Isolation of human hepatic microsomes and their inhibition by cimetidine and ranitidine

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Abstract
Human hepatic microsomes were isolated from wedge biopsies of the liver from 13 patients undergoing abdominal surgery. Ultrasonic homogenisation was used to increase the yield of microsomal monooxygenase activity (7-ethoxycoumarin O-deethylase, NADPH-cytochrome c reductase), resulting in a 30% higher total enzyme activity per g liver than preparation by other techniques. In 4 individual microsomal preparations the influence of cimetidine and ranitidine on Michaelis-Menten kinetics of O-deethylation and of reductase activity were studied. Without the H2-receptor blocking drugs, enzyme kinetics of O-deethylation with a Km of 51.0±16.4 µM (n=3) were obtained using Lineweaver-Burke plots. Both, cimetidine and ranitidine inhibited the O-deethylation; cimetidine had a five-fold higher inhibitory affinity (Ki 1.01 and 3.94 mM) to the monooxygenase than ranitidine (Ki 4.96 and 17.70 mM) in the uninduced liver. However, in liver from a patient with induced enzyme activity (Km=478.0 µM), the Ki of ranitidine was similar to that of cimetidine (Ki ran 3.57 versus Ki cim 2.49 mM). The reductase activity was not inhibited by ranitidine and only marginally so by cimetidine. The results suggest that in human hepatic microsomes oxidative drug metabolism is inhibited by both H2-receptor antagonists. However, the inhibitory potency of the compounds seems to depend on the individual isozyme pattern of the hepatic microsomes. Thus, while cimetidine is an relatively nonspecific enzyme inhibitor, ranitidine might more selectively inhibit induced drug metabolizing enzymes.