Cyanogenic Glycosides and Biogenetically Related Compounds in Higher Plants and Animals

Abstract

Abstract Cyanogenesis describes the ability of living organisms to liberate hydrogen cyanide from stored cyanogenic glycosides upon tissue damage by hydrolysis and/or decomposition. It has been described for more than 3000 species of higher plants. Chemically, cyanogenic glycosides are glycosides of α‐hydroxynitriles (cyanohydrins). Cyanogenic glycosides together with plant glycosidases and hydroxynitrile lyases form a preformed defence system. The structures are biogenetically related to only a few precursor amino acids. In the biosynthetic pathway, two multifunctional P450 enzymes and a glucosyltransferase act in a sequence. Biogenetically, glucosides of β‐ and γ‐hydroxynitriles are closely related. Cyanogenic glycosides and their related nitriles also may serve as storage forms for reduced nitrogen. A recycling pathway has been proposed to recover reduced nitrogen for primary metabolism. Many food plants are cyanogenic, and great efforts are made to optimise their detoxification. The presence of cyanogenic glycosides in the animal kingdom appears to be restricted to arthropods. Some insects, often aposematically coloured, synthesise cyanogenic glycosides de novo and/or sequester them from their host plants. Cyanogenic glycosides are secondary metabolites known from more than 3000 different higher plant species. Cyanogenic plants are able to liberate hydrogen cyanide from their cyanogenic glycosides upon disruption of plant tissue. Cyanogenic glycosides and their corresponding degrading enzymes are part of a preformed defence system. Thus, they can be regarded as phytoanticipins. Cyanogenic glycosides are glycosides of α‐hydroxynitriles, derived from five proteinogenic amino acids (Phe, Tyr, Val, Ile and Leu) and from the nonproteinogenic amino acid cyclopentenyl glycine. Acalyphin is apparently derived from nicotinic acid. A biogenetically related class consists of β‐ and γ‐hydroxynitrile glucosides, apparently derived from the aliphatic amino acids (Val, Ile and Leu). Up to now more than 100 different glycosylated α‐, β‐ and γ‐hydroxynitriles are known from higher plants and arthropods. Additional roles and functions of cyanogenic glycosides include storage of reduced nitrogen, transportation of nitrogen and the turnover of nitrogen into primary metabolism. The presence of cyanogenic glycosides in animals appears to be restricted to arthropods. Some insects are strongly associated with their cyanogenic host plants. They sequester the cyanogenic glycosides from these plants and additionally carry out de novo biosynthesis of these compounds. Convergent evolution in plants and insects has resulted in two identical biosynthesis pathways: two multifunctional P450 enzymes and a glucosyltransferase, acting sequentially.