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Role of Active Oxygen Species in the Photodestruction of Microsomal Cytochrome P-450 and Associated Monooxygenases by Hematoporphyrin Derivative in Rats¹

Department of Dermatology, Case Western Reserve University, and Veterans Administration Medical Center, Cleveland, Ohio 44106

The cytochrome P-450 in hepatic microsomes prepared from rats pretreated with hematoporphyrin derivative was shown to be rapidly destroyed in the presence of long-wave ultraviolet light. The photocatalytic destruction of the heme-protein was dependent on both the dose of ultraviolet 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 oxygen since quenchers of singlet oxygen, including 2,5-dimethylfuran, histidine, and ß-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, whereas inhibitors of the hydroxyl radical, including benzoate, mannitol, and ethyl alcohol, did afford protection. These results indicate that 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.

¹ Supported in part by NIH Grants CA 31069 and ES-1900 and funds from the Veterans Administration.

² To whom requests for reprints should be addressed, at Veterans Administration Medical Center, 10701 East Boulevard, Cleveland, OH 44106.

Received 8/ 2/84. Accepted 11/ 5/84.