Specific binding of polychlorinated biphenyls to rat liver cytosol protein

Specific binding of polychlorinated biphenyls (PCBs) to rat liver cytosol protein has been detected using the 3H-labeled PCB probe 2,2',4,4',5,5'-hexachlorobiphenyl (6-CB). Binding of 6-CB to cytosol protein is displaced by its non-radioactive congener, is of high affinity (Kd approximately 3 nM) and is saturable (maximal binding capacity Bmax approximately 600 pmol/mg protein). 6-CB binding is not found in liver cytosol of animals fed a PCB-supplemented diet (500 ppm PCB for 5 days). Binding is also in vitro inhibited by high concentrations of triglyceride. PCB congeners such as 3,3',4,4',5-pentachlorobiphenyl as well as the thyroid hormones 3,5,3',5'-tetraiodothyronine and 3,5,3'-triiodothyronine (the latter hormone with an order of magnitude lower affinity) compete for the PCB binding site. On the other hand, a number of biochemically important compounds including the PCB core compound biphenyl and the hormone ligands dexamethasone and estradiol, as well as 2,3,7,8-tetrachlorodibenzo-p-dioxin, do not compete for the 6-CB binding site. The data provide the first evidence of specific binding of unmetabolized PCB congeners to distinct binding sites in rat liver cytosol.