Catalyzation of cocaine N ‐demethylation by cytochromes P4502B, P4503A, and P4502D in fish liver

Abstract Cocaine N‐demethylation by microsomal cytochrome P450s is the principal pathway in cocaine bioactivation and hepatotoxicity. P450 isozymes involved in N‐demethylation of cocaine have not been elucidated yet and they differ from species to species. In humans and mice, P4503A contributes to cocaine N‐demethylase activity, whereas in rats, both P4503A and P4502B participate. In the present study, contribution of different P450 isozymes to cocaine N‐demethylase activity was studied in vitro with fish liver microsomes. The specific cocaine N‐demethylase activity was found to be 0.672 ± 0.22 nmol formaldehyde formed/min/mg protein (mean ± SD, n = 6). Cocaine N‐demethylase exhibited biphasic kinetics, and from the Lineweaver–Burk plot, two K m values were calculated as 0.085 and 0.205 mM for the high‐ and low‐affinity enzyme. These results indicate that N‐demethylation of cocaine in mullet liver microsomes is catalyzed by at least two cytochrome P450 isozymes. Inhibitory effects of cytochrome P450 isozyme‐selective chemical inhibitors, ketoconazole, cimetidine, SKF‐525A, and quinidine, on cocaine N‐demethylase activity were studied at 50, 100, and 500 μM concentrations of these inhibitors. At 100 μM final concentrations, ketoconazole (P4503A inhibitor), SKF‐525A (inhibitor of both P4502B and P4503A), and cimetidine (P4503A inhibitor) inhibited N‐demethylation activity by 73, 69, and 63%, respectively. Quinidine, P4502D‐specific inhibitor, at 100 μM final concentration, reduced N‐demethylation activity down to 64%. Aniline, a model substrate for P4502E1, did not alter N‐demethylase activity in the final concentration of 100 μM. IC 50 values were calculated to be 20 μM for ketoconazole, 48 μM for cimetidine (both specific P4503A inhibitors), 164 μM for quinidine (P4502D inhibitor), and 59 μM for SKF‐525A (inhibitor of both P4503A and P4502B). The contribution of P4502B to cocaine N‐demethylase activity in mullet liver microsomes was further explored by the use of purified mullet cytochrome P4502B in the reconstituted system containing purified mullet P450 reductase and lipid. The turnover number was calculated as 4.2 nmol HCOH/(min nmol P450). Overall, these results show that P4503A and P4502B are the major P450s responsible for N‐demethylation of cocaine, whereas contribution of P4502D is a minor one, and P4502E1 is not involved in the N‐demethylation of cocaine in mullet liver microsomes. © 2003 Wiley Periodicals, Inc. J Biochem Mol Toxicol 17:169–176, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jbt.10075