Purification and characterization of 3-mercaptopyruvic acid S-conjugate reductases

Three kinds of 3-mercaptopyruvic acid S-conjugate reductase (MPR-I, MPR-II and MPR-III) were purified from rat liver cytosol. These enzymes reduced 3-mercaptopyruvic acid S-conjugates derived from cysteine conjugates and some endogenous alpha-keto acids to the corresponding alpha-hydroxy acids in the presence of either NADH (for MPR-I and MPR-II) or NADPH (MPR-III), while simple aldehydes or ketones did not significantly induce substrate activity. The molecular weight of the present enzymes was about 80 kDa composed of two subunits of the same molecular weight. Km values of MPR-I, MPR-II and MPR-III were 0.38, 0.06 and 0.29 mM for S-(4-bromophenyl)-3-thiopyruvic acid, respectively, and 0.15 mM for NADH (MPR-I, MPR-II) and NADPH (MPR-III). Vmax values of MPR-I, MPR-II and MPR-III for this substrate were 5.3, 20 and 13 nmol/min/mg, respectively. The sulphydryl-modifying agents inhibited the enzyme activities of all the three reductases. Based on the properties including substrate selectivity for alpha-keto acids derived from aromatic amino acids, we assumed that MPR-II and aromatic alpha-keto acid reductase are the same enzyme, while enzymes similar to MPR-I and MPR-III have not been reported. From the viewpoints of metabolism of xenobiotics, these enzymes are likely to be important in biotransformation of cysteine conjugates to 3-mercaptolactic acid S-conjugates.