Oxidation of the supplemental methionine source L-2-hydroxy-4-methylthiobutanoic acid by pure L-2-hydroxy acid oxidase from chicken liver.

The peroxisomal enzyme L-2-hydroxy acid oxidase A (EC 1.1.3.1) was isolated from chicken liver to better evaluate its part in the utilization of the L isomer of supplemental DL-hydroxy-4-methylthiobutanoic acid by birds fed diets containing the methionine hydroxy analogue. The 650-fold purified enzyme, a 169 kDa protein composed of four apparently identical subunits, exhibited a specific activity of 1.3 mumol glycolate oxidized.min-1.mg protein-1. Glycolate (Km = 0.10 mmol/L) was actually a better substrate than L-2-hydroxyisocaproate (Km = 0.63 mmol/L), L-2-hydroxy-4-methylthiobutanoate (Km = 1.73 mmol/L) and L-lactate (Km = 10.13 mmol/L). Under all substrate concentrations tested, the enzyme activity toward L-2-hydroxyisocaproate and L-2-hydroxy-4-methylthiobutanoate was 55 and 17%, respectively, of that toward glycolate. Although the highly purified enzyme was unable to oxidize D-lactate, D-methionine, L-methionine, L-mandelate and beta-phenyl-L-lactate, the latter two aromatic substrates were significantly oxidized by the first ammonium sulfate precipitate obtained during the isolation procedure, supposedly because of the presence of L-2-hydroxy acid oxidase isozyme B. Because the hepatic tissue concentration of glycolate, the physiological substrate for the enzyme, was rather low (10 mumol/L) as compared to the concentration of the methionine hydroxy analogue, one can expect that the conversion of L-2-hydroxy-4-methylthiobutanoate to 2-keto-4-methylthiobutanoate prior to L-methionine formation might proceed at a substantial rate in chickens fed the supplemental methionine source.