Ligand binding of organic sulfides to microsomal cytochrome P-450

Octyl methyl-, butyl methyl- and pentamethylene sulfide react with about 50% of oxidized cytochrome P-450 in liver microsomes from phenobarbital-pretreated rats by formation of optical difference spectra with maxima at 435 and 552 nm and concomitant shifts in the electron paramagnetic resonance spectrum. Reduction by NADPH or sodium dithionite yielded a Soret absorption band at 449 nm and alpha and beta bands at 573 and 545 nm, respectively. The ligand metyrapone and the substrate n-octane competitively inhibited the formation of these difference spectra and pentamethylene sulfide was a strong competitive inhibitor of the 0-deakylation of 7-ethoxycoumarin. These results indicate a direct ligand binding of the sulfides to cytochrome P-450 with concomitant blocking of the hydrophobic substrate binding site. Some sulfides did not interact as ligands but as substrates, in variation, however, with the source of microsomes.