Polychlorinated biphenyls (PCBs) are persistent organic pollutants that are carcinogenic and cause a variety of adverse health effects to the nervous, reproductive and endocrine systems. They bind to cytochrome P450 (CYP) enzymes to undergo phase I biotransformation to form hydroxylated metabolites. The present study investigates the hypothesis that the binding of multiple ortho substituted PCBs depends on the number of chlorine substituents and the chlorine substitution pattern. It also explores if hydroxylation alters the binding of these compounds.
To test this hypothesis the spectral interactions of hepatic microsomal CYP enzymes with 2,2',3,4',6-pentachlorobiphenyl (PCB91), 2,2',3,5',6-pentachlorobiphenyl (PCB95), 2,2',3,3',4,6'-hexachlorobiphenyl (PCB132), and 2,2',3,4',5',6-hexachlorobipheny (PCB149) and their 5-hydroxylated (5-OH) metabolites were investigated. Hepatic microsomes were prepared from rats treated with phenobarbital (PB), dexamethasone (DEX), corn oil (CO) and saline (SL). The difference spectra of these microsomes in the presence of increasing concentrations of PCBs were recorded.
All four PCB congeners and their metabolites (except for 5-OH PCB 149 with the DEX microsomes) elicited type I difference spectra with the PB and DEX microsomal preparations. No difference spectrum was observed with the CO and SL microsomes. With the parent PCBs, there was no distinct trend with the maximal absorbance change, δA max with the PB and the DEX microsomes. The spectral dissociation constant (K ss app) was lowest for PCB 91 and it had the highest relative binding efficiency (= K ss app / δA max) with PB and DEX microsomes. This suggests that PCB congeners and metabolites that have more unsubstituted carbon atoms at the meta position bind better to the CYP enzymes.
The highest binding efficiency was observed with the PB microsomes both with the parent PCBs and their metabolites. This suggests that all the four PCB congeners and metabolites bind preferentially to the CYP2B enzymes (induced by PB) than to the CYP3A enzymes (induced by DEX). Also the binding efficiencies of the parent compounds were found be higher than their metabolites implying that hydroxylation of the parent PCB congener reduces its binding to the CYP enzymes.
From the structure activity relationships and the effect of chlorination studied on PCB-CYP interactions in this project, it was found that the chlorine substitution pattern, but not the number of chlorine substituents play an important role in the binding of multiple ortho chlorine substituted PCBs to CYP enzymes.