RESUMO
The major form of glutathione transferase from the marsupial Antechinus stuartii has been purified and characterized as an Alpha class enzyme (Ast GST A1-1) with distant sequence relationships to other class Alpha sublines, compatible with the early origin of marsupials. Amino acid replacements toward the closest enzyme characterized (chicken, form A3) involve no less than 79 positions (36%). At the active site, as deduced from comparisons with the known tertiary structure of the corresponding human enzyme, over half of the residues (8 of 15) ascribed to substrate binding interactions are exchanged although the general character of that site is conserved, while only 1 of 11 positions ascribed to interactions with GSH is exchanged. Class variability and species variability appear to coincide, with divergent segments centering around positions 33-49, 103-130 and 205-222. The pattern is reminiscent of that in similarly multiple MDR alcohol dehydrogenases. Both these enzyme families involved in cellular defense reactions have diverged considerably.