Significance of the steroid-induced type I spectral change in steroid C-21 hydroxylase system of bovine adrenocortical microsomes

The hydroxylyzable steroid 17-hydroxyprogesterone as well as the nonhydroxylyzable steroid androst-4-ene-3,17-dione induce Type I spectral change in cytochrome P-450, the oxygen activating component of the C-21 hydroxylase system. The data presented show quantitative relationship between the Type I spectral change and (1) the steroid-dependent NADPH oxidation; (2) the steroid-dependent increase in the steady-state level of P-450·CO and (3) the rate of C-21 hydroxylation in the case of 17-hydroxyprogesterone. The results indicate that the Type I spectral change is a reflection of the amount of the cytochrome activated for redox reactions and is independent of steroid hydroxylation.