EVIDENCE FOR BINDING OF LIDOCAINE TO TWO CATALYTICALLY DIFFERENT SITES OF LIVER MICROSOMAL CYTOCHROME P-450

Abstract

Addition of lidocaine to liver microsomes results in biphasic type I spectral titration curves. A high-affinity and a low-affinity binding phase exist. In this study we have found that microsomes from female rats have a marked high affinity phase, which can hardly be observed with microsomes from female guinea-pigs. Male rats were intermediate. On incubation of lidocaine at concentrations of 1 μM or less with female rat liver microsomes a larger fraction of the drug was aromatically hydroxylated than deethylated. The opposite was true for guinea-pig liver microsomes, and microsomes from male rats were intermediate. The ratio between the formation of deethylated and hydroxylated metabolite increases with the amount of added lidocaine in all microsomes. These data suggest that the two spectral phases represent different binding sites of cytochrome P-450 with a certain “catalytic specificity” - the “high affinity site” catalyzing aromatic hydroxylation and the “low affinity site” deethylation. Observed differential effects of pH and MgCl2 concentration on aromatic hydroxylation and deethylation further strengthens the view that these reactions are catalyzed by different entities of cytochrome P-450.