METABOLIC KINETICS OF PSEUDORACEMIC PROPRANOLOL IN HUMAN LIVER-MICROSOMES - ENANTIOSELECTIVITY AND QUINIDINE INHIBITION

The enantioselective formation kinetics of 4-hydroxypropranolol (4-HOP ), 5-hydroxypropranolol (5-HOP), and desisopropylpropranolol (DIP) wer e characterized over a wide substrate concentration range (1-1000 mu M ) in human liver microsomes using deuterium-labeled pseudoracemic prop ranolol. Existing data suggest that several microsomal cytochrome P-45 0 enzymes are involved in the oxidative metabolism of propranolol in h umans. Biphasic kinetics were ob- served in the formation of all three metabolites, indicating the involvement of at least two enzymes in ea ch pathway. The RIS ratios for the formation of all three metabolites varied with respect to the substrate concentration, lending further su pport to the contribution of two or more enzymes with differing K-M's and enantioselectivity. The high affinity 4-hydroxylation process show ed a strong R-enantioselectivity. The low-affinity component of 4-hydr oxylation 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 IS ratio showed an initial increase followed by a decrease as the prop ranolol concentration increased beyond 200 mu M. Formation of DIP was R-enantioselective at low substrate concentrations, whereas an opposit e enantioselectivity was observed at high propranolol concentrations. The metabolism of propranolol in the presence of nanomolar concentrati ons of quinidine (a selective inhibitor of P-450 2D6) was studied at c oncentrations of pseudoracemic propranolol in the high- and low-affini ty regions. A significant inhibition of 4- and 5-hydroxylation was obs erved, whereas N-dealkylation was not affected by quinidine. The inhib ition of 4-hydroxylation was slightly enantioselective toward R-enanti omer. Quinidine had no significant effect on the low-affinity componen t for 4-hydroxylation. Although the inhibition of 4- and 5-hydroxylati on at the high affinity site was extensive, complete inhibition was no t achieved even at the highest quinidine concentration (10 mu M). Data could be fitted to a mixed-type inhibition kinetics resulting from mu ltiple 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 P450 enzymes other than 2D6.