Cytochrome P450 isozymes involved in propranolol metabolism in human liver microsomes: The role of CYP2D6 as ring-hydroxylase and CYP1A2 as N- desisopropylase

Oxidative metabolic pathways of propranolol consist of naphthalene ring- hydroxylations (at the 4-, 5-, and 7-positions) and side-chain N- desisopropylation in mammals. We characterized cytochrome P450 isozymes responsible for propranolol metabolism, especially N-desisopropylation and 5- hydroxylation, in human liver microsomes. 4-Hydroxy, 5-hydroxy-, and N- desisopropylpropranolol were detected as primary metabolites, whereas 7- hydroxypropranolol was in trace amounts. Good correlations were obtained for activities of propranolol 4- and 5-hydroxylases with immunochemically determined CYP2D6 content, whereas correlations of these activities with CYP1A2, CYP2C, or CYP3A4 content were relatively low. The activities also correlated highly with debrisoquine 4-hydroxylase, compared with other metabolic activities such as phenacetin O-deethylase, hexobarbital 3'- hydroxylase, and testosterone 6β-hydroxylase, which are typical reactions for CYP1A2, CYP2C, and CYP3A4, respectively. Propranolol N-desisopropylase activity in the samples highly correlated with CYP1A2 content and phenacetin O-deethylase activity, but not with the other P450 isozyme contents or metabolic activities. Quinidine, a specific inhibitor of CYP2D6, inhibited propranolol 4- and 5-hydroxylase activities selectively and in a concentration-dependent manner. α-Naphthoflavone, a potent inhibitor of CYP1A2, inhibited all of the propranolol oxidation activities, and the IC₅₀ value for N-desisopropylase activity was much smaller than the values for ring-hydroxylase activities. Antibody directed to CYP2D inhibited propranolol 4- and 5-hydroxylase activities by 70% at an antibody/microsomal protein ratio of 1.0. Anti-CYP2C9 antibody did not inhibit any activity determined. These results indicate that propranolol 5-hydroxylation, as well as 4- hydroxylation, is mainly catalyzed by CYP2D6 in human liver microsomes. Furthermore, CYP1A2, rather than S-mephenytoin 4-hydroxylase, seems to be responsible for propranolol N-desisopropylation.