title: Positive effectors of the binding of an active site-directed amino steroid to rabbit cytochrome P-450 3c.
abstract: 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.
authors.affiliation: Department of Basic and Clinical Research, Scripps Clinic and Research Foundation, La Jolla, California 92037.
authors.forename: E F
authors.initials: EF
authors.lastname: Johnson
authors.forename: G E
authors.initials: GE
authors.lastname: Schwab
authors.forename: L E
authors.initials: LE
authors.lastname: Vickery
mesh_terms: D000818:Animals; D001571:Benzoflavones / Q000494:pharmacology; D001665:Binding Sites; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D011283:Pregnenes / Q000378:metabolism*; D011284:Pregnenolone / Q000494:pharmacology; D011374:Progesterone / Q000378:metabolism*; D011485:Protein Binding; D011817:Rabbits; D012293:Rifampin / Q000494:pharmacology; D013053:Spectrophotometry
chemical_list: D001571:Benzoflavones; D065607:Cytochrome P-450 Enzyme Inhibitors; D011283:Pregnenes; D011374:Progesterone; C011512:alpha-naphthoflavone; D011284:Pregnenolone; C046654:22-amino-23,24-bisnor-5-cholen-3-ol; D003577:Cytochrome P-450 Enzyme System; D012293:Rifampin