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The metabolism of 3-methylthiopropionate in rat liver homogenates.

Earlier studies demonstrated that 3-methylthiopropionate is an intermediate in the transaminative pathway of methionine catabolism. Therefore, the metabolism of [methyl-“Cland 3-methyl[3sS]thiopropionate was studied in a rat liver homogenate system in an effort to identify additional intermediates of this pathway. In addition to carbon dioxide and sulfate, methanethiol and hydrogen sulfide were identified as intermediates of 3-methylthiopropionate metabolism. The identification of one unknown compound as hydrogen sulfide was indicated by its ability to form methylene blue when it was trapped in a zinc acetate solution and then treated with N, N-dimethyl-p-phenylenediamine and ferric chloride. Tests with a hydrogen sulfide-sensitive indicator paper also were positive. Confirmation was accomplished by cochromatography with hydrogen sulfide when the unknown gas was injected into a gas chromatograph equipped with a flame photometric detector. Identification of another volatile intermediate formed during oxidation of 3-methylthiopropionate as methanethiol was based on cochromatography of authentic methanethiol with a compound released either from a sodium hydroxide trap by acidification or from a mercuric acetate trap by flash exchange. Samples of head space gas were injected into a gas chromatograph equipped with a flame photometric detector or a flame ionization detector. Confirmation was by mass spectroscopy. Oxidation of the methyl carbon of 3-methylthiopropionate to carbon dioxide occurred in liver and, to a slight extent, in kidney homogenates. No oxidation of 3-methylthiopropionate to carbon dioxide was detected in homogenates prepared from brain, skeletal muscle, or heart. However, in addition to the liver, a limited amount of hydrogen sulfide formation from 3methylthiopropionate was observed in kidney and heart homogenates. The appearance of hydrogen sulfide and methanethiol, two extremely toxic compounds, as intermediates in this pathway offers further support for the hypothesis that the transaminative pathway of methionine catabolism may be related to the marked toxicity of methionine.