Cyanogenic Glucosides

Many plants store cyanogenic glucosides, which on removal of the glucose can decompose to release hydrogen cyanide. As HCN is a powerful toxicant to all haem groups containing complexed iron (present in both plants and insects), it is remarkable how such compounds can be safely sequestered by plants or insects. More than 40 cyanogenic glucosides are known in plants, mandelonitrile glucoside, prunasin (Figure 10.7), and its epimer sambunigrin, are typical examples. A glucosidase cleaves the prunasin when the plant is damaged to give glucose and mandelonitrile. The released mandelonitrile is either cleaved spontaneously, at acid pH, or by oxynitrilase in alkaline medium, giving benzaldehyde and HCN.

Among animals, this type of defence is practised by millipedes, centipedes and insects only. The larvae of the Australian beetles of Paropsis and Chrysophtarta, feeding on Eucalyptus leaves produce mandelonitrile and prunasin. When freeze-dried insects were treated with P-glucosidase and nitrilase, HCN was released (Figure 10.7). The larvae mandelonitrile (3-glucoside mandelonitrile benzaldehyde or prunasin

Figure 10.7 Conversion ofplant-produced prunasin to HCN in Australian beetles mandelonitrile (3-glucoside mandelonitrile benzaldehyde or prunasin

Figure 10.7 Conversion ofplant-produced prunasin to HCN in Australian beetles have special glands that secrete the HCN, but eggs, pupae and adults also release some HCN but without specific glands.

In beetles and millipedes, which do not live on plants that produce cyanogenic glycosides, the compound is almost certainly made by the insect or arthropod itself. In millipedes mandelonitrile is stored in an inner chamber of their defensive glands, which is separated from the outer chamber which holds enzymes that catalyze the breakdown of mandelonitrile to benzaldehyde and HCN. A muscle controls the valve between the two chambers.

The millipede Oxidus gracilis converted racemic [2-14C]phenylalanine to HCN, but did not make HCN from [2-14C]tyrosine. The steps from phenylalanine to mandelonitrile have been studied in the millipede Harpaphe haydeniana (Plate 13) by feeding a wide range of potential precursors, from which the biosynthetic scheme shown in Figure 10.8 was constructed. Phenylalanine was much less well incorporated into mandelonitrile than N-hydroxyphenylalanine, but that may be because there are many competing uses for phenylalanine. Phenylpyruvic acid oxime was also incorporated, but not as well as phenylacetaldehyde oxime, which leaves the two possible routes open in the middle of the sequence.

Cyanogenic species are much more common among Lepidoptera, and many of them are able to synthesize the cyanogens. The bright red-and-black burnet moth Zygaena trifolii obtains the cyanogenic glycosides linamirin and lotaustralin from Lotus corniculatus. This species is also

mandelonitrile

Figure 10.8 The investigated route to mandelonitrile in the millipede Harpaphe haydeniana. The actual route was not firmly established, but with small possible variation, it is the same as the route in plants mandelonitrile

Figure 10.8 The investigated route to mandelonitrile in the millipede Harpaphe haydeniana. The actual route was not firmly established, but with small possible variation, it is the same as the route in plants h3cn| „ ^cooh ch-ch h3c' . i nh valine

Fe2+

2 02 NADPH

H3C\, ,0-glucose glucose H3CV,0H Fe2+

linamarin h3c h3Cn, ch h3c'

h3c-cv ^cooh ch-ch h3c' i nh2 isoleucine

H3C-C^ ^-glucose h3c c=n lotaustralin

Figure 10.9 The biosynthesis of the cyanogenic glycosides linamarin and lotaustralin from amino-acids in the burnet moth. The dot on nitrogen and the prime and double prime on carbon represent15N and13C respectively. Labelling showed that these atoms were retained in place during the synthesis able to synthesize more linamirin and lotaustralin from valine and isoleucine (Figure 10.9). This may be a case of co-evolution according to the suggestions of J. M. Pasteels. Other cyanogenic species do not feed on cyanogenic plants at all. The ability to make and store these compounds seems general in the heliconiine tribe of butterflies. About one third is stored in the haemolymph and two thirds in the integument. Biosynthetic studies in both Zygaena and Heliconius species showed that larvae fed with uniformly labelled 13C-valine or isoleucine were able to synthesize linamarin and lotaustralin respectively. Studies showed that the carbon skeleton was retained intact except for the carboxyl group.

For release of HCN, the glucose is enzymically cleaved by linamarinase and the cyanohydrin spontaneously decomposes (Figure 10.10). The cyanide once released can be detoxified as in plants, by conversion to cyanoalanine and asparagine (Figure 10.10), or by the enzyme rhoda-nese, which converts cyanide into the relatively innocuous thiocyanate. The formation and decomposition of HCN in millipedes and insects is therefore a remarkable example of parallel evolution, where insects and plants both produce HCN and benzaldehyde in similar ways and avoid the toxic effect of HCN by converting it to the same cyanoalanine. It is not easy to perform tests for the presence of rhodanese and so it is uncertain how widely it occurs in insects.

It is the view of at least one author, L. Kassarov, that the cyanogenic effect has no influence on rejection of butterflies and moths by birds and animals. The release of HCN is too slow to protect the insect. Not even the bitterness of the compounds is responsible, since the compounds are h3C\ ,0-glucose linamarinase H3CX pH

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