Abstract

The enantioselective formation kinetics of 4-hydroxypropranolol (4-HOP), 5-hydroxypropranolol (5-HOP), and desisopropylpropranolol (DIP) were characterized over a wide substrate concentration range (1-1000 microM) in human liver microsomes using deuterium-labeled pseudoracemic propranolol. Existing data suggest that several microsomal cytochrome P-450 enzymes are involved in the oxidative metabolism of propranolol in humans. Biphasic kinetics were observed in the formation of all three metabolites, indicating the involvement of at least two enzymes in each pathway. The R/S ratios for the formation of all three metabolites varied with respect to the substrate concentration, lending further support to the contribution of two or more enzymes with differing KM's and enantioselectivity. The high-affinity 4-hydroxylation process showed a strong R-enantioselectivity. The low-affinity component of 4-hydroxylation also exhibited a preference for R-(+)-propranolol, although to a lesser degree than the high-affinity component. A similar pattern of enantioselectivity was observed for 5-hydroxylation, except that R/S ratio showed an initial increase followed by a decrease as the propranolol concentration increased beyond 200 microM. Formation of DIP was R-enantioselective at low substrate concentrations, whereas an opposite enantioselectivity was observed at high propranolol concentrations. The metabolism of propranolol in the presence of nanomolar concentrations of quinidine (a selective inhibitor of P-450 2D6) was studied at concentrations of pseudoracemic propranolol in the high- and low-affinity regions. A significant inhibition of 4- and 5-hydroxylation was observed, whereas N-dealkylation was not affected by quinidine. The inhibition of 4-hydroxylation was slightly enantioselective toward R-enantiomer. Quinidine had no significant effect on the low-affinity component for 4-hydroxylation. Although the inhibition of 4- and 5-hydroxylation at the high-affinity site was extensive, complete inhibition was not achieved even at the highest quinidine concentration (10 microM). Data could be fitted to a mixed-type inhibition kinetics resulting from multiple high-affinity hydroxylases. Our in vitro results indicate that formation of 4-HOP and 5-HOP is mediated by more than one P-450 enzyme with major contribution from P-450 2D6, whereas the formation of DIP is catalyzed by two or more P-450 enzymes other than 2D6.