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

II. Sulfide and Sulfoxide Oxygenases in Microsomes

Abstract

The second step in the enzymatic process responsible for the novel metabolic pathway of foreign mercaptans leading to methylsulfonyl metabolites was shown to be sulfoxidation, subsequent to S-methylation. By using [³⁵S]methyl tetrahydrofurfuryl sulfide (MTFS) and [³⁵S]methyl tetrahydrofurfuryl sulfoxide (MTFSO) as substrates, the occurrence and involvement of both sulfide and sulfoxide oxygenases were demon-strated in rat liver microsomes. Both activities required NADPH and O₂. The reaction products were isolated and identified as MTFSO and its sulfone, respectively. The apparent Michaelis constants were 6.7×1O^-4_M for MTFS and 9.1×1O^-5_M for NADPH with sulfide oxygenase and 5.6×10^-3_M for MTFSO and 5.0×10^-5_M for NADPH with sulfoxide oxygenase, respectively. p-Chloromercuribenzoate, p-chloro-mercuribenzenesulfonate, HgCl₂, and menadione strongly inhibited both oxygenases. Polyanions, such as inorganic phosphate, pyrophosphate, sulfate, and ATP stimulated both enzyme activities, especially that of sulfoxide oxygenase. One atom of¹⁸O₂ was incorporated into the products in both enzyme reactions. No appreciable incor-poration was observed from H₂¹⁸O. These results indicate that both enzyme systems are typical mono-oxygenases.