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Toxicon 38 (2000) 1136 www.elsevier.com/locate/toxicon

Plant cyanogenic glycosides Janos VetterDepartment of Botany, University of Veterinary Sciences, Budapest, 1400 Pf. 2. Hungary Received 12 February 1999; accepted 9 March 1999

Abstract The cyanogenic glycosides belong to the products of secondary metabolism, to the natural products of plants. These compounds are composed of an a-hydroxynitrile type aglycone and of a sugar moiety (mostly D-glucose). The distribution of the cyanogenic glycosides (CGs) in the plant kingdom is relatively wide, the number of CG-containing taxa is at least 2500, and a lot of such taxa belong to families Fabaceae, Rosaceae, Linaceae, Compositae and others. Dierent methods of determination are discussed (including the indirect classical photometrical and the new direct chromatographic ones). The genetic control of cyanogenesis has no unique mechanism, the plants show variation in the amount of the produced HCN. The production of HCN depends on both the biosynthesis of CGs and on the existence (or absence) of its degrading enzymes. The biosynthetic precursors of the CGs are dierent L-amino acids, these are hydroxylated then the N-hydroxylamino acids are converted to aldoximes, these are turned into nitriles. The last ones are hydroxylated to a-hydroxynitriles and then they are glycosilated to CGs. The generation of HCN from CGs is a two step process involving a deglycosilation and a cleavage of the molecule (regulated by b-glucosidase and a-hydroxynitrilase). The tissue level compartmentalisation of CGs and their hydrolysing enzymes prevents large-scale hydrolysis in intact plant tissue. The actual level of CGs is determined by various factors both developmental and ecological ones, which are reviewed too. The last part of the present work demonstrates the biological roles of CGs in plant physiological processes and in plant defence mechanisms as well. The eect of CGs (HCN) on dierent animals, the symptoms of poisonings are discussed to cows, sheep, donkeys, horses and chicks. Finally, the poisonous eects of cassava (Manihot esculenta ) roots are summarised on experimental animals and on the human organism. # 1999 Elsevier Science Ltd. All rights reserved.

0041-0101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 1 - 0 1 0 1 ( 9 9 ) 0 0 1 2 8 - 2

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nos Vetter / Toxicon 38 (2000) 1136 Ja

Contents 1. 2. 3. 4. 5. 6. 7. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Occurrence, structure, distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Estimation assays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Cyanogenesis: genetic background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Cyanogenesis: biosynthesis and transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Regulation of CG level in plants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Biological eects of CGs . . . . . . . . . . . . . . 7.1. Role in plant physiological processes. 7.2. Role in plant defence mechanism . . . 7.3. Eects on animals . . . . . . . . . . . . . . 7.4. Eects on human organism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 17 18 18 20

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1. Introduction The cyanogenic glycosides (abbreviated: CG) may be dened chemically as glycosides of a-hydroxinitriles and belong to the secondary metabolites of plants (to natural products). They are amino acid-derived plants constituents, present in more than 2500 plant species. Hydrogen cyanide (HCN) as a product of their hydrolysis was rstly isolated from plant in 1802 by Scrade (from bitter almond and from the leaves of peach). The release of HCN by plants was rstly ascribed to a particular compound by Robiquet and Charlard who isolated amygdalin from bitter almonds. On enzymatic hydrolysis cyanogen glycosides yield the aglycone (that is an a-hydroxynitrile) and the sugar moiety. The aglycones can be grouped into aliphatic and aromatic compounds; the sugar is mostly D-glucose, but can be other sugars too, for example gentibiose, primeverose or other (see Table 1). 2. Occurrence, structure, distribution Whereas most plants produce a small amount of cyanide associated with ethylene production, between 312000 plant species produce sucient quantities of cyanogenic compounds (McMahon et al., 1995). The CGs are glycosides of ahydroxynitriles, all known compounds are b-linked, mostly with D-glucose. The structure of some cyanogenic glycosides and some examples of their occurrence

nos Vetter / Toxicon 38 (2000) 1136 Ja

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are given in Table 1. The enzymic hydrolysis produces the aglycone and the sugar moiety. The CGs can be grouped according to chemical nature of substituents, namely aliphatic, aromatic groups and into the glycosides with a free ahydroxynitrile. Some of these CGs are better known than the others because the carrying plant species (group) has a greater practical importance, several economically important plants are highly cyanogenetic (linamarin in Manihot esculenta, Linum usitatissimum, Trifolium repens, dhurrin in Sorghum species, amgydalin in rosaceous plants, lotaustralin in Lotus corniculatus, etc.). The linamarin and lotaustralin have a relatively broad distribution in the plant kingdom, having been demonstrated in the following plant families: Compositae, Euphorbiaceae, Linaceae, Papaveraceae and Fabaceae (Leguminosae). A similar wide distribution has been observed for prunasin in six families (Polypodiaceae, Myrtaceae, Rosaceae, Saxifragaceae, Scrophulariaceae and Myoporaceae). Sambunigrin, vicianin, amygdalin, which are closely related to prunasin, have been demonstrated in three (Caprifoliaceae, Mimosaceae, Oleaceae), in two (Polypodiaceae, Fabaceae) and only in one (Rosaceae) families, respectively. The more common distribution pattern is that a particular cyanogenic compound will occur in one or two families. Conversely, it is generally true, that, with few exceptions, only one or two characteristic glycoside will occur in a given plant family (Poaceae: dhurrin; Compositae: linamarin; Polypodiaceae: prunasin and vicianin, Rosaceae: amygdalin and prunasin). The cyanogenic compounds of plants belong undoubtedly to secondary plant metabolites which have or can have a chemotaxonomical character. Let's see their taxonomical distribution from this point of view. The majority of these families belongs to the Angyospermatophyta, but there are some exceptions (Polypodiaceae/Pteridophyta/, Taxaceae/ Gymnospermatophyta). Both the class Dicotyledonopsida and Monocotyledonopsida have plant families with cyanogenetic compounds, but the most families belong to the dicots. The families Saxifragaceae, Rosaceae, Mimosaceae, Fabaceae, Myrtaceae, Linaceae and Euphorbiaceae are in the subclass Rosiidae, other cyanogenous families are in subclasses Ranunculidae (family Papaveraceae), in Lamiidae (families Caprifoliaceae, Sambucaceae, Oleaceae), Asteridae (family Compositae). The occurrence or missing of the cyanogenetic compounds has probable chemotaxonomical importance too, but these relations are less, or not, documented or investigated. CGs have been reported from many members of the three subfamilies of Fabaceae, but some of these reports should be reconrmed (Seigler et al., 1989). Cyanogenic members of the Papilionoideae have been reported from 18 tribes (the ability to produce HCN upon hydrolysis), but the compounds responsible have been isolated only from a few of these tribes. Linamarin and lotaustralin have been found in many species of the tribes Loteae and Trifoliae, probably occur in the Coronilleae. Vicianin occurs in seeds of several Vicia species. At least one member of the Crotalarieae appears to contain prunasin. Cyanogenic compounds from the Galegeae have been reported, but their chemical nature is unclear. Species of the Indigofereae appear to contain prunasin or sambunigrin, whereas those of the Phaseoleae contain linamarin and lotaustralin.

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Table 1 The chemical structure of some cyanogenic glycosides (according to Tapper and Reay, 1973) Substituent Glycoside Sugar Conguration at C1 Occurrence

(A) Glycosides with aliphatic substitutes R1R'1CH3linamarinD-glucose

Linum spp. Trifolium spp.

R1CH3-, R'1CH3CH2lotaustralin aciapetalin R1(CH3)2CH

D-glucose D-Glucose

nos Vetter / Toxicon 38 (2000) 1136 Ja

Lotus spp. Maniholt spp. Acacia spp.

R1HCO2CH1CH(CO2HCH2)C R1H, R'1H R1OH, R'1H R1OH, R'1H

D-glucose D-glucose D-glucose D-Glucose D-glucose

Triglochinin deidaclin tetraphyllin A tetraphyllin B Gynocardin

Triglochin spp. Deidamia spp. Tetrapathaea spp. Tetrapathaea spp. Gynocardia spp. Pangium spp.

Table 1 (continued ) Substituent Glycoside Sugar Conguration at C1 Occurrence

(B) Glycosides with aromatic substituents phenyl prunasin

D-glucose

D

Prunus spp.

D D D L L D

nos Vetter / Toxicon 38 (2000) 1136 Ja

L

C. Glycosides with a free a-hydroxynitrile

phenyl phenyl phenyl phenyl p-hydroxyphenyl p-hydroxyphenyl p-Hydroxyphenyl p-Glucosyloxyphenyl

amygdalin lucumin vicianin sambunigrin dhurrin taxiphyllin Zierin Proteacin p-Glucosyloxymandelonitrile

gentiobiose primeverose vicianose D-glucose D-glucose D-glucose D-Glucose D-Glucose

Prunus spp. Lucuma spp. Vici

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