S-Alkylcysteinase: Enzymatic Cleavage of S-Methyl-L-cysteine and Its Sulfoxide*

S-Methylcysteine has been isolated from the nonprotein nitrogen fractions of cabbage, turnips, and several other crucifers, and the structure of the amino acid was established by chemical studies and by comparison with a synthetic material (14). Subsequently, this compound has been found in garlic and onion (5)) kidney bean seeds (6)) Neurospora crassa (7)) and the hemolymph of Prod&a ericlenia (8). It has been isolated also as a peptide with glutamate, y-glutamyl-S-methylcysteine, from various leguminous plants (9, 10) and garlic (11). Although, in contrast to methionine, S-methylcysteine does not undergo oxidation to the corresponding sulfoxide at room temperature (3), the latter compound was isolated from several plants (l-5). Wolff, Black, and Downey (12) purified an enzyme from yeast which catalyzed the synthesis of S-methylcysteine from L-serine and methyl mercaptan, but this reaction was shown to be essentially irreversible. The enzymatic degradation of this amino acid has been reported to be catalyzed by another enzyme system or systems occurring in a microorganism (13, 14) and in the seeds of Albizzia Zophantha (15). Little information is available, however, concerning the mechanism of this reaction(s) or the properties of the enzyme(s) concerned. S-Methylcysteine sulfoxicle has been shown to be converted to pyruvic acid, ammonia, and methyl methanethiosulfinate by alliinase purified from garlic (16) and Bacillus subtilis (17). The present paper describes the partial purification and properties of a new enzyme obtained from cells of a strain of Pseudomonas cruciviae grown with S-methyl-L-cysteine as the major carbon and nitrogen source. This enzyme catalyzes the stoichiometric conversion of S-methyl-L-cysteine to methyl mercaptan, pyruvic acid, and ammonia (Reaction l), and requires pyridoxal phosphate as a cofactor.