Purification and properties of β-cyano-l-alanine synthase from Vicia angustifolia

β-Cyano- l -alanine synthase was purified ca 1000-fold to homogeneity from the immature seeds of Vicia angustifolia . The purified enzyme has an apparent M r of 54 000 consisting of two identical subunits. The subunits contain one molecule of pyridoxal 5′-phosphate each. The K m value is 3.6 mM for l -cysteine and 0.5 mM for cyanide. β-Cyano- l -alanine synthase from V. angustifolia also catalyses the formation of some S -substituted l -cysteines and some heterocyclic β-substituted alanines from l -cysteine or O -acetyl- l -serine as an additional catalytic activity. Significant differences were found between this enzyme and β-cyano- l -alanine synthases from other sources. The amino acid composition of the purified enzyme, and also the occurrence and distribution of β-cyano- l -alanine synthase and cysteine synthase activities in some higher plants are given.