Irreversible Binding and Metabolism of Propranolol by Human Liver Microsomes--relationship to Polymorphic Oxidation

Studies were performed to investigate the irreversible binding and oxidative metabolism of propranolol in human liver microsomes and the relationship of binding and metabolism to the polymorphic oxidation of debrisoquine. Incubation of microsomes with 14C-labelled propranolol in the presence of a NADPH-generating system gave rise to irreversible binding which increased linearly with time and became saturated at high substrate concentrations. The extent of binding was decreased by the exclusion of cofactors, boiling, anaerobic conditions, and the addition of reduced glutathione and SKF-525A. Trichloropropene oxide had a negligible effect on cofactor-dependent binding. However, debrisoquine, antipyrine and phenacetin decreased binding to a considerable extent. The latter compound abolished cofactor-dependent binding completely at the concentration used (1 mM). Electrophoresis of microsomes which had been incubated with tritiated propranolol revealed that binding was probably occurring to a large number of proteins particularly in the 40,000-90,000 molecular weight range. Glutathione, debrisoquine and antipyrine did not inhibit the 4'-hydroxylation and N-deisopropylation of propranolol. In contrast, phenacetin exerted a very potent inhibitory action on both routes of metabolism. It is concluded that a product or products of propranolol oxidation bind irreversibly but non-selectively to human liver microsomal protein, the enzyme system responsible for the activation of propranolol appears to be related more closely to the cytochrome P-450 system which metabolizes phenacetin than to that metabolising debrisoquine, and radiolabelled propranolol is not a sufficiently specific probe for studying these cytochrome P-450 systems.