Dethiomethylation of Methionine by an Extract of Rumen Protozoa: General Substrate Specificity

Abstract

Substrate specificity for dethiomethylation by rumen microorganisms has been examined in vitro. Several methionine analogs have been tested for their ability to inhibit methanethiol production in a protozoal extract from rumen fluid. Ethionine has been shown to be as effective an inhibitor of methionine methyl-14C dethiomethylation as unlabelled methionine. Homocysteine, N-acetyl methionine and α-methyl methionine have also been shown to inhibit the production of methanethiol-14C from methionine methyl-14C. Methionine hydroxy analog, homoserine, S-methyl cysteine and S-ethyl cysteine were shown to be very poor inhibitors of the dethiomethylation of methionine even though S-methyl cysteine and S-ethyl cysteine gave rise to larger quantities of alkyl mercaptan than did methionine or ethionine. It is suggested that there are at least two enzymes in the rumen which produce alkyl mercaptans from sulfur amino acids.