Cyanide metabolism in higher plants: III. The biosynthesis of β-cyanolanine

Abstract

Seedlings of blue lupine, sorghum, and common vetch which convert H¹⁴CN extensively into the amide carbon of asparagine have been shown to utilize L-cysteine-3-¹⁴C as the source of the other 3 carbon atoms of asparagine. The presence of HCN in the atmosphere in which the plants are grown greatly stimulates the conversion of ¹⁴C in the cysteine into asparagine. L-Serine-1-¹⁴C is also converted into asparagine by sorghum seedlings, and the amount of conversion is stimulated 3-fold by HCN. Seedlings of common vetch also convert L-serine-1-¹⁴C into the β -cyanoalanine moiety of the lathyritic dipeptide γ-glutamyl-β -cyanoalanine, but this conversion is not affected by the presence of HCN. Buffered extracts of acetone powders of seedlings of sorghum, blue lupine, common vetch, and bird's-foot trefoil catalyze the formation of β -cyanoalanine and H₂S from l-cysteine and HCN. The enzyme responsible, which has been tentatively named β -cyanoalanine synthase, may be exhibited by incubating radioactive L-cysteine or H¹⁴CN with the extracts and the other unlabeled substrate and measuring the β -cyanoalanine-¹⁴C formed. The enzyme may also be assayed by measuring colorimetrically the H2S formed. H¹⁴C¹⁵N is converted by sorghum seedlings to the amide group of asparagine without any separation of the C and N atoms of the cyanide group.