Purification and specificity of the C-S-lyase of Albizzia lophanta

Abstract

Purification of the l-cysteine C-S-lyase of Albizzia lophanta seed endosperm has resulted in a 200-fold increase in specific activity as compared with the original extract. The enzyme is stable above pH 6.0 and below 60°, is activated by pyridoxal phosphate, and acts optimally on S-ethyl- l-cysteine at pH 8.0. A survey of the specificity pattern of the enzyme with 40 potential substrates demonstrate that whereas the l-cysteine moiety, unaltered except for addition of oxygen to the sulfur atom and substitution of the hydrogen of the thiol group, is an absolute requirement for susceptibility to enzyme action, all or almost all S-substituted l-cysteine derivatives tested may serve as substrates, although there is considerable variation in the rate of hydrolysis to free thiol, pyruvate, and ammonia. In general it appears that those S-substituents whose structure suggest interference (either by hydrogen bonding or by steric hindrance) with the formation of a hydrogen bond (or chelate) between the hydroxyl at the 3-position of pyridoxal phosphate and the cysteine-derived group of the Schiff's base are poor substrates. The utility of C-S lyases in the preparation, elucidation of structure and their possible role in the metabolism of sulfur compounds of biochemical interest are discussed.