Purification and properties of a recombinant sulfur analog of murine selenium-glutathione peroxidase.

We previously constructed plasmids for synthesis of glutathione-peroxidase (GPx) mutants in an Escherichia coli expression system. In these recombinant proteins either cysteine (Cys]GPx mutant) or serine (Ser]GPx mutant) were present in place of the active-site selenocysteine (SeCys) of the natural enzyme. We have now investigated GPx activity of Cys]GPx and Ser]GPx mutants. Enzyme assays performed on preparations of these partially purified proteins demonstrated that the Cys]GPx mutant exhibited a significant GPx activity, unlike the Ser]GPx mutant. Purification of Cys]GPx was performed in two steps of ion-exchange chromatography giving a 98% homogenous protein in 50% yield. The purified Cys]GPx protein was shown to be a symmetrical tetramer by the means of gel-filtration HPLC and SDS/PAGE. Two isoelectric points were found (6.8 and 7.2) which may reflect two different oxidation states of the mutant protein. The GPx activity of the Cys]GPx mutant was optimal at pH 8.5. The Cys]GPx mutant had a specific activity approximately 1000-fold smaller than that of the natural enzyme, and was very easily inactivated by hydroperoxides. Inhibition of the activity with iodoacetate determined a pKa of 8.3, presumably that of the active-site cysteine. Unlike that of SeGPx, the GPx activity of Cys]GPx was only slightly inhibited by mercaptosuccinate. We discuss hypothetical mechanistic constraints of either catalytic cycle, which may explain such results.