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  • Title: Cytochrome P450 isozymes involved in propranolol metabolism in human liver microsomes. The role of CYP2D6 as ring-hydroxylase and CYP1A2 as N-desisopropylase.
    Author: Masubuchi Y, Hosokawa S, Horie T, Suzuki T, Ohmori S, Kitada M, Narimatsu S.
    Journal: Drug Metab Dispos; 1994; 22(6):909-15. PubMed ID: 7895609.
    Abstract:
    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 6 beta-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. alpha-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.(ABSTRACT TRUNCATED AT 250 WORDS)
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