Oxidative deamination of carboxyethyl-cysteine and carboxymethyl-homocysteine.

Snake venom L-aminoacid oxidase, while inactive on 4C and 5C dicarboxylic alpha-amino acids, may oxidize their 6C and 7C homologues alpha-aminoadipic and alpha-aminopimelic acid. It has been demonstrated that the enzyme also oxidizes S-carboxyethyl-L-cysteine and S-carboxymethyl-L-homocysteine, two analogues of alpha-aminopimelic acid with the gamma or delta methylene groups substituted by a sulfur atom. As oxidation products the corresponding ketoacids were obtained. Thus substrate specificity of L-aminoacid oxidase for dicarboxylic alpha-aminoacids seems highly dependent on the carbon chain length, whereas substitution of a methylene group by a sulfur atom seems to have almost no effect. On the other hand, D-aspartate oxidase from beef kidney (active on 4C to 6C dicarboxylic alpha-aminoacids) cannot oxidize the 7C alpha-aminopimelic acid but is active on the S-carboxyethyl-D-cysteine analogue. The other aminopimelic analogue, S-carboxymethyl-D-homocysteine, is a very poor substrate. Thus the substrate specificity of D-aspartate oxidase, highly dependent on chain length, is also influenced by chain conformation. Furthermore, oxidation of either compound under study, L-aminoacid oxidase and D-aspartate oxidase, leaves unaffected the thioether bond.