Evidence that the Yb subunits of hepatic glutathione transferases represent two different but related families of polypeptides

Abstract

Three soluble rat liver glutathione (GSH) transferases A, C and one referred to as 'D', all of which are dimers of Yb subunits [Bass et al. (1977) Biochim. Biophys. Acta, 492, 163-175], have been compared with respect to C-terminal amino acids and tryptic peptide maps. GSH transferases A and 'D' gave different tryptic peptide maps and different C-terminal amino acids, lysine and proline respectively. In each case the number of tryptic peptides is about half of that expected from their lysine and arginine content, and there are 2 mol C-terminal amino acid/mol enzyme. This indicates that GSH transferases A and 'D' represent two different Yb homodimers, which we refer to here as Y1bY1b and Y2bY2b respectively. GSH transferase C is the corresponding heterodimer Y1bY2b since it gives all the tryptic peptides which arise from GSH transferase A and GSH transferase 'D' and also contains both C-terminal lysine and proline. These results provide a structural basis to similar conclusions drawn by Mannervik and Jensson [(1980) J. Biol. Chem. 257, 9909-9912] based on enzymic and immunological comparisons. Tryptic peptide maps show that GSH transferases A and 'D' have considerable homology since there are 23 peptides common to both, 12 peptides unique to A and 8 peptides unique to 'D'. Even so GSH transferase A is selectively induced by a phenobarbitone regime. It is, therefore, concluded that Y1b and Y2b are derived from separate but related genes. A similar conclusion has been drawn concerning the Ya and Yc subunits [Beale et al. (1982) Eur. J. Biochem. 126, 459-463], and a comparison of amino acid compositions, presented here, further suggests a genetic relationship between both pairs of subunits.