REGULATION MECHANISM OF THE CATALYTIC ACTIVITY OF BOVINE ADRENAL CYTOCHROME P-450(11-BETA)

In our previous paper (Ikushiro et al. (1992) J. Biol. Chem. 267, 1464), two catalytic states were proposed for bovine adrenocortical P-450(11 beta) at 37 degrees C: one in liposome membranes and the other in liposome membranes containing P-450(scc). Similar reaction characteristics were observed at 5 degrees C and all the experiments in this study were performed at 5 degrees C. P-450(11 beta)-proteoliposomes had relatively low 11 beta-hydroxylase activity and could catalyze aldosterone formation from ll-deoxycorticosterone. Relatively high 11 beta-hydroxylase activity was observed in P-450(11 beta)-proteoliposomes containing P-450(scc) and in Tween-20 solubilized P-450(11 beta), in which no aldosterone formation could be detected. Optical titration indicated binding of corticosterone to P-450(11 beta) to be much weaker in the Tween-20 solubilized state than in proteoliposomes. Corticosterone competitively inhibited 11 beta-hydroxylation reaction of P-450(11 beta)-proteoliposomes, but neither in P-450(11 beta)-proteoliposomes containing P-450(scc) nor in the Tween-20 solubilized system. The binding of corticosterone to P-450(11 beta) was concluded quite weak in proteoliposomes in the presence of P-450(scc) and in the Tween-20 solubilized state. Aldosterone formation thus was not possible in these systems. Inability of the bovine adrenocortical zonae fasciculata and reticularis to produce aldosterone may be due to the weak binding of