Conversion of 2-hydroxy-4-(methylthio)butanoic acid to L-methionine in the chick: a stereospecific pathway

Conversion of 2-hydroxy-4-(methylthio)butanoic acid (HMB) to L-methionine (L-Met) was studied by using chick liver homogenates. The first step was found to be stereospecific with different enzymes for the D- and L-isomers of HMB. L-HMB was the substrate for L-2-hydroxy acid oxidase, a peroxide-producing flavo-enzyme found in peroxisomes of liver and kidney. The enzyme for D-HMB, identified as mitochondrial D-2-hydroxy acid dehydrogenase, had not been previously described in the chick. This enzyme was found in every tissue tested including intestinal mucosa and skeletal muscle. Thus, D-HMB could be used by any organ for protein synthesis, like L-Met itself. These results provide a biochemical explanation of equimolar incorporation of HMB and DL-methionine (DL-Met) into chick hepatocyte protein in that the two HMB enzymes can simultaneously convert both HMB isomers to L-Met while only one enzyme, D-2-amino acid oxidase, converts D-Met to L-Met.