Spectral analysis of CYP11A interaction with inhibitory drugs. The concentration of CYP11A1 was 0.4 μM, and the buffer was 40 mM KPi, pH, 7.2, containing 1 mM EDTA. Numbers above or below the spectra indicate the wavelengths of absorption maxima or minima. A and C, absolute spectra; solid and dashed lines represent CYP11A1 spectrum in the absence and presence of ketoconazole (A) and posaconazole (B), respectively. Inhibitors’ concentrations were 15 μM (ketoconazole) and 10 μM (posaconazole), equal to 10 Kd of for the studied drug (1.5 μM and 1.0 μM, respectively) for substrate-free CYP11A1. B and D, difference spectra.