Indirect Correlation between Hydroxylation Activities and Oxidation-Reduction of Cytochrome P-450

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Abstract
The oxidation-reduction potential (E′0) of cytochrome P-450 was measured spectrophotometrically in rabbit liver microsomes and in a partially purified particle preparation, and a value of −285mV was obtained in both cases for P-450 (Fe111)/P-450 (Fe11CO) in the pH range from 6 to 8. A potential of −310mV was determined for NADPH-cytochrome c reductase purified from the same source. Xanthine oxidase [EC 1.2.3.2] and hypoxanthine, added exogenously, could reduce cytochrome P-450 in liver microsomes. This artificial electron donor system could also support the microsomal hydroxylation of o-chloroaniline. However, cytochrome P-450 in the particle preparation was not reduced by the xanthine oxidase system unless a viologen dye was added. Purified ferredoxin-NADP+ reductase [EC 1.6.99.4], NADPH-adrenodoxin reductase, and microsomal NADPH-cytochrome c reductase could also catalyze the reduction of cytochrome P-450 by NADPH in the particle preparation when supplemented by a viologen dye or a non-heme iron protein (ferredoxin or adrenodoxin). The reduction of cytochrome P-450 by these reductase systems was not accompanied by the hydroxylation of o-chloro-aniline by the particle preparation. Based on these observations, the mechanism of microsomal hydroxylation reaction is discussed.

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