ROLE OF ACTIVE OXYGEN SPECIES IN THE PHOTODESTRUCTION OF MICROSOMAL CYTOCHROME-P-450 AND ASSOCIATED MONOOXYGENASES BY HEMATOPORPHYRIN DERIVATIVE IN RATS

Abstract

The cytochrome P-450 in hepatic microsomes prepared from rats pretreated with hematoporphyrin derivative [used as a sensitizer in tumor phototherapy] was rapidly destroyed in the presence of long-wave UV light. The photocatalytic destruction of the heme-protein was dependent on both the dose of UV light and of hematoporphyrin derivative administered to the animals. The destructive reaction was accompanied by increased formation of cytochrome P-420, loss of microsomal heme content and diminished catalytic activity of cytochrome P-450-dependent monooxygenases such as aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase. The specificity of the effect on cytochrome P-450 was confirmed by the observation that other heme-containing moieties such as myoglobin and cytochrome c were not susceptible to photocatalytic destruction. The destruction of cytochrome P-450 was a photodynamic process requiring O2 since quenchers of singlet O2, including 2,5-dimethylfuran, histidine and .beta.-carotene, each substantially diminished the reaction. Scavengers of superoxide anion such as superoxide dismutase and of H2O2 such as catalase did not protect against photodestruction of cytochrome P-450; inhibitors of the hydroxyl radical, including benzoate, mannitol and ethyl alcohol, did afford protection. Lipid-rich microsomal membranes and the heme-protein cytochrome P-450 embedded therein are potential targets of injury in cells exposed to hematoporphyrin derivative photosensitization.