THE ROLE OF ALPHA-CLASS GLUTATHIONE S-TRANSFERASES AGAINST OXIDATIVE STRESS

Cholesterol 7α- and 7β-hydroperoxides are mutagenic and cytotoxic steroids existing as minor components in rat skin and regarded as good aging markers in the rat. Rat liver and skin cytosols had high and very low activities of the reduction of cholesterol 7-hydroperoxides to the corresponding steroidal alcohols, respectively, in the presence of GSH. The GSH-dependent reduction of the steroid hydroperoxides in rat liver was catalyzed mainly by GSTs with the subunit A1(2) of the Alpha class, but not catalyzed by any other isoform of GST existing in rat liver. Se-containing GSH peroxidase (Se-GSH Px) also played a partial role, to a lesser extent than rat (r) GSTA1-3, in the GSH-dependent reduction of the hydroperoxides in rat liver cytosol. However, rat skin cytosol contained no detectable level of rGSTsAI-1(2) and Al-3, but contained a very low level of Se-GSH Px. The above facts strongly suggest that the age-dependent linear increase in the dermal levels of the cholesterol hydroperoxides as the aging markers in the rat is mainly responsible for the absence of rGSTs bearing the subunit A1(2) and for the very low level of Se-GSH Px in the skin. gpGSTA1-1 existed at a concentration of 1% of cytosolic protein in the liver of guinea pigs, which are well known to lack Se-GSH Px in their tissues. A comparative study using rGSTA1-2 and Theta-class rGSTT2-2 indicated gpGSTA1-1 to have the highest GSH Px activity toward cumene hydroperoxide of all known mammalian GSTs and also to have Px activity toward polyunsaturated fatty acid hydroperoxides (PUFAOOHs) comparable to rGSTT2-2, with the highest activity toward PUFA-OOHs of rGSTs. No detectable level of the GSTA4-4 ortholog highly active toward 4-hydroxy-2(E)-nonenal (4-HNE) was found in the liver of guinea pigs. All gpGST isoforms isolated from hepatic cytosol showed modest GSH-conjugating activity toward 4-HNE.