Mechanism of the cytochrome P-450 catalyzed isomerization of hydroperoxides

Relationship between the metabolism of butylated hydroxytoluene (BHT) and lung tumor promotion in mice

The widely used antioxidant butylated hydroxytoluene (BHT, 2,6-di-tert-butyl-4-methylphenol) produces acute pulmonary toxicity in mice, and also enhances the multiplicity of lung tumors in mice when chronically administered following a single dose of a carcinogen such as urethane. Evidence strongly indicates that the pulmonary effects of BHT are caused by one or more of its reactive metabolites, particularly the hydroperoxide or quinone methide products. The former, BHT-OOH (2,6-di-tert-butyl-4-hydroperoxy-4-methylcyclohexa-2,5-dienone+ ++), is later converted to free radicals by cytochrome P-450, and evidence implicating this pathway in BHT-OOH-induced cytotoxicity has been obtained using isolated rat hepatocytes. Pulmonary microsomes from mice effectively hydroxylate BHT to BHT-BuOH [6-tert-butyl-2-(hydroxy-tert-butyl)-4-methylphenol]; this metabolite was several-fold more effective than BHT as a lung tumor promoter, substantially more pneumotoxic than BHT in vivo, and more toxic to isolated rat hepatocytes and mouse bronchiolar Clara cells in vitro. These effects may be a result of oxidation of BHT-BuOH to the corresponding quinone methide, which is a highly electrophilic. The tumor promoting effects of BHT in mouse lung may be a result of selective cytotoxicity or altered signal transduction caused by radical-generating hydroperoxide and/or electrophilic quinone methide metabolites.

Cumene hydroperoxide-dependent oxidation of NNN'N'-tetramethyl-p-phenylenediamine and 7-ethoxycoumarin by cytochrome P-450. Comparison between the haemoproteins from liver and olfactory tissue

The interaction of cytochromes P-450 of the liver and olfactory epithelium of male hamsters with cumene hydroperoxide (CHP) has been characterized with regard to the ability of CHP to (1) support 7-ethoxycoumarin-O-de-ethylase (ECOD) activity, (2) support the oxidation of NNN'N'-tetramethyl-p-phenylenediamme (peroxidase activity) and (3) cause inactivation of cytochrome P-450. In the liver, CHP was found to support both ECOD and peroxidase activities while causing only minimal inactivation of cytochrome P-450. In contrast, in the olfactory epithelium CHP was virtually unable to support ECOD activity, peroxidase activity was 4-fold greater than in the liver, and extensive inactivation of cytochrome P-450 occurred. The reasons for these differences have been investigated with particular reference to the mode of cytochrome P-450-catalysed decomposition of CHP, that is, via homolytic or heterolytic cleavage of the hydroperoxide dioxygen bond. In both tissues, cumenol (2-phenylpropan-2-ol) was the major product of CHP decomposition detected. The radical scavenger nitrosobenzene inhibited cumenol formation by 84% in the olfactory epithelium, but by only 38% in the liver. This may indicate that dioxygen-bond scission occurs predominantly homolytically in the nasal tissue, whereas there is a balance between homolysis and heterolysis in the liver. It is suggested that the inability of CHP to support ECOD activity in the olfactory epithelium and the extensive inactivation of cytochrome P-450 that it causes both stem from decomposition of the hydroperoxide occurring homolytically rather than heterolytically in this tissue.