Identification of 3-methylthiopropionic acid as an intermediate in mammalian methionine metabolism in vitro.

The oxidative metabolism of methionine by a transaminative pathway was studied in rat and monkey tissue homogenates. Methionine was shown to undergo transamination followed by subsequent oxidation of its a-keto acid, a-keto-y-methiolbutyrate, to carbon dioxide in rat and monkey liver homogenates. Considerable transamination of methionine was observed in monkey kidney homogenates, but little of the a-keto acid was oxidized to carbon dioxide. The addition of saturating levels of S-adenosyl-L-methionine as a diluting pool to the homogenates had no effect on the recovery of radioactive carbon dioxide from L-[methyl-‘“Cl methionine. However, the addition of 3-methylthiopropionate to the incubations inhibited oxidation of 10 mM L-[methyZ-‘4C]methionine to carbon dioxide by 85% in the rat and by about 50% in the monkey liver homogenate. The expanded 3-methylthiopropionate pool was isolated by anion exchange chromatography to determine if the pool became labeled during the experiment. The isolated radioactive product co-chromatographed with standard 3-methylthiopropionate on paper chromatography. Larger amounts of this isolated radioactive product from rat and monkey liver homogenates were purified by cellulose column chromatography and analyzed by gas-liquid chromatography. The chromatograms that were obtained were essentially identical with the elution profile of a reference sample of 3methylthiopropionate subjected to the same isolation procedure. Mass spectra of the peak suspected of being 3-methylthiopropionate in the samples obtained from rat and monkey liver homogenates were identical with that of authentic 3-methylthiopropionate. The majority of radioactivity injected into the column was also shown to be associated with 3-methylthiopropionate when the labeled precursor was L-[methyZ-‘4C]or [:“S]methionine, but not when it was L-[l-‘4C]methionine. These results show that the expanded 3-methylthiopropionate pool became labeled during the experiments, indicating that 3-methylthiopropionate is an intermediate in methionine catabolism in rat and monkey liver in uitro. This pathway appears to account for a major portion of methionine oxidation in uitro.