Comparison of the binding sites of GSH S-transferases of the Y(a)- and Y(b)-subunit classes: Effect of glutathione on the binding of bile acids

We have previously observed that the Y(a) subunit-containing glutathione (GSH) S-transferases from rat liver exhibit a common high affinity binding site for lithocholic acid, bilirubin, and sulfobromophthalein (BSP) (1984. J. Lipid Res. 25: 1177-1183). Subsequently we found that cholic acid and its amidates bound to a site on the Y(a) subunit separate for the lithocholic acid/bilirubin site (1986. J. Lipid Res. 27: 955-966). We now have extended this work by showing that amidates of lithocholic acid as well as chenodeoxycholic acid and its amidates competitively displace [¹⁴C]lithocholic acid from the Y(a) subunit. GSH did not inhibit binding of any of the ligands to the high affinity Y(a) site, but did inhibit binding to the cholic acid site on the Y(a) subunit. We have also defined the binding sites and effects of GSH on the Y(b) class of subunits. Lithocholic, chenodeoxycholic, and cholic acids (and amidates) shared a common site on the Y(b) subunit or Y'(b) subunit, whereas BSP and bilirubin were bound at a different site. Both the bile acid and organic anion sites on the Y(b) subunit were inhibited by GSH. The inhibition by GSH in all cases (Y(a) cholic acid site or Y(b) bile acid or bilirubin sites) was saturable, of the competitive type, and incomplete at maximal GSH concentrations, suggesting that when GSH binds to its distinct substrate site, it induces a conformational change in the proteins affecting the other binding sites.