The relationship between binding to cytochrome P-450 and metabolism of n-alkyl carbamates in isolated rat hepatocytes

Abstract

The binding constants of an homologous series of n-alkyl (C 2-C 10) carbamates ▪ to the cytochrome P-450 of suspensions of isolated, viable rat hepatocytes have been measured. All the carbamates except ethyl and propyl carbamate produced type I difference spectra and their binding affinities (1/ K s ) were found to be directly dependent upon their lipophilicity. These binding affinities were similar to those determined in rat liver microsomes. Maximum development of the binding spectrum in hepatocytes was always within one second of the addition of each carbamate, indicating that for these carbamates membrane permeability was not rate limiting for access to, and metabolism by, cytochrome P-450 and that much of the cells' cytochrome P-450 was unoccupied by endogenous substrates. The major metabolites of C 4-C 8 carbamates were unconjugated ω-1 oxidation products. Below hexyl carbamate only the ω-1 hydroxylated metabolite was observed but for the more lipophilic carbamates the keto metabolite was also a major product. The same products were found in blood after i.p. dosing of rats with hexyl carbamate. A direct relationship was observed between the affinity constant of the carbamate for cytochrome P-450 and the total rate of oxidative metabolism in the ω-1 position. Hydrolysis of the carbamate group was a minor metabolic pathway in contrast to the in vivo situation.