Beschreibung:
<jats:p>The rates of the NADPH‐dependent formation of superoxide radicals and hydrogen peroxide have been measured in liver microsomes from phenobarbital‐pretreated rats. Correcting a quenching of O<jats:sub>2</jats:sub><jats:sup>−</jats:sup> radicals by microsomes, a stoichiometry of O<jats:sub>2</jats:sub><jats:sup>−</jats:sup> to H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> close to 2:1 was obtained. This, and the fact that pseudo‐substrates of microsomal cytochrome P450 like perfluoro‐<jats:italic>n</jats:italic>‐hexane and perfluorinated cyclohexane did not increase H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> formation in a catalase‐inhibited assay, rules out a two‐electron reduced oxygen species as the source of H<jats:sub>2</jats:sub>C<jats:sub>2</jats:sub>.</jats:p><jats:p>The rates of O<jats:sub>2</jats:sub><jats:sup>−</jats:sup> as well as H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> generation in the presence of 7‐ethoxycoumarin were equally inhibited by carbon monoxide (75%) and resulted in photochemical action spectra with a maximum reactivation at 450 nm. Using the same conditions the monooxygenation was inhibited to a high degree (83%) but without exogenous substrate the inhibition of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> formation dropped to 55%.</jats:p><jats:p>It was concluded that most of the O<jats:sub>2</jats:sub><jats:sup>−</jats:sup> originated from the oxycomplex of cytochrome P450 and that substrates can modify the rates of its decomposition and sensitivity to carbon monoxide. No correlation of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> formation or of substrate monooxygenation with the optical substrate binding spectra could be observed. From the pH dependence a proton‐assisted decomposition of oxy‐cytochrome P450 appears likely. H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> formation was only slightly decreased at 20 μM dioxygen suggesting that H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> formation via cytochrome P450 should also occur <jats:italic>in vivo.</jats:italic></jats:p>