II. Isozyme specificity of rat liver glutathione S-transferases in the formation of PGF2α and PGE2 from PGH2☆

Abstract When prostaglandin H 2 (PGH 2 ) was incubated with a mixture of glutathione S -transferases (GSTs) obtained from S -hexylglutathione affinity chromatography, as much as 40% of it was transformed into a prostanoid whose R f value corresponded to that of the standard PGF 2α . The reaction product was identified as PGF 2α by cochromatography with a standard on TLC and HPLC. The stereochemistry of the hydroxyl groups on C-9 and C-11 of the cyclopentane ring was confirmed by mass-spectral analysis of the butylboronate derivative of the reaction product. Neither PGE 2 nor PGD 2 could substitute for PGH 2 in the reaction mixture, indicating that the mechanism of formation of PGF 2α is a direct two-electron reduction of the endoperoxide moiety and not through a reduction of the keto group on PGE 2 or PGD 2 . Individual GST isozymes exhibited distinct differences in their catalytic rates of formation of PGF 2α from PGH 2 . Among various GSTs, isozyme IV, a homodimer of Ya size subunit showed the highest activity with a V max value of approximately 6000 nmol · min −1 · mg −1 . In general, the isozymes containing Ya and Yc subunits exhibited relatively high activity toward PGH 2 , indicating that it is the non-selenium-dependent glutathione peroxidase activity associated with the GSTs that might be responsible for the reduction of PGH 2 to PGF 2α . Interestingly, isozyme IV also exhibited the highest PGE 2 forming activity with a V max value of ~3000 nmol · min −1 · mg −1 followed by isozyme I, a homodimer of Yb subunit, which had a V max value of 420 nmol · min −1 · mg −1 . Based on these results, it appears that the GSTs play an important role in the biosynthesis of classical PGs. Therefore, it is conceivable that the tissue-specific formation of PGF 2α and PGE 2 might, in part, be due to the relative distribution of these enzyme activities in a given tissue. Our results have not only confirmed the previously published reports (E. Christ-Hazelhof et al. (1976) Biochim. Biophys. Acta 450 , 450–461), but also have characterized the specificity of GST isozymes in the formation of PGF 2α .