The role of transamination in methionine oxidation in the rat.

The role of transamination as the initial step in catabolism of methionine in the rat was investigated. [Methyl-14C] or [1-14C]-L-Methionine was added to tissue homogenates and transamination was determined from the counts recovered in a precipitable phenylhydrazone following treatment of the samples with 2,4-dinitrophenylhydrazine. Transamination of methionine was detected in homogenates of liver, kidney, heart, brain, spleen, skeletal muscle, and small intestines. The product of methionine transamination in the liver was identified as alpha-keto-gamma-methiolbutyrate. Approximately the same tissue distribution was observed for the conversion of the methyl or carboxyl carbon of methionine or alpha-keto-gamma-methiolbutyrate to CO2. alpha-Keto-butyrate could be used as a co-substrate for transamination, but inhibited oxidation of methionine apparently by competing for oxidation of alpha-keto-gamma-methiolbutyrate. S-Adenosyl-L-methionine was not a substrate for transamination in the liver homogenate system nor did it inhibit transamination of methionine. Amino-oxyacetic acid inhibited transamination and oxidation of methionine, but not oxidation of alpha-keto-gamma-methiolbutyrate. These observations are consistent with transamination being an initial step in methionine catabolism and an alternate pathway for methionine oxidation which does not involve its activation to S-adenosyl-L-methionine.