Metabolism of Sulfur Amino Acids by Rumen Microorganisms1

The production of methanethiol and ethanethiol from L-methionine, s-methyl-l-cysteine, ethionine, and s-ethyl-l-cysteine during in vitro fermentation by rumen microorganisms was determined by employing gas-liquid chromatography to analyze the head-space gas produced by the fermentations. Each substrate with rumen fluid was incubated at 39 C in serum bottles equipped to permit syringe sampling of the head-space gas evolved. All substrates were used at a concentration of 1.67 mg/ml rumen fluid. It was found that methanethiol was formed during the in vitro fermentation of s-methyl-l-cysteine and of methionine and that ethanethiol was produced from s-ethyl-l-cysteine and from ethionine. When dimethyl thetin chloride was used as substrate, dimethyl sulfide was formed. s-ethyl-l-cysteine inhibited production of methanethiol from s-methyl-l-cysteine in the early stages of fermentation, but inhibition was largely overcome as fermentation proceeded. Some indication of utilization of methanethiol was obtained and appeared to be related to diet. This is in accord with the production of methanethiol by the reaction: methionine → s-methyl-l-cysteine → methanethiol. However, it is possible that both methionine and s-methyl-l-cysteine were directly dethiomethylated.