Cyanogenesis in Higher Plant and Insects

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Cyanogenesis in HigherPlants and InsectsMatthias Lechtenberg, University of Muenster, Muenster, Germany

Cyanogenesis describes the ability of plants to store cyanogenic glycosides, which, on

tissue damage, undergo hydrolysis with the release of toxic hydrogen cyanide. This

defensive mechanism is widely distributed in plants and also occurs in some insects.

Introduction

The termcyanogenesis describes the ability of organisms toliberate free prussic acid after the breakdown of hydrogencyanide (HCN)-storing compounds (so called cyanogens)by catalysis of cleaving enzymes. Often the disruption oftissue or wounding initiates this process by abolition of thecompartmentation of substrates (cyanogenic glycosides,

cyanogenic lipids) and enzymes (b-glycosidase, hydroxy-nitrile lyases, esterases).

Cyanogenic compounds are mostly regarded as defencecompounds. Cyanogenesis has been described for over2500 species of higher plants and lots of examples areknown from insects. Many basic foodstuffs containcyanogenic glycosides or breakdown products.

Structures

Figure 1 shows the general formula for cyanogenic glyco-

sides. All known cyanogenic glycosides are derivatives ofa-hydroxynitriles (cyanohydrins). These unstable com-pounds are stabilized by b-glycosidic bonded sugars orsugar chains. In all documented cases b-d-glucose is thefirst sugar attached to the aglycone. As the R1 and R2residues are often different, two epimeric forms areprobable. Usually, both forms are known from naturalsources but they seldom occur in the same plant (insect) oreven in related species (Nahrstedt, 1992).

Today, more than 60 cyanogenic glycosides are knownfrom higher plants (Seigler and Brinker, 1993). Thestructures are classified into biogenetic groups accordingto their (in most cases putative) precursor amino acids.

Six major groups can be derived: the phenylala-nine group (including cyanogenic glycosides bearing ameta-hydroxylation at their aromatic ring), the tyrosinegroup, the valine/isoleucine group, the leucine group, the

cyclopentenyl glycine group and the nicotinic acid groupFigure 2 shows one prominent example of each group witthe amino acid precursor.

Cyanogenesis: Degradation ofCyanogenic Glycosides

The action of specific plant or insect b-glucosidases ocyanogenic glycosides leads to fairly unstable cyanohydrins which may decompose enzymatically (catalysed by hydroxynitrile lyase) or nonenzymatically (depending othe pH) into HCN and a corresponding carbonycompound. This process is called cyanogenesis. Figure shows the decomposition of linamarin. Cyanogenesinvolves large amounts of HCN liberated after the breakdown of cyanogens; the occurrence of low amounts o

prussic acid is due to the normal metabolism of plants, foinstance during the formation of ethylene or the action ohorseradish peroxidase on amino acids.

Related Structures

Closely related to the cyanogenic glycosides of the leucingroup is the small group of cyanogenic lipids. Four typeare known today. Instead of glycosylation, in cyanolipidthe a-hydroxy group of the cyanohydrin is esterificatewith fatty acids.

A further structurally related group consists of thnoncyanogenic nitrile glucosides. Often, the misleadinterm cyanoglucosides is used. These compounds are bglucosides ofb- or g-hydroxynitriles. After hydrolysis othe glucosidic linkage, no free cyanohydrins arise. Nevertheless, these nitriles seem to be able to liberate HCN undecertain conditions. Thus sarmentosin epoxide (structurallrelated to cyanogenic glycosides of the valine/isoleucingroup) or osmaronin epoxide (related to the leucine groupare responsible for the weak cyanogenesis of Sedum

Article Contents

Secondary article

. Introduction

. Structures

. Cyanogenesis: Degradation of Cyanogenic Glycoside

. Related Structures

. Distribution

. Biogenesis in Plants

. Toxicology

. PlantInsect Interaction

C

CN

R1

R2OSugar

Figure 1 General formula for known cyanogenic glycosides.

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net


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sarmentosum (Crassulaceae) and Osmaronia cerasiform(Rosaceae) respectively. The cyclohex(en)ylcyanomethy

lene glucosides form the largest group of noncyanogeninitrile glucosides and are putatively derived from tyrosine

A third group consists of two compounds usualldiscussed together with the cyanogenic glycosides: the frecyanohydrins 4-glucosyloxy-mandelonitrile and its 4-Ocaffeic acid ester (trivial name: nandinin), both apparentlderived from tyrosine.

Distribution

Cyanogenesis has been recorded in all major vascular plan

taxa inat least 550 genera and 130 families.As most reportare based on simple qualitative tests with HCN test strip(e.g. FeiglAnger or picrate test papers), in most cases thresponsible cyanogenic principle remains undetermined.

Among the division Pteridophyta, the ferns accumulatcyanogenic glycosides of the phenylalanine group. Withithe Spermatophyta, in members of the gymnosperms onltaxiphyllin (compare with Figure 2) has been reported tdate. Within the Magnoliophytina, cyananogenesis occurin many families. The dicots are very heterogeneous wit

R

OGlc

H

CN

Prunasin (R= H)Taxiphyllin (R=OH)

1. Phenylalanine group2. Tyrosine group

H2N

O OH

R

Phenylalanine (R=H)Tyrosine (R= OH)

H2N

O OH

Leucine

4. Leucine group

OGlcH

CN

Heterodendin

O

H2N

OH

Cyclopentenylglycine

NC OGlc

Deidaclin

5. Cyclopentenyl glycine group

N

HOOC

Nicotinic acid

6. Nicotinic acid grou

NHO

GlcO

OCH3

O

CH3

CN

Acalyphin

Valine

3. Valine/isoleucine group

Isoleucine

H2N

O OH

H2N

O OH

OGlcCN

Linamarin

OGlcCN

Lotaustralin

Figure 2 Prominent examples of each biogenetic group of cyanogenic glycosides (blue) with amino acid precursors (red).

HO CN

CH3

+OHHO

OHO

OH

OH

CH3 + HCN

H3C

O

OHO

OHO

OH

CH3

CN

CH3OH

(1) H2O

(2)

CH3

Figure 3 Enzymatic hydrolysis of linamarin: (1) b-glucosidase; (2)hydroxynitrile lyase.

Cyanogenesis in Higher Plants and Insects

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regard to the reported structures, whereas cyanogenicmembers of the monocots are characterized mainly bytyrosine-derived glycosides.

Sometimes cyanogenic and acyanogenic phenotypes ofthe same species occur. This polymorphism has been wellstudied for white clover (Trifolium repens). The expressionof cyanogenesis in white clover is controlled by two

independent gene loci.Cyanogenesis has been described in bacteria, cyanobac-

teria, microalgae and fungi. It is also known in arthropods:the occurrence of cyanogenic glycosides and noncyano-genic nitrile glycosides has been reported in Lepidopteraand Coleoptera.

Biogenesis in Plants

Among the biosynthetic studies of cyanogenic glycosides,the biosynthesis of dhurrin (2S-b-d-glucopyranosyloxy-2-(4-hydroxy)phenylacetonitrile) in Sorghum bicolor is oneof the best investigated examples. l-Tyrosine is thebiogenetic precursor of dhurrin. Two steps in thebiosynthesis of dhurrin are catalysed by multifunctionalmembrane-bound cytochrome P450 enzymes. The first(P450tyr) catalyses the conversion of tyrosine to Z-p-hydroxyphenylacetaldoxime, the second (P450ox) theconversion of Z-p-hydroxyphenylacetaldoxime to p-hy-droxymandelonitrile. In vivo, this cyanohydrin is convertedinto the cyanogenic glycoside dhurrin by a solubleglucosyltransferase (Mller and Seigler, 1999; Selmar,1999).

Toxicology

The cyanogenic glycosides are potential toxins because oftheir ability to liberate HCN after hydrolysis. Thussymptoms of acute toxicity of cyanogenic glycosides afteringestion of cyanogenic plant material correlate with thoseof an acute HCN intoxification. Sometimes intact cyano-genic glycosides show only low acute toxicity because oftheir slow hydrolysis under the conditions in the gastro-intestinal tract. Additionally, detoxification mechanismsof humans and animals are able to detoxify up to 60 mgHCNper h. Nevertheless raised plasma levels of the human

detoxification products rhodanide and cyanocobalaminmay lead to severe disease. Thus the daily consumption ofeven subacute amounts of cyanogenic glycosides withcyanogenic food plants leads to chronic cyanide intoxifica-tion. The most important cyanogenic food plant is cassava(Manihot esculenta), which provides energy to more than500 million people, and great efforts are made in the

optimization of its detoxification. Other examples foprominent cyanogenic food plants are lima beans, flaseeds, bamboo shoots, sorghum, bitter almonds, passiofruits and apricot kernels (Nahrstedt, 1993; Jones, 1998)

Plant

Insect InteractionAs mentioned above, cyanogenic glycosides are considereto be defence compounds for plants and insects. Ainteresting example of a herbivore that feeds on cyanogenic plant and uses cyanogenic glycosides aprotecting agents is found in lepidopterous insects (InsectLepidoptera). Some moths of the genus Zygaena are ablto accumulate linamarin and lotaustralin. The larvae feeon Lotus corniculatus, which contains the same compounds. Incorporation experiments have shown that thlarvae are able to synthesize both linamarin and lotaustralin, or, alternatively, may sequester them from their ho

plant.

References

Jones DA (1998) Why are so many food plants cyanogenic?Phytochem

istry 47(2): 155162.

Mller BL and Seigler DS (1999) Biosynthesis of cyanogenic glycoside

cyanolipids, and related compounds. In: Singh B (ed.)Plant Amin

Acids: Biochemistry and Biotechnology, pp. 563609. New Yor

Marcel Dekker.

Nahrstedt A (1992) The biology of the cyanogenic glycosides: ne

developments. In: Mengel K and Pilbeam DJ (eds) Nitroge

Metabolism of Plants, pp. 249269. Oxford: Clarendon Press.

Nahrstedt A (1993) Cyanogenesis and foodplants. In: van Beek TA an

Breteler H (eds) Phytochemistry and Agriculture Proceedings of thPhytochemical Society of Europe, pp. 107129. Oxford: Clarendo

Press.

Seigler DS and Brinker AM (1993) Characterisation of cyanogen

glycosides, cyanolipids, nitroglycosides, organic nitrocompoundsa

nitrile glucosides from plants. In: Dey PM and Harborne JB (seri

eds) Methods in Plant Biochemistry, vol. 8. Waterman PG (ed

Alkaloids and Sulphur Compounds, pp. 51131. London: Academ

Press.

Selmar D (1999) Biosynthesis of cyanogenic glycosides, glucosinolat

and nonprotein amino acids. In: Wink M (ed.)Biochemistry of Plan

Secondary Metabolism, pp. 79150. Sheffield: Academic Press.

Further Reading

Ballantyne B and Marrs TC (eds) (1987) Clinical and Experiment

Toxicology of Cyanides. Bristol: Wright.

LechtenbergM andNahrstedt A (1999)Cyanogenic glycosides. In:Ik

R (ed.) Naturally Occurring Glycosides, pp.147191.Chichester:Joh

Wiley.

Vennesland B, Conn EE, Knowles CJ, Westley J and Wissing F (ed

(1981) Cyanide in Biology. London: Academic Press.

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Cyanogenesis in Higher Plant and Insects