Glutathione S-transferases of the bovine retina. Evidence that glutathione peroxidase activity is the result of glutathione S-transferase

We have purified two isoenzymes of glutathione S-transferase from bovine retina to apparent homogeneity through a combination of gel-filtration chromatography, affinity chromatography and isoelectric focusing. The more anionic (pI = 6.34) and less anionic (pI = 6.87) isoenzymes were comparable with respect to kinetic and structural parameters. The Km for both substrates, reduced glutathione and 1-chloro-2,4-dinitrobenzene, bilirubin inhibition of glutathione conjugation to 1-chloro-2,4-dinitrobenzene, 1-chloro-2,4-dinitrobenzene inactivation of enzyme activity and molecular weight were similar. However, pH optimum and energy of activation were found to differ considerably. Retina was found to have no selenium-dependent glutathione peroxidase activity. The total glutathione peroxidase activity fractionated with the transferases in the gel-filtration range of mol.wt. 49000 and expressed activity with only organic hydroperoxides as substrate. Only the more anionic isoenzyme expressed both transferase and peroxidase activity.