Molecular Pharmacology, Vol 12, 933-944, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

Metabolism of Diethyl p-Nitrophenyl Phosphorothionate (Parathion) by a Reconstituted Mixed-Function Oxidase Enzyme System: Studies of the Covalent Binding of the Sulfur Atom

¹ Center in Environmental Toxicology, Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232

During the metabolism of parathion by a reconstituted mixed-function oxidase system from rat liver, the sulfur-containing portion of the molecule becomes covalently bound, predominantly, if not exclusively, to cytochrome P-450. Companion experiments using ¹⁴C-labeled parathion indicated that more than 95% of the sulfur bound to cytochrome P-450 is free of the remainder of the parathion molecule. Thus the bound sulfur is the sulfur atom released in the metabolism of parathion to paraoxon. Investigations concerning the nature of the covalent bond between the sulfur atom and cytochrome P-450 suggested that about 50% becomes bound to the side chain of the cysteine in the cytochrome P-450 apoenzyme, forming a hydrodisulfide. Binding of the sulfur to cytochrome P-450 of the reconstituted system apparently leads to cross-linking of the proteins of the reconstituted system to form high molecular weight complexes. Examination of the binding of sulfur to the macromolecules of intact rat liver microsomes on incubation with parathion revealed that the majority of the atomic sulfur is bound to protein(s) precipitated by an antibody to rat liver cytochrome P-450. This binding of sulfur to the macromolecules of intact microsomes also leads to the formation of high molecular weight complexes as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

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