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 hydroxy analogue, in protein synthesis was investigated in vitro using pure L-2-hydroxy acid oxidase A from chicken liver. The reaction yielded no more than 20% of the corresponding alpha-keto acid, the well-known intermediate in methionine metabolism, 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 therefore concluded that chicken liver L-2-hydroxy acid oxidase, a peroxisomal 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 conversion of the methionine hydroxy analogue into alpha-keto acid and 3-methylthiopropionate by this NADH:flavin oxidoreductase-decarboxylase activity is described.