Purification and characterization of a novel glutathione transferase from Serratia marcescens

Four forms of glutathione transferase were resolved from the cytosol of Serratia marcescens CIP 6755 by GSH-affinity chromatography followed by isoelectric focusing. The major isoenzyme, named Sm-GST-7.3, is composed of two subunits each with a molecular mass of 22 kDa and has an isoelectric point at pH 7.3. Sm-GST-7.3, appears to be distinct from Pm-GST-6.0, previously characterized from Proteus mirabilis AF 2924 as indicated by its substrate specificity, immunological reactivity, subunit molecular mass as well as by its N-terminal amino acid sequence. None of the antisera raised against a number of human, rat and mouse GSTs cross-reacted with Sm-GST-7.3 indicating major structural differences between them and bacterial GST. This is further supported by the fact that the N-terminal sequence of Sm-GST-7.3 also differs significantly from the known sequences of mammalian GSTs of alpha, mu and pi classes. In addition, comparison with the known N-terminal amino acid sequences of helminth, plant and insect GSTs demonstrate that the latter enzymes are distantly related (less than 25% identity) to the Sm-GST-7.3. Immunoblotting experiments performed with antisera raised against Sm-GST-7.3 indicate that a GST immunologically identical to Sm-GST-7.3 is present in a number of other bacterial strains. All together the results obtained suggest that Sm-GST-7.3 is distinct from any known GST, including microbial and mammalian GSTs.