P-Cyanoalanine

The biochemical problem has been traced to the presence in the seeds of P-cyanoalanine and of its decarboxylation product P-aminopropionitrile. 

The human health aspects of this substitution are unclear, but V. sativa is known to produce the neurotoxic amino acids p-cyanoalaninc and y-glutamyl-P-cyanoalanine (Ressler, 1962 Ressler et al., 1969), the favism factors vicine and... 

Ressler, C., Nigam, S.N. and Giza, Y.H. (1969) Toxic principle in vetch. Isolation and identification of y-L-glutamyl-L-P-cyanoalanine from common vetch seeds. Distribution in some legumes. J. Am. Chem. Soc. 91, 2758-2764. 

Another catabolic pathway is transamination (Fig. 24-25, rection h) to 3-mercaptopyravate. The latter compound can be reductively cleaved to pyruvate and sulfide. Cysfeine can also be oxidized by NAD and lacfafe dehydrogenase to 3-mercaptopyruvate. An interesting PLP-dependent p-replacement reaction of cysfeine leads to P-cyanoalanine, the lathyritic factor (Box 8-E) present in some plants. This reaction also detoxifies the HCN produced during the biosynthesis of ethylene from ACC. 

Fig. 16.15. Conversion of HCN to p-cyanoalanine and asparagine (modified from Conn, 1979b used with permission of the copyright owner, Academic Press, Orlando, FL).

Under certain conditions y-glutamylaspartic is eluted from the amino acid analyzer in two bands. Whether one or two peaks occur is determined by the volume of buffer in which the peptide is applied to the resin the phenomenon therefore probably reflects differences in elution behaviour of the species at different stages of ionization. None of the other a- and y-glutamyl derivatives which have been investigated showed this type of behaviour 136, 137), although similar problems have been encountered with y-glutamyl-P-cyanoalanine (275). 

P-Cyano-L-alanine and y-L-glutamyl-P-cyano-L-alanine are present in seeds of various species of Vida and Lathyrus (see Table 2), including V. sativa and V. angustifolia. The concentrations of the y-glutamyl derivative is much higher in seedlings than in seeds of V. sativa 274), and P-cyanoalanine is nearly- or totally absent from the seedlings of V. 

Nigam. S. N.. and W. B. McConnell Incorporation of serine-U- C into P-cyano-alanine andy-glutamyl-p-cyanoalanine in Vida sativa. Can. J. Biochem. 46,1327 (1968). 

Ressler, C., Y. H. Giza, and S. N. Nigam P-Cyanoalanine, product of cyanide fixation and intermediate in asparagine biosynthesis in certain species of Lathyrus and Vicia. J. Amer. Chem. Soc. 91,2766 (1969). 

Tschiersch, B. Occurrence ofy-glutamyl-P-cyanoalanines. Tetrahedron Lett. 1964,747. 

P-Cyanoalanine can be transformed into y-glutamyl f -cyanoalanine by y-glutamyl transferase. Other enzymes involved in cyanide catabolism are cyanide hydratase and rhodanase. Cyanide hydratase, found in many fungi, catalyses the hydration of cyanide to formamide (H2N-CH=0). Rhodanase found in bacteria, plants and animals catalyses the conversion of cyanide into thiocyanate (R-S C N), which is also known as rhodanide. 

The assumption that cysteine and not O-acetylserine is the immediate precursor for p-cyanoalanine is supported by tracer experiments with incorporation of [ RC]cyanide, [ C]serine, and [ C]cysteine into asparagine... 

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