Enzymatic Studies on the Metabolism of the Tetrahydrofurfuryl Mercaptan Moiety of Thiamine Tetrahydrofurfuryl Disulfide

I. Microsomal S-Transmethylase

Abstract

Enzyme systems responsible for the novel metabolic pathway of foreign mercaptans leading to the formation of methylsulfonyl metabolites were investigated in rat tissues. The first step was shown to be S-transmethylation. Tetrahydrofurfuryl mercaptan was methylated to its methyl sulfide by liver homogenates in the presence of S-adenosylmethionine. The activity was highest in liver, followed by kidney and small intestine. The hepatic activity was located exclusively in microsomes. The apparent Michaelis constants were 2.5×1O^-4_M for the acceptor substrate and 1.0×1O^-4_M for S-adenosylmethionine. The activity was completely inhibited by p-chloromercuribenzoate, p-chloromercuribenzenesulfonate or HgCl₂, whereas it was enhanced by GSH, cysteine or ascorbate.