title: Role of nonprotein thiols in enzymatic reduction of 2-nitroimidazoles.
abstract: The role of nonprotein thiols (NPSH) in the enzymatic reduction of the nitro function in 2-nitroimidazoles (2-NI) has been investigated. The addition of NPSH has been shown previously to protect cells from the hypoxic cytotoxicity of 2-NI, whereas depletion of NPSH enhances the hypoxic cytotoxicity. In this report, we have investigated the effects of thiol depleting agents, N-ethylmaleimide (NEM) and diethyl maleate (DEM), on the enzymatic reduction of the nitro group. Cytosolic and microsomal fractions of rat hepatic tissue and xanthine oxidase were employed as sources of nitro reductases. Addition of NPSH caused an enhancement in the reduction of the nitro group of 2-NI; cysteine was significantly more effective than glutathione (GSH) in stimulating the enzymatic reduction. The reduction of the nitro function was decreased markedly in the presence of NEM or DEM. Addition of cysteine or GSH reversed the inhibition with NEM. Both NEM and DEM also attenuated the enhancement of reduction observed after the addition of NPSH. These results suggest that the addition of NPSH facilities the reduction of the nitro function to the reduced intermediates that may be inactivated by an excess of NPSH, whereas the depletion of NPSH allows the accumulation of the toxic nitro radicals causing increased cytotoxicity.
authors.affiliation: Department of Pharmacology, Tulane University School of Medicine, New Orleans, LA 70112.
authors.forename: K H
authors.initials: KH
authors.lastname: Wong
authors.forename: K C
authors.initials: KC
authors.lastname: Agrawal
mesh_terms: D000818:Animals; D003545:Cysteine / Q000494:pharmacology*; D003600:Cytosol / Q000201:enzymology; D005978:Glutathione / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D009593:Nitroimidazoles / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D011919:Rats, Inbred Strains; D013439:Sulfhydryl Reagents / Q000494:pharmacology
chemical_list: D009593:Nitroimidazoles; D013439:Sulfhydryl Reagents; D005978:Glutathione; D003545:Cysteine