Positive effectors of the binding of an active site-directed amino steroid to rabbit cytochrome P-450 3c

The binding of the amino steroid, 22-amino-23,24-bisnor-5-cholen-3 beta-ol (22-ABC), to rabbit liver cytochrome P-450 3c was studied using purified P-450 3c and liver microsomes prepared from rifampicin-treated B/J rabbits. 22-ABC binds to purified cytochrome P-450 3c producing a type II spectral change reflecting the coordination of the amine with the heme iron of the protein. In the absence of allosteric effectors, the binding is characterized by a Ks of 5 microM. In the presence of alpha-naphthoflavone or progesterone, the Ks decreases to 0.8 microM, indicating that these two compounds serve as positive effectors of the binding of 22-ABC to cytochrome P-450 3c. The antibiotic rifampicin induces cytochrome P-450 3c in rabbit liver microsomes, and the benzo(a)pyrene hydroxylase, estradiol 2-hydroxylase, and progesterone 6 beta-hydroxylase activities of these microsomes are stimulated by alpha-naphthoflavone. Moreover, the progesterone 6 beta-hydroxylase activity catalyzed by these microsomes exhibits a dependence on substrate concentration that is consistent with activation of the enzyme by the substrate, progesterone. The magnitude of the type II spectral change elicited by 22-ABC for microsomes prepared from rifampicin-treated B/J rabbits is greater than that observed for microsomes from untreated rabbits. For microsomes from rifampicin-treated rabbits, the apparent binding constant for 22-ABC was decreased 5-fold in the presence of alpha-naphthoflavone. We propose that the effects of alpha-naphthoflavone and progesterone on the binding of 22-ABC to cytochrome P-450 3c mimic the effects of the two positive effectors on the metabolism of substrates by increasing the affinity of the enzyme for substrate.