The in vitro binding of 2,2', 5,5'-tetrachlorobiphenyl metabolites to rat liver microsomal proteins.

The in vitro association of tritium labeled 2,2', 5,5'-tetrachlorobiphenyl (TCB) with the microsomal fraction isolated from rat livers has been investigated in a metabolizing system. It was found that the control microsomes, capable of only minimal TCB metabolism, had 92% of the total microsomal radioactivity associated with lipids, while only 61% was associated with the phenobarbital induced microsomal lipid. The radioactivity per mg microsomal protein was the same for both induced and noninduced microsomes; however, very important qualitative differences were found. Only the proteins of the induced system contained a protein (s) (MW = 45,000 g/mole) capable of specifically binding a TCB metabolite. This binding required metabolism and was TCB concentration dependent. The specificity of this association was confirmed by dialysis and this data could be analyzed by the Scatchard-Klotz equation. These calculations allowed the evaluation of the number of binding sites (38 micron moles/g of total microsomal protein) and the apparent binding constant (1.4 x 10(7) l/mole). These data are consistent with strong noncovalent interaction of a 2,2', 5,5'-TCB metabolites, but do not exclude the possibility of covalent binding of other non-dialyzable metabolites.