Abstract
The electron paramagnetic (EPR) properties of a transient species detected during a cytochrome P450 (P450)-mediated peroxidative reaction have been compared with those of peroxidases Compound I and model metalloporphyrins. The reaction which was studied with cholesterol-specific P450scc and (20 R)-20-hydroperoxycholesterol, occured without enzyme denaturation. The resulting transient species, which reached its maximum after 50 s reaction, was characterized by a one-line EPR spectrum, g = 2.0035, Δ 1 2 = 1.08 mT . The decay of this radical was concomitant with the production of (20 R)-20,21-dihydroxycholesterol. The reaction (almost 100% yield) conserved the stereo-specificity of the natural pathway. We suggest this intermediate is a candidate for the in vivo ultimate oxidant in the P450-mediated hydroxylation process. The comparison of the observed EPR spectrum with those of peroxidase Compound I and related synthetic models allows us to propose a Fe IV porphyrin II cation radical structure for the intermediate.
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