Purification and properties of β-cyano-l-alanine synthase from Spinacia oleracea

β-Cyano-l-alanine synthase was purified ca 6200-fold to homogeneity from the leaves of spinach (Spinacia oleracea). The purified enzyme has an apparent Mr of 60 000 and can be dissociated into identical subunits of Mr 30 000. The subunits each contain one molecule of pyridoxal 5′-phosphate. The Km value is 2.3 mM for l-cysteine and 0.73 mM for cyanide. β-Cyano-l-alanine synthase from S. oleracea also catalyses the formation of some S-substituted l-cysteines and some heterocyclic β-substituted alanines from l-cysteine or O-acetyl-l-serine. The specificity of these additional catalytic activities of the purified enzyme are compared with those of cysteine synthase purified from the same plant, and with those of β-cyano-l-alanine synthase purified from other sources. Some other properties, including the amino acid composition of the purified enzyme, are also described.