Detoxification of Molinate Sulfoxide: Comparison of Spontaneous and Enzmatic Glutathione Conjugation Using Human and Rat Liver Cytosol

Abstract

Previous lab studies implicated the sulfoxidation pathway of molinate metabolism to induce testicular toxicity. Once molinate is metabolized to molinate sulfoxide, it undergoes further phase II metabolism either spontaneously, enzyme catalyzed, or both to form glutathione-conjugated molinate. This study compared the metabolic capability of rat and human liver cytosol to form a glutathione (GSH)-conjugated metabolite of molinate. The GSH conjugation of molinate sulfoxide in rat cytosol was described by the constants Km of 305 μM and Vmax of 4.21 nmol/min/mg cytosol whereas the human values were 91 μM and 0.32 nmol/min/mg protein for Km and Vmax, respectively. At the same 1 mM GSH concentration, the in vitro bimolecular nonenzymatic rate constant of 3.02 × 10⁻⁶ μM ⁻¹ min⁻¹ was calculated for GSH conjugation of molinate sulfoxide. Specific activity for rat and human glutathione transferase was calculated to equal 1.202 ± 0.25 and 0.809 ± 0.45 μmol/min/mg protein, respectively by 1-chloro-2,4-dinitrobenzene (CDNB) assay. Compared to a conventional GSH depletion model (BSO + DEM combination), molinate alone was nearly as effective in reducing GSH levels by approximately 90 and 25% in liver and testes, respectively. The impact of molinate sulfoxide's ability to adduct glutathione transferase and inhibit the production of the glutathione conjugated metabolite was examined and found to be negligible.

We thank Dr. Carol Green and the Department of Drug Metabolism and Pharmacokinetics, SRI International, and Dr. Marion Miller, Department of Environmental Toxicology, UC California, Davis, for their generous contributions of time, resources, and mentorship. This work was supported by the California Rice Research Board (RP-7) and by the National Institute of Environmental Health Sciences Training Grant 5-T32-EOS- 7059.