Metabolism-related spectral characterization and subcellular distribution of polychlorinated biphenyl congeners in isolated rat hepatocytes

The disposition and biotransformation of 4,4'-dichlorobiphenyl (4-DCB), 2,2',3,3',6,6'-hexachlorobiphenyl (236-HCB), and 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB) were studied in isolated rat hepatocyte suspensions. The polychlorinated biphenyls (PCBs) were taken up rapidly by the cells but incompletely metabolized. Metabolism followed first-order Michaelis-Menten kinetics for 20 min and plateaued by 60 min, at which point only 32% of 4-DCB (0.005 to 100 microM) and 60% of 236-HCB (0.001 to 100 microM) were metabolized, while metabolism of 245-HCB was not detected (0.1 to 200 microM). Kinetic studies revealed that both 4-DCB and 236-HCB were metabolized by two Michaelis-Menten processes, displaying high- and low-affinity binding. Readdition of congener once metabolism plateaued resulted in a reinitiation of metabolism with the same proportion of metabolites produced. The termination of metabolism was not due to destruction of the mixed-function oxidases or to depletion of cofactors. The metabolism of PCB congeners is influenced by the affinity of the congener for cytochrome P-450 and partitioning of the congener within the hepatocyte. Analysis of absorbance differences (delta absorbance 390-240 nm) of equimolar concentrations of congener (100 microM) revealed that 236-HCB displayed the greatest affinity of binding to cytochrome P-450 followed by 4-DCB, while 245-HCB showed virtually no binding. Microsomal preparations demonstrated equivalent but greater absorbance values. Subcellular distribution of 14C-labeled congener and its metabolites showed that the majority of radioactivity appeared in the cytosolic fraction, representing 70% of the dose added for each congener. Cytosolic binding of congener and metabolites may influence both the availability of congener to cytochrome P-450 and the excretion rate of metabolites from the cell.