Rotation of cytochrome P450SCC (CYP11A1) in proteoliposomes studied by delayed fluorescence depolarization

The rotational diffusion of cytochrome P450SCC (CYP45011A1) was investigated in proteoliposomes measuring the time-resolved delayed fluorescence anisotropy of diiodofluorescein iodoacetamide covalently and specifically attached to Cys264 of P450SCC. Rotation strongly depends on the lipid composition of the membrane, especially on the cardiolipin content. In proteoliposomes having a lipid composition similar to the inner membrane of bovine adrenal mitochondria P450SCC is rotating uniaxially with a relaxation time phi rot about 53 microseconds and almost no immobile P450 is present. Addition of high KCl concentration has no effect. The results and the absence of any intramembrane particles observed by freeze-fracturing indicate that P450SCC probably exists in the liposomal membrane as oligomer not penetrating the bilayer. It is suggested to bind tightly within the outer monolayer with large parts exposed to the aqueous solution surrounding the membrane.