,

Poaceae, Arecaceae and Rutaceae [44]. Vasicine and related

alkaloids form the important members of this group. Some

example of quinazoline alkaloids are vasicine, ferbifugine,

glomerin, and homoglomerin. Their primary biosynthetic

precursor is anthranilic acid, but phenyalanine can also be a

candidate in some cases [5].

3.1.11. Phenanthro-indolizidine alkaloids: Only 5 alkaloids

possessing both phenanthrene and indolizidine nuclei are

known at present. They are present in members of

Asclepiadaceae, Apocyanaceae and Moraceae [46]. These

alkaloids are derived from ornithine, phenylalanine, and

tyrosine. The most commonly known compound of this group

is tylophorine [153].

3.1.12. Purine alkaloids: They are found in the families like

Rubiaceae, Malvaceae, Fabaceae, and Theaceae [46]. These

are implicated in protection against UV-B radiation. They are

biosynthesised via N-methyltransferases. The basic roles of

these purine alkaloids in plants are restriced to providing

defence against herbivores and pathogens and allelopathy [7].

3.1.13. Pyridine-piperidine alkaloids: About 125 alkaloids in

this group are known. The important alkaloids of this group

include arecoline, lobeline, pelletierine, coniine, piperine,

nicotine, and anabasine found primarily in Arecaceae,

Piperaceae, Puniaceae, Apiaceae, Lobeliaceae, Solanaceae,

Palmae, and Campanulaceae [46,125].

3.1.14. Pyrrolizidine alkaloids: About 360 pyrrolizidine

alkaloids are found in plants. These are primarily restricted to

the Boraginaceae, Asteraceae, Fabaceae, Orchidaceae,

Apocyanaceae, Ranunculaceae and Scrophulariaceae [46].

These are derived from ornithine and sometimes from

arginine. Structurally they have two 5-membered rings fused

together with a nitrogen atom at a common junction of both

rings. They are biosynthesized in the roots and from there are

transported to other parts of the plant. Most alkaloids of this

group are potential poisons [5,153].

3.1.15. Steroidal alkaloids: It is a large group of about 250

alkaloids. Most of them occur as glycosides and resemble

saponins. They are practically confined to families Liliaceae,

Ranunculaceae, Buxaceae, Apocyanaceae, and Solanaceae.

Their synthesis is presumed to start from cholesterol and likely

occurs in the cytosol [83]. These alkaloids have a 21-, 24-, or

27-carbon heterocyclic skeleton and are structurally divided

into solanidanes, spirosolanes, and jervanes [86].

3.1.16. Tropane alkaloids: Tropane alkaloids form a small

family of about 40 members belonging to families Solanaceae,

Erythroxylaceae, Euphorbiaceae, Convolvulaceae,

Lentibulariaceae, Brassicaceae, Rhizophoraceae, Asteraceae,

and Proteaceae [46]. Tropane alkaloids are derived from

arginine or ornithine via condensation with three acetate or

malate units. Roots are the biosynthetic organs for these

alkaloids from whence they are transported to the aerial plant

parts. (S)-tropic acid, derived from phenylalanine, upon

esterification, also synthesizes many tropane alkaloids

[46,153].

3.1.17. Tropolone alkaloids: About 25 alkaloids in this group

are known which are confined to Liliaceae, Verbenaceae, and

Capparaceae [46]. Colchicine is an important compound

possessing significant medicinal importance (Table 1).

Benzocycloheptanotropolone is the main structural ring in

colchicine. These compounds are sensitive to light and when

exposed, convert to their lumi derivatives [171].

The structures of the one of the representative compounds

from the above-mentioned gropus are given in Figure (1).

3.2. Terpenoids

Terpenes include more than 30,000 lipid-soluble compounds.

A majority of them are volatile and are responsible for the

plants‟ fragrance. The term „terpene‟ was initially used to

describe a mixture of hydrocarbons with a molecular formula

C10H16 occurring in the essential oils obtained from plant

tissues. „Terpenoids‟ is a more general term to describe the

hydrocarbons with the general formula (C5H8)n as well as their

oxygenated, hydrogenated, and dehydrogenated derivatives.

The 5-carbon building blocks of terpenes in plants are the

interconvertible isomers isopentenyl pyrophosphate (IPP) and

dimethylallylpyrophosphate (DMAPP). They are condensed

by prenyltransferases to give rise to a number of products like



geranyl, farnesyl, geranylgeranyl, pyrophosphates, squalene,

and phytoene, which are the precursors of terpenes. Their

subsequent modifications (like oxidation, cyclization, and

skeletal rearrangement) give rise to various isoprenoids [27].

The terpenoids range from monoterpenes (C10) to tetraterpenes

(C40) depending upon the number of C5 units present in their

structure. They are found prominently in leaf glandular

trichomes, bud exudates, and bark resins. In plants they

function as electron carriers (quinones), plant hormones (side

chain of cytokinins, abscisic acid, gibberellins, and

brassinosteroids), photosynthetic pigments (chlorophyll,

phytol, and carotenoids), and essential oils [80]. They act as

feeding deterrents, pollination attractants, defence compounds

photoprotectants, free-radical scavengers, and signalling

molecules [100,129]. They are exploited for their aromatic

qualities and pharmacological properties such as antibacterial,

anti-inflammatory, inhibition of cholesterol synthesis,

sedatives, etc. [127,179].

[46] has classified the terpenoids into the following detailed

categories as given below. The medicinal properties of this

class of compounds are outlined in Table (2).

Figure 1: Structures of one of the representative compounds of various categories of alkaloids



3.2.1. Monoterpenes: These are a group of C10 isoprenoids

constituting about 1000 metabolites formed by head-to-tail,

head-to-head, or tail-to-tail condensation of two isoprene

units. Most of them are colourless volatile oils [110]

containing volatile open chain and cyclic compounds like

menthol, limonene, geraniol, linalool, and pinene. They act as

pollinator attractants, and antioxidants [110].

3.2.2. Sesquiterpenes: More than 1200 sesquiterpenes are

known today. They comprise three isoprene units and occur as

volatile oils and bitter principles of many plants.

Sesquiterpene lactones occur primarily in Compositae,

Asteraceae, Apiaceae, and Magnoliaceae and act as feeding

deterrants, phytoalexins and antibiotics [46].

3.2.3. Diterpenes: These comprise of more than 3000

different compounds occurring in almost all plant families.

Biosynthesised in the plastids, they arise from the metabolism

of geranylgeranyl pyrophosphate (GGPP). They act as growth

hormones, defence compounds, and occur as phytol side chain

of chlorophylls, tocopherols and phylloquinone [27].

The above three categories of compounds, i.e. mono-, sesqui-,

and diterpenes together form the majority of essential oils and

are distributed in more than 2000 plant species belonging to

some 60 families (e.g. Rutaceae, Myrtaceae, Umbelliferae,

Labiateae, Compositeae, Pinaceae, to name a few). They have

several important functions in plants like protection against

predators (microorganisms, fungi, insects, and herbivores),

attraction of pollinators and dispersal of spores, inhibitors of

germination and growth, and ameliorating the effects of

abiotic stress on plants, amongst others [36].

3.2.4. Triterpenoids:

3.2.4.1. Triterpenoids and steroids: These are C30 carbon

chains derived from the head-to-head condensation of two

molecules of FPP (farnesyl pyrophosphate) to form squalene

which is the precursor for sterols, important membrane

constituents, via squalene synthase [18]. Triterpene derivatives

such as glycosides or fatty acid esters are also accumulated by

plant cells, for example, saponins [163] which can be

exploited as sources of drugs (like liquorice and ginseng) as

these possess medicinal and cytoprotective activities (Table 2)

[71].

3.2.4.2. Limonoids and Quassinoids: These C30 members

may be alipahatic, tetracyclic, or pentacyclic. Tetracyclic

members include limonoids primarily confined to order

Rutales. The families producing these compounds include

Meliaceae, Rutaceae, Cneoraceae, and Simaroubaceae. They

protect the plants against herbivory and have pharmacological

importance [19].

Quassinoids are highly oxygenated triterpenes from

Simaroubaceae family. C20 quassinoids are especially

prominent in pharmacological investigations and are divided

into tetracyclic and pentacyclic categories [66].

4.2.4.3. Glycosides: These compounds are usually found

bonded to mono- or oligosaccharide or to uronic acid. They

are further categorized into five major types:

3.2.4.3.1. Cardiac glycosides: They occur in plant families

such as Apocyanaceae, Asclepiadaceae, Fabaceae,

Celastraceae, Scrophulariaceae, Solanaceae, Hycinthaceae,

Asclepiadaceae, Asteraceae, Moraceae, Apocyanaceae,

Asparagaceae and Leguminoseae, Ranunculaceae,

Sterculiaceae, Cruciferae, Tiliaceae and Brassicaceae,

Liliaceae [92]. The core structure consists of a steroid nucleus

with a five membered lactone ring (cardenolides) or a six

membered lactone ring (bufadienolides) and sugar moieties

[119]. As the name indicates, the majority of the compounds

of this group have cardioprotective properties (Table 2).

3.2.4.3.2. Cyanogenic glycosides: They are common in

families like Rosaceae, Sapindaceae, Fabaceae, Linaceae

Euphorbiaceae, Papavaraceae, and Asteraceae, but rare in

others. They protect the plants against herbivores due to their

bitter taste and release of toxic hydrogen cyanide (HCN) on

wounding/tissue disruption [183]. Most of these compounds

are derived from tyrosine, phenylalanine, valine, leucine, and

isoleucine and have also been described as nitrogen storage

compounds [32].

3.2.4.3.3. Glucosinolates: Families containing glucosinolates

include Bataceae, Brassicaceae, Bretschneideraceae,

Cruciferae, Capparidaceae, Caricaceae, Euphorbiaceae,

Gyrostemonaceae, Limnanthaceae, Moringaceae,

Pentadiplandraceae, Phytolaccaceae, Pittosporaceae,

Resedaceae, Salvadoraceae, Tovariaceae, and Tropaeolaceae

[16]. They are sulphur containing compounds and depending

upon their precursor amino acids. These can be divided into

three groups: aliphatic (from alanine, leucine, isoleucine,

valine, and methionine), benzenic (from phenylalanine or

tyrosine), and indolic (from tryptophan). They provide defence

against herbivores [47].

3.2.4.3.4. Saponins: These C30 constituents are abundant in

nature. They get their name from soapwort plant (Saponaria),

the root of which was used historically as a soap. Depending

upon the structure of aglycone, they are categorised into

steroidal and the pentacyclic triterpenoids types. They occur in

the families like Liliaceae, Caryophyllaceae, Sapindaceae,

Polygalaceae, Sapotaceae, Phytolaccaseae, Ranunculaceae,

Papaveraceae, Linaceae, Rutaceae, Myrtaceae, Cucurbitaceae,

Araliaceae, Oleaceae, Rubiaceae, and Compositeae [46]. They

protect the plants from pathogens and ruminants [48].

3.2.5. Carotenoids: These are a group of about 700 lipophilic

pigments. They are tetraterpenoids (containing 40 C atoms)

with eight C5 isoprene units linked in a regular head-to-tail

manner except in the centre of the molecule where the order is

tail-to-head so that the molecule is symmetrical. Carotenoids

can be classified into carotenes and xanthophyll depending



upon the absence or presence of additional O-containing

groups. GGPP is also the biosynthetic precursor of the

carotenoids, which carry out essential functions in the life

cycle of green plants [57]. They act as accessory pigments in

the light-harvesting steps of photosynthesis and provide

colouration to flowers and fruits. Carotenoid concentrations

have been known to increase under UV-B radiation [85]. Due

to the presence of the conjugated double bond system, they are

capable of absorbing light in the range of 350-500nm [13].

Xanthophylls are functional in the light harvesting complex of

the chloroplasts. They quench triplet excited states in

chlorophyll by dissipating excess excitation energy via non-

photochemical quenching. Carotenoids in plants are also

precursors for the synthesis of the hormone abscisic acid [74].

3.2.6. Polyterpenes: Comprising of many isoprene units,

these have a molecular weight of over 1,00,000 and are the

constituents of rubber (Z-isomer of poly-isoprene) and gutta

percha (E-isomer of poly-isoprene). The rubber-like

substances of many other plants have similar composition.

Their biosynthesis occurs in the latex vessels or lacticifers.

The sticky nature of the latex helps in would healing of the

plants and checks the insect- and pathogen attack [14,153].

3.2.7. Resins: These are lipid-soluble complexes comprising

of volatile fraction (di- and triterpenoid compounds) as well as

non-volatile fraction (mono- and sesquiterpenoids). They

harden on exposure to air [15]. Oleoresin terpenes are

synthesised from isopentenyl diphosphate. These do not

attenuate UV-B radiation, but resin droplets may cause

scattering or reflection of UV-B radiation from the stem

surface. Families of plants producing resins include Pinaceae,

Podocarpaceae, Apiaceae, Cupressaceae, Arecaceae,

Platanaceae, Plumbaginaceae, Rutaceae, Guttiferae,

Cannabaceae, Moraceae, Dipterocarpaceae, Anacardiaceae,

Araucariaceae, Sapindaceae, etc. [175]. They protect the

plants against insect/pathogen attack, attract insects for

pollination, as well as prevent herbivory [186,21].

Some of the representative compounds from the above-

mentioned gropus are given in Figure (2).


terpenoids.

TERPENOIDS

/ TERPENES Some Source(s) Active Ingredients Medicinal Properties References

Monoterpenes

Valeriana

officinalis,

Tanacetum

parthenium, Thuja

occidentalis,

Quercus ilex,

Salvia leucophylla

Valerian, borneol,

camphor, limonene,

pinene, carene, linalool,

sabinene, camphene,

thujene, geraniol,

carvone, nerol, ocimene,

α-phenandrene, menthol,

citral

Tranquilizers, sedative,

antimicrobial, anti-leukemic,

laxative, hypotensive, analgesic,

anaesthetic, expectorant, anti-

puritic, antiseptic, diuretic,

sedative, spasmolytic, anti-

inflammatory

Heinrich et al., 2004,

Gurib-Fakim 2006,

Kashani et al., 2012

Sesquiterpenes

Chrysanthemum

parthenium,

Ageratum

conyzoids,

Anthemis nobilis,

Artemisia cina, A.

maritima,

Santalum album,

Pogostemon

Parthenolide, artemisinin

Antibacterial, antiviral, anti-

tumour, antimalarial, antitumour,

anti-leukemic, cytotoxic,

antifungal, remedy for

gastrointestinal problems

Bruneton 1999,

Castleman 2003,

Heinrich et al., 2004




cablin, Inula

racemosa

Diterpenes

Ginkgo biloba,

Taxus brevifolia,

Andrographis

paniculata,

Jatropha

glandulifera,

Caesalpinia crista

Taxol, gingkolides,

cafestol, kahweol,

Treatment of cardiovascular

diseases, cerebrovascular

diseases, dementia, stimulates

blood circulation, cholesterol

reduction, sedatives, anti-

spasmodics, anticancer,

treatmentof hypertension and

obesity

Bruneton 1999,



Triterpenoids:

Triterpenoids

and steroids

Allamanda

cathartica,

Vernonia cinerea,

Gossypium

arboretum, G.

herbaceum,

Euphorbia

thymifolia, E.

nivulia, Ficus

bengalensis, F.

carica, F.

religiosa

Digitoxin, cembrene,

taxisin, cassaic acid,

Gibberellin A1, casbene,

podocarpic acid

Anti-cancer, anti-inflammatory,

anti-bacterial, anti-malarial, anti-

atherosclerotic, antioxidative,

decrease incidence of

cardiovascular diseases

Piironen et al., 2000,

Castleman 2003, Shi et

al., 2004, Sparg et al.,

2004

Limonoids and

Quassinoids

Picrasma excelsa,

Ailanthus excelsa,

Azadirachta

indica, Brucea

javanica

Limonin, azadarachtin,

bruceantin, arbruside E

Anti-cancer, anti-neoplastic, anti-

tumour, antimalarial, antiviral,

anti-inflammatory, anti-leukemic,

anti-feedant, insecticidal,

amoebicidal, antiulcer, herbicidal,

potent inhibitors of induced

inflammation and arthritis

Manners and Hasegawa

1999, George et al.,

2000, Boeke et al.,

2004



Glycosides:

Cardiac

glycosides

Digitalis

purpurea,

Calotropis

gigantea, C.

procera, Nerium

oleander,

Thevetia

peruviana,

Helleborus niger

Ouabain, digoxin,

digitoxin, digitalis,

oleandrin

Cardioprotective, treatment of

cancer

Newman et al., 2008,

Bernhoft, 2010

Cyanogenic

glycosides

Manihot

esculenta,

Passiflora edulis,

Macadamia

ternifolia, Prunus

persica, Holocalix

balanseae,

Nectranda

megapotamica

Linamarin, amygdalin,

linustatin, dhurrin,

prunasin, vicianin

Hypothyroidism, supress and

soothe dry coughs

Vetter 2000, Ganjewala

et al., 2010

Glucosinolates

Brassica spp.,

Raphanus spp.,

Lepidium sativus,

Tropaeolum

majus

Glucotropaeolin, allicin

Antiproliferative, anti-

inflammatory, anti-cancer,

antioxidant, antimicrobial,

decrease carcinogenic disorders,

hypothyroidism

Fahey et al., 2001,

Mithen 2006, Denny

and Buttriss 2007

Saponins

Smilax spp.,

Polygala senega,

P. tenuifolia,

Platycodon

grandiflorum,

Agave americana,

Asparagus

racemosus,

Glycyrrhiza

glabra, Bacopa

Bacopasaponin,

Giganteoside D and E,

Frondoside A,

Lotoidosides,

Macranthoside B

Anti-inflammatory, anti-

neoplastic, antibacterial,

antifungal, anti-cholesterolemic,

anticancer, regulate body

immunity

Rajput et al., 2007,

Podolak et al., 2010



monnieri

Carotenoids

Crocus sativus,

Nyctanthes arbor-

tristis, Bixa

orellana,

Indigofera

tinctoria, Allium

fistulosum

α- and Β-carotene,

zeaxanthin, lycopene, β-

cryptoxanthin, lutein

Antioxidant, anticancer,

cardioprotective, modulating gene

expression and immune response

Fraser and Bramley

2004, Heinrich et al.,

2004, Gurib-Fakim

2006, Kashani et al.,

2012

Polyterpenes

Hevea

brasiliensis,

Achras sapota

rubber, gutta-percha -------- Bernhoft 2010

Resins

Pistacia

lentiscus var. Chi

a, P.

terebinthus var. C

hia, Betula

pendula, Pinus

contorta, P.

banksiana

--------- Antimicrobial agents, for healing

wounds Bernhoft 2010

3.3. Phenolics

Phenolics are ubiquitous across the plant kingdom with about

10,000 structures identified to date. Structurally, they have at

least one hydrocarbon ring with one or more hydroxyl groups

attached. They range from simple low molecular weight

compounds (coumarins, benzoic acids and their derivatives) to

more complex structures (flavonoids, stilbenes, and tannins).

Flavonoids are the largest and the most diverse group, with

about 6,000 members. They share a common structure of two

6-carbon rings, with a 3-carbon bridge, which usually forms

the third ring. The modification of this basic structure gives

rise to chalcones, flavanones, isoflavones, flavan-3-ols, and

anthocyanins [25].

Phenolics play several important functions in plants. They

protect cellular membranes and tissues from lipid

peroxidation, act as pollinator attractants and UV-B absorbers,

repair DNA by electron transfer reactions, are involved in

plant growth and reproduction, and provide resistance against

pathogens and predators [151].

Phenolics are mainly derived from phenylalanine and tyrosine

via the phenylpropanoid pathway. Table 3 gives an overview

of this class of compounds [46], including their sources,

medicinal components, and pharmacological properties.

Phenolics have been categorised into the following classes:

3.3.1. Flavonoids: Flavonoid biosynthetic pathway is

conserved in plants. Enzymes such as isomerases, reductases,

hydroxylases, and several Fe2+/2-oxoglutarate-dependent

dioxygenases modify the basic flavonoid skeleton leading to

the different flavonoid subclasses [115] and transferases

modify the backbone so formed with sugars, methyl groups,

and/or acyl groups. This leads to the different classes having

different solubility, reactivity, and physiological activities

[25,56]. Variations in the heterocyclic ring, position and

number of substituents (hydroxyl groups, methyl groups, for

instance) cause flavonoids to be divided into flavones,

flavonols, flavanones, catechins, anthocyanidins, and

isoflavones. These are induced in response to UV-B radiation

and provide organ protection by absorbing in the wavelength

range of 280-350 nm. They also stimulate root nodule

formation, and offer disease resistance [134].

Flavonoids have been classified into following categories:



3.3.1.1. Anthocyanins: Anthocyanins are an important group

of colouring pigments in plants. They are responsible for

attraction of pollinators and seed dispersal by animals eating

fruits. They may from conjugates with hydroxycinnamic or

other organic acids. They are structurally related to flavones

and are derived from them by addition or substitution of

hydroxyl groups or by methylation or glycosylation [46,162].

3.3.1.2. Anthocyanidins: These are anthocyanins without

sugar moiety. They provide colours to flowers and fruits and

hence attract insects for pollination. They also provide plant

protection against excessive light. Anthocyanins and

anthocyanidins (e.g. cyanidin, delphinidin, malvidin, peonidin,

and pelargonidin) and their glycosides are widely distributed

in the medicinal herbs such as inflorescences of Prunella

vulgaris and flowers of Rosa chinensis [33]. Camellia sinensis

and Areca catechu contained proanthocyanidins and

leucoanthocyanidins respectively [33,166].

3.3.1.3. Aurones: Aurones, (Z)-2-benzylidenebenzofuran-3-

(2H)-ones, are a rare subclass of flavonoids occuring in nature.

They provide bright yellow colour of some important

ornamental flowers, such as snapdragon (Antirrhinum majus,

Scrophulariaceae) and plants belonging to the Asteraceae

family (e.g. Coreopsis sp., Comos sp., and Dahlia sp.) in their

glycosylated forms. They are biosynthesized from chalcones

by the key enzyme aureusidin synthase [126]. Aurones

primarily occur in the epidermal cells of the flowers, however

they may also be found in bark, wood, leaves, seedlings, and

nectar. Aurones have been shown to be derived from chalcone

[123].

3.3.1.4. Flavan-3-ols and Flavan-3,4-diols: Flavan-3-ols are

flavonoids which generally polymerise to give rise to

proanthocyanidins (PAs) which are stored in the vacuole.

They are highly potent antioxidant compounds [138]. Flavan-

3,4-diols (such as leucocyanidin), are precursors for

anthocyanin synthesis [20].

3.3.1.5. Chalcones: These are biosynthesised through a

combination of acetate-malonate and shikimate pathways.

They are the precursors of open chain flavonoids and

isoflavonoids. Chalcones are α,β unsaturated ketones

consisting of two aromatic rings (ring A and B) having diverse

substituents. In plants they act as pollinator attractants, and

provide defence against pathogens, and contribute to the

plants‟ medicinal properties (Table 3). They are found in

families like Asteraceae, Solanaceae, Anacardiaceae,

Caesalpiniaceae, Piperaceae, and Apiaceae [34].

Dihydrochalcones, 1,3-diphenylpropan-1-ones, natural

phenolics related to chalcones, are restricted to about 30 plant

families like Rosaceae, Rutaceae, Lauraceae, and

Leguminosae. Both chalcones and dihydrochalcones are

synthesised via common pathway involving chalcone synthase

[62].

3.3.1.6. Flavonols: These are flavonoids occurring as O-

glycosides and are widely distributed in plants. Examples

include myricetin, quercetin, and kaempferol (Table 3). They

absorb UV-B light in the 280-320 nm region and they are

thought to act as UV-B filters [1]. They also function as

reactive oxygen species (ROS) scavengers [2], regulate auxin

transport and cell growth processes [95]. Families possessing

these compounds include Ranunculaceae, and Apiaceae

amongst others.


phenolic compounds.

PHENOLICS Some Source(s) Active Ingredients Medicinal Properties References

Flavonoids Anticancer, anti-viral, anti-

allergic, anti-stress, estrogenic

antibiotic, antioxidant,

antidiarrheal, antiulcer, anti-

inflammatory

Lu et al., 2006, Sharma

2006, Agrawal 2011

Anthocyanins and

Anthocyanidins

Hibiscus rosa-

sinensis, Vaccinium

spp., Saussurea

medusa

Cyanidin,

delphinidin,

malvidin, peonidin,

pelargonidin,

petunidin

Antioxidant, anti-platelet,

chemopreventive, antimicrobial,

anticarcinogenic, proapoptotic,

neuroprotective,

cardioprotective, anti-

Youdim 2002, Scalbert et

al., 2005, Lucioli 2012



hepatotoxic, anti-lipolytic,

vasodilatory, enhance memory

Aurones Antirrhinum majus,

Coreopsis spp.,

Cosmos spp., Dahlia

spp.

Hispidol,

sulfuretin,

aureusidin, auresin,

maritimetin,

letopsin, bracetin

Anticancer, antimicrobial,

antioxidant, anti-hormonal, anti-

inflammatory, anti-leishmanial,

enzyme inducing/inhibitory

activities, potential for treating

malaria, diabetes, viral

infections, skin diseases,

Alzheimer‟s disease

Boumendjel 2003, Vogel

et al., 2008, Zwergel et

al., 2012

Flavan-3-ols,

Flavan-3,4-diols

Humulus lupuhts,

Sambucus nigra,

Camellia sinensis,

Onobrychis

viciifolia,

Pseudotsuga

menziesii, Mammea

longifolia

(+)-catechin, (−)-

epicatechin, (+)-

gallocatechin, (−)-

epigallocatechin,

(−)-

teracacidin,

proapigeninidins,

propelargonidins,

proluteolinidins,

procyanidins,

prodelphinidins,

and

protricetinidin

Antioxidant, antimicrobial,

antiviral,

anticarcinogen, anti-

inflammatory,

neuroprotective

Cheynier 2005, Aron

and Kennedy 2008

Chalcones Artocarpus nobilis,

Angelica

Keiskei, Dalbergia

odorifera, Alpinia

rafflesiana,

Nymphaea

caerulea, Piper

methysticum,

Glycyrrhyza

uralensis,

Artemisia

Morachalcones A,

B and C,

Flavokawain B

Antioxidant, cytotoxic,

anticancer, antimicrobial,

antiprotozoal, antiulcer,

antihistaminic, anti-

inflammatory, cytotoxic,

chemopreventive, anti-

angiogenic, anti-infective,

antimalarial, aldose reductase

inhibitory, anti-protozoal,

antibacterial, anti-filarial, anti-

fungal, anti-convulsant, anti-

Lin et al., 2002, Batovska

and Todorova 2010, Doan

and Tran 2011, Rahman

2011



absinthium tuberculosis, mammalian alpha

amylase, cyclo-oxygenase, and

mono-amine oxidase inhibitory

activity

Flavonols Aconitum

tanguticum,

Sutherlandia

frutescens, Acacia

nilotica, Terminalia

arjuna, Aloe

barbedensis,

Moringa oleifera,

Ficus religiosa

Quercetin,

kaempferol,

myricetin, morin,

galangin, and their

glycosides

Anti-mutagenic, antioxidant, ,

antimicrobial, anti-carcinogenic,

anti-hypertensive, anti-allergic,

anti-depressant, anti-diabetic,

enzyme-inhibitors,

neuroprotective,

cardioprotective,

chemopreventive,

Bruneton 1999, Cai et al.,

2004, Goutam and Delip

2006

Flavones Scutellaria

baicalensis,

Biophytum

umbraculum,

Gentiana triflora,

Torenia

hybrida

Luteolin, apigenin,

baicalein, chrysin,

diosmin, rutin,

sibelin and their

glycosides

Anti-tuberculosis, anti-

microbial, anti-

tumour, antioxidant, anti-

carcinogenic, anti-

inflammatory, anti-proliferative,

anti-

angiogenic, anti-estrogenic,

improvement of

blood circulation

Bruneton 1999, Huang

et al., 2009, Kashani et

al., 2012

Flavanones and

flavanonols

Harungana

madagascariensis,

Arachis

hypogaea,

Hemizonia

increscens,

Eriodictyon

glutinosum, Thymus

vulgaris, Sophora

flavescens

Naringenin,

hesperetin,

eriodictyol, fisetin,

taxifolin,

and their

glycosides

(e.g. naringin,

hesperidin, and

liquiritin)

Antioxidant, antiproliferative,

estrogenic,

radio-protective, anti-

inflammatory,

analgesic, anti-

hypercholesterolemic, anti-

carcinogenic, antimicrobial,

hepatoprotective, CNS

depressants

Bruneton 1999, Pietta

2000, Tamilselvam et

al., 2013

Isoflavones Psoralea corylifolia,

Trifolium pratense,

Genistein,

daidzein, glycitein,

Anticancer, neuroprotective,

cardioprotective

Cai et al., 2004, 2006,

Huang et al., 2009



Erythrina

variegate,

Pueraria lobata

fromononetin, and

their

glycosides

Neoflavonoids Vaccinium myrtillus,

Sambucus nigra,

Hibiscus

rosa-sinensis,

Fagopyrum

esculentum,

Camellia

sinensis

Melanin, daidzein,

genistein,

glycitein

Anticancer, cardioprotective Manach et al., 2004,

Cheynier 2005, Kashani et

al., 2012

Simple phenolics Clematis chinensis,

Paeonia

suffruticosa,

Matteuccia

struthiopteris,

Agrimonia

pilosa, Angelica

sinensis,

Tripterygium

wilfordii

Catechol,

resorcinol, eugenol,

phloroglucinol,

pyrogallol

Antiseptic, antibacterial,

antifungal, local

anaesthetic, antioxidant

Scalbert 1991, Cowan

1999

Phenolic acids Barringtonia

racemosa, Cornus

officinalis, Cassia

auriculata,

Polygonum

aviculare, Punica

granatum, Rheum

officinale,

Sanguisorba

officinalis,

Terminalia chebula,

Cinnamomum

cassia, Lawsonia

Gallic acid, p-

hydroxybenzoic

acid,

protocatechuic

acid,

vanillic acid,

syringic acid,

ferulic acid, caffeic

acid, p-coumaric

acid, chlorogenic

acid,

sinapic acid,

salicylic acid,

Anticancer, cardio-protective,

anti-ulcer,

cytotoxic, antioxidant,

antiseptic,

antimicrobial, anti-

inflammatory, anti-

tumour, anti-spasmodic, anti-

depressant,

treatment of dyspepsia

Cai et al., 2004, Luk et

al., 2007, Surveswaran

et al., 2007



inermis, Foeniculum

vulgare, Ipomoea

turpethum,

Lavandula

officinalis,

Rosmarinus

officinalis

protocatechuic

acid, vanillic

acid, syringic acid

Acetophenones Gnetum ula,

Artemisia anua,

Tannacetum

polycephalum,

Cynanchum bungei

Dextropropoxyphe

ne,

phenylpropanolami

ne

Anti-asthmatic, anti-

inflammatory,

antioxidant

Müller et al., 1999,

Kedar et al., 2010

Benzophenones Humulus lupulus,

Curcuma longa,

Capsicum annum,

Piper methysticum,

Securidaca

diversifolia,

Hypericum elegans

Benzophenones 1-

12

Anticancer, antibacterial,

antiviral, anti-

parasitic, antioxidant, anti-

inflammatory,

cytotoxic, anti-HIV, estrogenic

Ma et al., 2003,

Seidlova-Wuttke et al.,

2004

Xanthones Gentiana lutea,

Mangifera indica,

Madhuca indica

Gentisin,

mangiferin

Antioxidant, anti-tumour, anti-

inflammatory, anti-allergy, anti-

bacterial, anti-fungal, anti-viral,

anti-allergic, anti-malarial

Akao et al., 2008, Yang et

al., 2009, Shan et al.,

2011

Coumarins Aegle marmelos,

Feronia

limonia, Eclipta

alba,

Mammea americana

,, Torresea

cearensis, Justicia

pectoralis, Eclipta

alba, Pterodon

polygaliflorus,

Umbelliferone,

daphnatin,

warfarin,scopoletin

, scoparone,

fraxetin, 4-

methyldaphnetin,

dicoumarol,

marmelosin

anthelmintic, anti-asthmatic,

anti-coagulant, anti-tumour, anti-

viral, anti-inflammatory, anti-

oxidant, anti-microbial and

enzyme-inhibitory, anti-obesity,

anti-mutagenic, digestive,

astringent, stomachic, heart

tonic, hypoglycaemic,

spasmolytic, vasodilators

Daniel 2006,

Kontogiorgis et al.,

2007, Borges et al.,

2009



Hybanthus

ipecacuanha

Chromones Bergenia ciliata,

Ammi visnaga,

Eranthis hyemalis,

Baeckea frutescens,

Eucryphin,

cromolyn, 6,7-

dimethoxy-2,3-

dihydrochromone

Anti-inflammatory anti-viral,

anti-microbial, anti-cancer,

antioxidant, biocidal, anti-

spasmodic, anti-diabetic, anti-

ulcer, anti-HIV, wound healing,

immune stimulatory activities

agents

Kumar and Yusuf 2006,

Sharma et al., 2011

Stilbenes Reynoutria

japonica,

Polygonum

cuspidatum,

Rhodomyrtus

tomentosa,

Passiflora edulis

Resveratrol,

oxyresveratrol,

piceatannol,

petrostilbene,

pinosylvin

Chemoprotective, anticancer,

antioxidant, anti-ageing, anti-

angiogenesic, neuroprotective,

anti-fungal, immune modulation

Shankar et al., 2007,

Zhang and Björn 2009,

Kasiotis et al., 2013

Lignans Podophyllum

peltatum, P. emodi,

Phyllanthus amarus,

Linum

usitatissimum,

Schisandra

chinensis

Matairesinol,

enterolactone,

podophyllotoxin

Antimicrobial, antioxidant, anti-

inflammatory, anti-cancer,

antiviral, anti-pyretic, diuretic,

analgesic, anti-rheumatic, anti-

neoplastic, phytoestrogenic,

cathartic, immunosuppressive,

hepato-protective, cardio-

protective, treatment of

osteoporosis, rheumatoid

arthritis, gastric and duodenal

ulcers

Berhhoft 2010, Nagar et

al., 2011, Cunha et al.,

2012

Quinones:

Benzoquinones Neoregelia laevis,

Diospyros

crassiflora, Embelia

schimperi,

Polygonatum

falcatum

Coenzyme Q Antioxidant, anti-inflammatory,

anti-cancer, treatment of

mitochondrial diseases, prevent

atherosclerosis, retinal cell

apoptosis

Skulachev et al., 2009,

Lulli et al., 2012, Perez-

Sanchez et al., 2012



Naphthoquinones Plumbago

zeylanica,

Rhinacanthus

nasutus, Tabebuia

avellanedae

Plumbagin,

ubiquinone,

plastoquinone K

vitamins,

juglone

Antibiotic, anti-viral, anti-

inflammatory, anti-pyretic, anti-

proliferative, cytotoxic, anti-

allergic, anti-asthmatic

Babula et al., 2009

Anthraquinones Embelia ribes,

Plumbago indica,

Cassia tora, C.

fistula, C.

augustifolia,

Hypericum

perforatum, Rumex

crispus

Hypericin, emodin,

rhein

Anti-inflammatory, anti-

depressant,

antimicrobial, anti-dermatic,

anthelmintic,

purgative, tonic, treatment of

gout,

rheumatism, leukaemia,

responsible for

peristaltic colon movement and

water and

electrolyte secretion

Cai et al., 2006, Huang

et al., 2007

Tannins

Condensed

tannins

Rhus javanica,

Quercus lusitanica,

Geranium

thunbergii, Tellima

grandiflora, Mimosa

tenuiflora,

Caesalpinia

pyramidalis,

Agrimonia pilosa,

Procyanidin,

prodelphinidins

Antioxidant, anti-cancer, anti-

HIV, anti-

diarrhoea, anti-inflammatory,

antibacterial

Gurib-Fakim 2006,


Hydrolysable

tannins

Neoregelia laevis,

Diospyros

crassiflora, Embelia

schimperi,

Polygonatum

falcatum

Gallotannins,

ellagitannins

Anti-diarrhoea, antidote in

poisoning by heavy metals


Bernhoft 2010, Kashani et

al., 2012



Figure 2: Structures of one of the representative compounds of various categories of terpenes/ terpenoids

3.3.1.7. Flavones: These compounds bear very close

resemblance to flavonols. Most of them occur as 7-O-

glycosides though various substitutions are also possible

including methylation, hydroxylation, O- and C- alkylation

and glycosylation [40]. They differ from flavonols in lacking

a 3-hydroxyl substitution; this affects their UV absorption

properties. Flavones normally occur in herbaceous plants.

They act as UV filters and prevent light penetration to internal

tissues. They are mainly distributed in Labiateae, Apiaceae,

Asteraceae, and so forth such as the roots of Scutellaria

baicalensis, inflorescences of Chrysanthemum morifolium,

and aerial parts of Artemisia annua. Examples include

luteolin, apigenin, baicalein, chrysin, and their glycosides

(Table 3) [77].

3.3.1.8. Flavanones and flavanonols: Flavanones (also

termed dihydroflavones) are a minor subclass of plant

flavonoids. The basic chemical structure involves two benzene

rings (A and B) linked by a third heterocyclic, saturated ring.

Flavanones occur in several plant families including Libiatae,

Annonaceae, Acanthaceae, Compositae, Leguminosae,

Rosaceae and also Rutaceae [26]. They are synthesised from

chalcones via chalcone-flavanone isomerase. Examples

include naringenin, hesperetin, eriodictyol, and their

glycosides (e.g. naringin, hesperidin, and liquiritin) [77].

Flavanonols may act as precursors to anthocyanins involving

enzymes like dihydroflavonol 4-reductase (DFR),

anthocyanidin synthase (ANS), and anthocyanidin 3-glycosyl

transferase (3GT) [178].

3.3.1.9. Isoflavones: These compounds comprise the largest

group of isoflavonoids. They differ from flavones in the

location of the phenyl group which is attached to the third

position of the heterocyclic ring rather than the second. They

are present in families such as Leguminoseae,

Chenopodiaceae, Polygalaceae, and Iridaceae [51]. They play



a role in nitrogen-fixation and act as defence compounds.

Some important isoflavones are genistein, daidzein, glycitein,

fromononetin, and their glycosides (Table 3) [77].

3.3.1.10. Neoflavonoids:

Neoflavonoids are derived from isoflavonoids and depending

upon their structural variations are further categorised into 4-

phenyl coumarins, dalbergiones, and aryl chromans.

Neoflavonoids are concentrated in many plant families:

Guttiferae, Leguminoseae, Rubiaceae, Passifloraceae,

Polypodiaceae, and Compositeae [46].

3.3.2. Simple Phenolics: These are C6 compounds. Simple

phenolics are substituted phenols with one of the functional

groups being the hydroxyl group. These consist of a singly

substituted phenolic ring with either alcoholic, aldehydic, or

carboxylic acid groups. These are toxic to microorganisms but

their occurrence is rare in plants [149].

3.3.3. Phenolic Acids: Phenolic acids comprise two parent

structures: hydroxycinnamic and hydroxybenzoic acids. The

former include ferulic acid, caffeic acid, p-coumaric acid,

chlorogenic acid, and sinapic acid while the latter includes

gallic acid, p-hydroxybenzoic acid, protocatechuic acid,

vanillic acid, and syringic acid [34]. Phenolic acids occur in

either free or conjugated forms, usually as esters or amides.

These are widely distributed in the plant kingdom. They play a

role in plant growth and reproduction, in rhizospheric plant-

microbe interactions, and act as defence compounds against

stresses [172]. They are also involved in plant nutrient uptake,

protein synthesis, photosynthesis, as structural components

and in allelopathy. A majority of them occur in bound form to

the structural components of the plants (cellulose, protein,

lignin) or to larger polyphenols (flavonoids) or smaller organic

molecules (e.g. glucose, quinic, maleic or tartaric acids) or

other natural products (e.g. terpenes) [144]. They are

synthesised via phenylpropanoid pathway, as by-products of

the monolignol pathway and as breakdown products of lignin

and cell wall polymers in vascular plants [113].

3.3.4. Acetophenones: These are derived from phenylalanine

[50]. They are aromatic ketones and some occur as glycosides.

Families possessing these compounds include Loranthaceae,

Gnetaceae and Urticaceae. Their usage as drugs is limited to

raw material in the synthesis of several pharmaceuticals

(Table 3) [90].

3.3.5. Benzophenones: Benzophenones are the precursors of

xanthones. They are low molecular weight compounds

containing two phenol rings and various hydroxyl groups.

These are prominent in the families Hypericaceae, Olacaceae,

Clusiaceae, Gentianaceae Polygalaceae, Moraceae and

Guttiferae. These are capable of absorbing UV-A (315-

400nm) and UV-B (280-315 nm) radiation and this property

and are in the production of sunscreens and skin lotions [165].

3.3.6. Xanthones: Xanthones (dibenzo- γ-pyrone) have nearly

one thousand known naturally occurring derivatives. They are

biosynthesised via mixed shikimate-acetate pathway [131].

Simple xanthones contain simple substituents such as

hydroxy, methoxy or methyl [116]. These compounds are

found in families such as Gentianaceae, Guttiferae,

Logoniaceae, Podostemaceae, Hypericaceae, Moraceae,

Polygonaceae and Clusiaceae [46].

3.3.7. Coumarins: These are aromatic constituents of plants

(C6-C3 derivatives), numbering more than 1300 and almost all

are O-substituted. They are found in families such as

Apiaceae, Asteraceae, Fabaceae, Moraceae, Rosaceae,

Rubiaceae and Solanaceae Thymeliaceae, Rutaceae,

Umbelliferae, and Euphorbiaceae [46]. Their pharmacological

and biochemical properties depend upon their pattern of

substitution. The simple coumarins occur as hydroxylated,

alkoxylated, alkylated and glycosylated derivatives of the

parent compound [98].

3.3.8. Chromones: These compounds are synthesized via the

acetic acid pathway involving the condensation of five acetate

molecules. They are similar to coumarins in structure and

properties. They are usually characterized by a methyl group

at C2 and oxygen at C5 and C7. Families possessing chromone

alkaloids include Meliaceae, Rubiaceae, and Fabaceae. They

play antioxidative and defensive roles in plants [46,170].

3.3.9. Stilbenes: They are derived from malonyl CoA and 4-

Coumaroyl CoA via enzyme stilbene synthase from the

phenylpropanoid pathway. Examples include picetannol and

resveratrol which accumulate under UV-B [185]. They occur

in plants as monomers and oligomers. They function as

defence compounds, phytoalexins, allelopathic agents, and

respond to oxidative stress generated by UV irradiation

[38,68]. These are produced by a number of unrelated plant

families like Pinaceae, Cyperaceae, Vitaceae, Polygonaceae,

Betulaceae, Fabaceae, and Poaceae [38].

3.3.10. Lignans and Lignin: They are formed via general

phenylpropanoid and the monolignol pathway [177], and are

composed of two phenylpropanoid units forming a C18

skeleton with various functional groups. Lignans are

widespread in the plant kingdom. p-coumaroyl alcohol,

coniferyl alcohol, and sinapyl alcohol are the main precurosrs

of all lignins and lignans. Lignins in gymnosperms are

primarily derived from coniferyl alcohol, and to a lesser

extent, p-coumaryl alcohol, whereas angiosperm lignins

contain coniferyl and sinapyl alcohols in roughly equal

proportions. These lignin constituents are synthesised from

phenylalanine via various cinnamic acid derivatives [63]. The

main function of lignin in plants includes cell wall

strengthening, facilitating water and minerals to be conducted

through the xylem. The presence of lignin reduces cell wall

digestibility and blocks the growth of pathogens [177].



3.3.11. Quinones: Quinones are aromatic rings with two

ketone substitutions, ubiquitous in nature and occur in free

state as well as in the form of glycosides. More than 500

compounds of the group are known about half of which are

found in higher plants. These are primarily confined to the

bark and roots [46]. Quinones have been known to act as plant

growth regulators [153]. Depending upon the number of rings

in their structure, quinones are usually divided into

benzoquinones, naphthoquinones, and anthraquinones.

3.3.11.1. Benzoquinones: Benzoquinones are a class of small

molecules that contain two carbonyl groups on a benzene ring.

In plants, these are found in families such as Myrsinaceae,

Connaraceae, Geraniaceae, Liliaceae, Leguminoseae

Umbelliferae, Labiateae, Ranunculaceae, Compositeae,

Cupressaceae, Pyrolaceae, Primulaceae, and Apocyanaceae. In

this category, plastoquinones and ubiquinones are

physiologically significant as being important for the

processes of photosynthesis and respiration, respectively

[153,46].

3.3.11.2. Naphthoquinones: These are derived from

naphthalene, and occur as 1,4-naphthaquinones, and rarely

1,2-naphthaquinones. The families comprising these

compounds include Bignoniaceae, Droseraceae,

Plumbaginaceae, Boraginaceae, Juglandaceae,

Dioncophyllaceae, Avicenniaceae, Acanthaceae,

Boraginaceae, Ebenaceae, Droseraceae, Balsaminaceae,

Steraliaceae, Nepenthaceae, and Ulmaceae. They are

synthesised via the shikimate/ succinyl CoA combined

pathway and the shikimate/mevalonate pathway [9]. They

prevent plants from pathogen attacks. They have also been

known to possess phytotoxic properties [153].

3.3.11.3. Anthraquinones: Anthraquinones comprise the

largest group of natural quinones usually found in the form of

glycosides or in carboxylated state. They function as

protective plant compounds against insects and pathogens

[153]. They occur in families such as Rubiaceae, Rhamnaceae,

Leguminoseae, Polygonaceae, Bignoniaceae, Verbenaceae,

Scrophulariaceae, and Liliaceae, amongst others [46].

Anthraquinone is an aromatic organic compound and

sometimes contribute to their colouring pigments [109].

3.3.12. Tannins: These are basically found in families like

Leguminoseae, Anacardiaceae, Combretaceae,

Rhizophoraceae, Myrtaceae, and Polinaceae but are

distributed widely throughout the plant kingdom. These are

synthesised via phenylpropanoid pathway and are commonly

found combined with alkaloids, polysaccharides, and proteins

[70]. Structurally, they possess 12 to 16 phenolic groups and

5 to 7 aromatic rings per 1,000 units of relative molecular

mass [99]. With the exception of some high molecular weight

compounds, these are soluble in water. Their molecular weight

usually ranges from 500 to 20,000. Plant tannins also serve as

defences against microorganisms and herbivores. These

usually classified into two classes: hydrolysable tannins

(gallo- and ellagi-tannins) and condensed tannins

(proanthocyanidins) [33].

3.3.12.1. Condensed tannins: These are also termed

proanthocyanidins and are synthesised via polymerization of

flavan-3-ol units. They are frequent constituents of woody

plants and amongst the most abundant polyphenols in the plant

kingdom. The most common members of the PAs are the

procyanidins (PCs), composed of the monomeric flavan-3-ols

(+)-catechin and/or (-)-epicatechin [79]. They play protective

roles in plants acting against pathogens, herbivores, and UV-B

radiation [33].

3.3.12.2. Hydrolysable tannins: These comprise of a

monosaccharide core associated with several catechin

derivatives [99]. They are comparatively less stable than

condensed tannins and are readily degraded into smaller

molecules [15]. Gallic acid is the principle phenolic unit of

hydrolysable tannins [58]. They also contribute to astringent

quality of tannin-containing beverages [46].

Figure 3A and Figure 3B shows the structures of the one of

the representative compounds from the above-mentioned

groups.

4. Summary and Future Perspectives With the demand for herbal medicines on the rise, it is only

obvious that the sources of these medicines, the medicinal

plants, will also be consumed in much higher quantities. This

scenario, however, raises concerns about the exploitation of

these resources and their unsustainable use. To counter the

overexploitation of these natural resources, following

measures needs to be brought into action:

4.1. Conservation of medicinal plants and their sustainable

and judicial use: Medicinal plants, not only in India, but the

world over, have been facing increased risk of extinction due

to habitat destruction, bioprospecting or biopiracy, and over-

harvesting. According to A Native Plant Conservation

Campaign Report, about 15,000 plants face extinction as a

result of habitat loss and overharvesting. Also, the current

extinction rates are likely to result in the loss of a potential

major drug every two years [145]. To counter overexploitation

of natural resources, and consequent threat to biodiversity,

sustainable practices have been recommended and several

worldwide organisations have established guidelines for

collection and sustainable cultivation of medicinal plants [91].



4.2. Enhancement of the compound of interest in the

plant(s)/ plant part(s) via the following:

4.2.1. Biotic/ abiotic elicitors: Various biotic (such as

bacterial and fungal infestation and herbivory) and abiotic

(such as temperature, UV-B exposure, and water deficit)

elicitors have been reviewed by [128] which influence the

levels of secondary metabolites in plants. It has been observed

that under duress, the secondary metabolites in plants, along

with the bioactive compound(s) of interest, usually increase.

Thus, it can be safely reasoned that if more amount of the

desired bioactive component can be harvested from a unit

amount of plant tissue, the resulting number of plants required

to obtain the same amount of the bioactive component would

reduce. This may prove to be effective in easing the pressure

on the harvest rates of medicinal plants in the long run.

4.2.2. Genetic and metabolic engineering: Genetic

engineering holds vast potential for the transformation of

plants suited for particular needs. This may include

manipulating the existing pathways, overcoming the rate-

limiting steps, and/or over-inducing the desired genes and

pathways to enhance the production of the required

pharmaceutical compound from the plant. Another approach

can be the synthesis of a novel compound in a plant by

introduction of appropriate heterologous genes. Genetic

manipulations in some medicinal and aromatic plants have

been reviewed by [61]. They have also touched upon the

concept of „metabolons‟ or metabolic channelling. The

targeted genetic interventions might be advantageous if the

mechanisms and principles underlying the formation of

metabolons are more thoroughly understood. These may

facilitate the accumulation of medicinal compounds of

commercial importance at the cost of less or non-commercial

ones and in turn might accelerate the evolutionary processes in

plants besides being expedient for health purposes.

4.2.3. Cell and tissue culture: Via cell and/or tissue culture

many of the secondary metabolites, for instance anthocyanins,

alkaloids such as vincristine and vinblastine etc. have been

produced. Since these systems are free from the interfering

compounds such as pectins, excess polysaccharides, and

enzymes, they allow easy, rapid, and efficient isolation of the

desired compound(s). These cultures also provide the

advantages of faster growth, proliferation, and metabolism

compared to the field-grown plants [143]. For instance,

Ramani and Jayabaskaran (2008) observed the enhanced

production of catharanthine and vindoline from C. roseus cell

cultures, when irradiated with UV-B for 5 min.

The majority of the vast wealth of herbal natural compounds

and their sources is yet to be discovered, and the compounds

already in place are yet to be utilized to their full potential. To

achieve this objective, and to prevent the loss of biodiversity,

medicinal plants and their bioactive compounds need to be

revered and conserved.



Figure 3A: Structures of one of the representative compounds of various categories of phenolic compounds



Figure 3B: Structures of one of the representative compounds of various categories of phenolic compounds

5. Acknowledgement

I am grateful to my Ph.D. supervisor, Prof. S.B. Agrawal, Dept. of Botany, Banaras Hindu University and to the Head, Department of

Botany and Coordinator, Centre of Advanced Study in Botany, Banaras Hindu University, for aiding in the research work pertaining

to this review and to the University Grants Commission (UGC), and Council of Scientific and Industrial Research (CSIR), New Delhi,

for financial assistance during my Ph.D. tenure.

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