Mammalian class Sigma glutathione S-transferases: catalytic properties andtissue-specific expression of human and rat GSH-dependent prostaglandin D-2 synthases

GSH-dependent prostaglandin D-2 synthase (PGDS) enzymes represent the only vertebrate members of class Sigma glutathione S-transferases (GSTs) identif ied to date. Complementary DNA clones encoding the orthologous human and ra t GSH-dependent PGDS (hPGDS and rPGDS, respectively) have been expressed in Escherichia coli, and the recombinant proteins isolated by affinity chroma tography. The purified enzymes were both shown to catalyse specifically the isomerization of prostaglandin (PG) H-2 to PGD(2). Each transferase also e xhibited GSH-conjugating and GSH-peroxidase activities. The ability of hPGD S to catalyse the conjugation of aryl halides and isothiocyanates with GSH was found to be less than that of the rat enzyme. Whilst there is no differ ence between the enzymes with respect to their K-m. values for 1-chloro-2,4 -dinitrobenzene, marked differences were found to exist with respect to the ir K-m for GSH (8 mM versus 0.3 mM for hPGDS and rPGDS, respectively). Usin g molecular modelling techniques, amino acid substitutions have been identi fied in the N-terminal domain of these enzymes that lie outside the propose d GSH-binding site, which may explain these catalytic differences. The tiss ue-specific expression of PGDS also varies significantly between human and rat; amongst the tissues examined, variation in expression between the two species was most apparent in spleen and bone marrow. Differences in catalyt ic properties and tissue-specific expression of hPGDS and rPGDS appears to reflect distinct physiological roles for class Sigma GST between species. T he evolution of divergent functions for the hPGDS and rPGDS is discussed in the context of the orthologous enzyme from chicken.