IN-VITRO OXIDATIVE DECARBOXYLATION OF L-2-HYDROXY-4-METHYLTHIOBUTANOIC ACID BY L-2-HYDROXY ACID OXIDASE-A FROM CHICKEN LIVER

The first step in the set of reactions responsible for the biological utilization of L-2-hydroxy-4-methylthiobutanoic acid, the methionine h ydroxy analogue, in protein synthesis was investigated in vitro using pure L-2-hydroxy acid oxidase A from chicken liver. The reaction yield ed no more than 20% of the corresponding alpha-keto acid, 2-keto-4-met hylthiobutanoic acid, the well-known intermediate in methionine metabo lism, and as much as 80% of the subsequent decarboxylation product, 3- methylthiopropionate, suggesting that L-2-hydroxy-4-methylthiobutanoic acid cannot be completely converted into methionine in vivo. It was t herefore concluded that chicken liver L-2-hydroxy acid oxidase, a pero xisomal enzyme requiring flavin mononucleotide as a coenzyme, also has an oxidative decarboxylation activity in vitro, which was found to be NADH-dependent. The mechanism possibly underlying the successive conv ersion of the methionine hydroxy analogue into alpha-keto acid and 3-m ethylthiopropionate by this NADH:flavin oxidoreductase-decarboxylase a ctivity is described.