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.
D000603:Amino Acids, Sulfur / Q000378:metabolism; D000625:Aminooxyacetic Acid / Q000494:pharmacology; D000818:Animals; D007651:Keto Acids / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008297:Male; D008715:Methionine / Q000031:analogs & derivatives / Q000378:metabolism*; D010084:Oxidation-Reduction; D010659:Phenylhydrazines; D051381:Rats; D012436:S-Adenosylmethionine / Q000378:metabolism; D014018:Tissue Distribution; D000637:Transaminases / Q000502:physiology*
D000603:Amino Acids, Sulfur; D007651:Keto Acids; D010659:Phenylhydrazines; D000625:Aminooxyacetic Acid; C014632:2-keto-4-methylthiobutyric acid; D012436:S-Adenosylmethionine; D008715:Methionine; D000637:Transaminases