Fate of free radicals generated during one-electron reductions of 4-alkyl-1,4-peroxyquinols by cytochrome P 450

Abstract

Free radicals resulting from the one-electron reduction and subsequent homolytic cleavage of oxygen-oxygen bonds by heme proteins are likely to be responsible for some aspects of the toxicity of organic hydroperoxides. In the present work, effects of the 4-alkyl substituent of 2,6-di-tert-butyl-4-alkyl-4-hydroperoxycytohexa-2,5-dienones (1,4-peroxyquinols) on radical production were investigated with microsomal cytochrome P-450 from rat liver. Quinoxy radicals from homolysis of the peroxyquinols underwent beta-scission to produce a quinone and an alkyl radical, and this process occurred with increasing frequency as the stability of the alkyl radical increased. The fate of benzyl and 2-phenylethyl radicals generated from the appropriately substituted peroxyquinols was investigated also. The former was converted to benzyl alcohol, benzaldehyde, and toluene and the latter to 2-phenylethanol, phenylacetaldehyde, ethylbenzene, styrene, and benzaldehyde. Oxygen-18 labeling studies demonstrated that 80-85% of the benzyl alcohol incorporated oxygen from the hydroperoxide and the balance from molecular oxygen. This indicates that the predominant reaction pathway involved recombination between the benzyl radical and the iron-bound hydroxyl radical of the P-450 intermediate complex. By contrast, about 50% of 2-phenylethanol from the 2-phenylethyl radical incorporated oxygen from water and the balance from O2. Two alternative mechanisms are proposed to explain the formation of 2-phenylethanol that contained oxygen from water and the concurrent formation of styrene: (a) oxygen exchange of the P-450 intermediate with water, followed by hydrogen abstraction and radical recombination reactions with the P-450 complex, or (b) oxidation of the radical to the 2-phenylethyl cation followed by proton elimination and hydration.

Free radical-derived quinone methide mediates skin tumor promotion by butylated hydroxytoluene hydroperoxide: expanded role for electrophiles in multistage carcinogenesis.

“…Although the identity of this second radical remains tentative, it is likely to be the semiquinone radical. It is well established that 4-alkyl hydroperoxide analogs of BHTOOH can undergo univalent reduction to produce alkoxyl radicals (32). These alkoxyl radicals subsequently undergo fragmentation through P-scission by which alkyl radicals and quinone intermediates are generated.…”

Cytochrome P450-mediated oxidation of pentafluorophenol to tetrafluorobenzoquinone as the primary reaction product

“…To circumvent this problem, incubations were performed under anaerobic conditions using CumOOH as an artificial oxygen donor to create the reactive (FeO)3+ species. The CumOOH-dependent P450 monooxygenase activity has been suggested to proceed either via heterolytic oxygen-oxygen bond cleavage, resulting in (FeO)3+ formation, or via homolytic oxygen-oxygen bond cleavage, resulting in (FeO-H)3+ and a peroxide radical that initiates the reaction by hydrogen abstraction (19)(20)(21)(22)(23). However, for hydroxylation of aromatic carbon atoms, the peroxidedriven cytochrome P450 reaction may very well proceed to a significant extent by the heterolytic cleavage, leading to the (FeO)3+ intermediate, which is also formed in the NADPH/oxygen-driven reaction (22,23).…”

“…For the CumOOH-driven reaction, independent of the mechanism for cleavage of the peroxide bond, the present data provide evidence for a reaction resulting in formation of TFBQ as the primary reaction product for the conversion of PFP. However, evidence exists (22,23) that a peroxide-supported cytochrome P450catalyzed aromatic hydroxylation proceeds at least in part by heterolytic cleavage of the peroxide bond and, thus, a P450(FeO)3+ intermediate similar to the one formed in the NADPH/oxygen-driven reaction. An IOB-driven cytochrome P450 reaction is generally accepted to proceed by the same cytochrome P450(FeO)3+ intermediate as formed in the NADPH/ oxygen-supported reaction (21,22).…”