Interaction of Polycyclic Hydrocarbons with Cytochrome P-450. I. Specific Binding of Various Hydrocarbons to P-448₁

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Interaction of various polycyclic aromatic hydrocarbons with P-448₁ from rabbit liver microsomes was demonstrated by measuring absorption, CD and fluorescence spectra of the hydrocarbons, the cytochrome and the complexes of both. On binding of hydrocarbons such as pyrene and benz[a]anthracene to P-448₁, prominent CD peaks appeared at around the wavelengths where the hydrocarbons possess absorption bands. Correspondingly, fluorescence of the hydrocarbons was quenched to various extents depending on the hydrocarbon examined but was restored when the protein structure was destroyed by denaturation. The absorption peaks of the bound hydrocarbons also shifted toward a longer wavelength. Such an appearance of CD bands, quenching of fluorescence emission or a shift of absorption peaks was not seen when the hydrocarbons were mixed with P-450₁ or albumin. Titration experiments indicated that hydrocarbons can bind specifically and tightly to P-4481 at a single binding site to form equimolar complexes. Complexes of 21 kinds of hydrocarbons, possessing 2 to 5 benzene rings spread in various directions, were isolated and their spectral properties were investigated. It was shown that the hydrocarbons bind to P-448₁ at the same site and compete with one another. Evidence was obtained that the hydrocarbons were bound at the substrate site in the hemecontaining domain of the monooxygenase, P-448₁, and metabolized by the aid of NADPH-cytochrome P-450 reductase in the presence of NADPH and O₂.