Novel Metabolic Pathways of Cysteine Conjugate and its Related Metabolites

Mercapturic acids (N-acetylcysteine conjugates) are not an ultimate excretion form in many compounds, but are converted to methylthio-containing metabolites, or S-substituted mercapto acetic acid or mercapto lactic acid via mercapto pyruvic acid. Methylthio-containing metabolites are generated by the action of N-deacetylases for mercapturic acids, β-lyase for cysteine conjugates and methyl transferases for thiols. Methylsulfides thus formed are subsequently oxidized to the methylsulfoxides or methylsulfones prior to excretion. In addition to above enzymes, reductases for methyl sulfoxide were also predominantly localized in the liver and kidney. In the formation of S-substituted mercapto acetic acid, likely participation of cysteine conjugate aminotransferase, thio-pyruvic acid oxidase and in some cases thio-lactic acid reductases is indicated.