Kinetics of benzylpenicillin metabolism in isolated rat hepatocytes

The metabolism of benzylpenicillin (PCG) in isolated rat hepatocytes was investigated. The evidence of metabolizing activity for PCG in hepatic cells was obtained as follows; the disappearance rate of PCG from the incubation medium followed Michaelis-Menten kinetics and was dependent on the cellular protein concentration, while PCG did not disappear when it was incubated with cells denaturated by heat. The rate of disappearance of PCG was reduced significantly in the presence of the structural analogue of PCG such as phenoxymethylpenicillin in the incubation medium. The major metabolite of PCG was identified, by high performance liquid chromatographic analysis, to be penicilloic acid (PA) of PCG. A kinetic model describing the intra- and extra-cellular concentrations of PCG and PA was developed. The proposed model fitted well the time course of changes in the concentration of PCG and PA. The clearance of the uptake of PCG by isolated hepatocytes was evaluated to be about 23-times greater than that of metabolism of PCG.