CYTOCHROMES P450 IN BENZENE METABOLISM AND INVOLVEMENT OF THEIR METABOLITES AND REACTIVE OXYGEN SPECIES IN TOXICITY

Cytochrome P450 (CYP) 2E1 was the most efficient CYP enzyme that oxidi zed benzene to soluble and covalently bound metabolites in rat and hum an liver microsomes. The covalent binding was due mostly to the format ion of benzoquinone (BQ), the oxidation product of hydroquinone (HQ), and was inversely related to the formation of soluble metabolites. In rats, inhalation of benzene 14 mg/liter of air) caused a rapid destruc tion of CYP2B1 previously induced by phenobarbital. The ability of ben zene metabolites to destroy liver microsomal CYP in vitro decreased in the order BQ > HQ > catechol > phenol. The destruction was reversed b y ascorbate and diminished by alpha-tocopherol, suggesting that HQ was not toxic, whereas BQ and semiquinone radical (SQ) caused the effect. in the presence of nicotinamide adenine dinucleotide phosphate, reduc ed (NADPH) the microsomes did not oxidize HQ to BQ, while the formatio n of superoxide anion radical from both HQ and BQ was markedly quenche d. Destruction of CYP in vitro caused by HQ or BQ was not mediated by hydroxyl radical formation or by lipid peroxidation. On the contrary, HQ and BQ inhibited NADPH-mediated lipid peroxidation. Ascorbate induc ed high levels of hydroxyl radical formation and lipid peroxidation, w hich were differentially affected by quinones, indicating different me chanisms. Despite reducing the toxicity of HQ and BQ, ascorbate appear ed to induce its own toxicity, reflected in high levels of lipid perox idation. Iron redox cycling played a significant role in the NADPH-ind uced hydroxyl radical formation but not in that caused by ascorbate; h owever, lipid peroxidation induced by NADPH or ascorbate was suppresse d by ethylenediaminetraacetate, indicating a crucial role of iron. Thu s, the data indicate that the quinones destroyed CYP directly and not via oxygen activation or lipid peroxidation.