Species difference in enantioselectivity for the oxidation of propranolol by cytochrome P450 2D enzymes

We examined and compared enantioselectivity in the oxidation of propranolol (PL) by liver microsomes from humans and Japanese monkeys (Macaca fuscata) . PL was oxidized at the naphthalene ring to 4-hydroxypropranolol, 5-hydrox ypropranolol and side chain N-desisopropylpropranolol by human liver micros omes with enantioselectivity of [R(+)> S(-)] in PL oxidation rates at subst rate concentrations of 10 mu M and 1 mM. In contrast, reversed enantioselec tivity [R(+)< S(-)] in PL 5-hydroxylation and N-desalkylation rates at the same substrate concentrations was observed in monkey liver microsomes, alth ough the selectivity was the same for PL 4-hydroxylation between the two sp ecies. All oxidation reactions of the PL enantiomers in human liver microso mes showed biphasic kinetics, i.e. the reactions could be expressed as the summation of a low-K-m phase and a high-K-m phase. Inhibition studies using antibodies and characterization of CYP2D6 enzymes expressed in insect cell s or human lymphoblastoid cells indicated that the enantioselectivity of PL oxidation, especially the ring 4- and 5-hydroxylations reflected the prope rties of CYP2D6 in human liver microsomes. In monkey liver microsomes, all of the oxidation reactions of S( -)-PL showed biphasic kinetics, whereas ri ng 4- and 5-hydroxylations were monophasic and side chain N-desisopropylati on was biphasic for R(+)-PL. Similarly, from the results of inhibition stud ies using antibodies and inhibitors of cytochrome P450 (P450), it appears t hat the reversed selectivity [R(+)<S(-)] of PL oxidation rates is catalyzed by CYP2D enzyme(s) in monkey liver at low substrate concentrations. These results indicate that different properties of P450s: belonging to the 2D su bfamily cause the reversed enantioselectivity between human and monkey live r microsomes. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.