Protection against Peroxynitrite by Selenoproteins

Abstract Cellular defense against excessive peroxynitrite generation is required to protect against DNA strand-breaks and mutations and against interference with protein tyrosine-based sig naling and other protein functions due to formation of 3-nitrotyrosine. We recently demon strated a role of selenium-containing enzymes catalyzing peroxynitrite reduction. Glutathione peroxidase (GPx) protected against the oxidation of dihydrorhodamine 123 (D H R) by perox ynitrite more effectively than ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one), a selenoor-ganic compound exhibiting a high second-order rate constant for the reaction with peroxynit rite, 2 × 106 M-1 S-1. The maintenance of protection by GPx against peroxynitrite requires GSH as reductant. Similarly, selenomethionine but not selenomethionine oxide exhibited inhibition of rhodamine 123 formation from DHR caused by peroxynitrite. In steady-state experiments, in which peroxynitrite was infused to maintain a 0.2 μᴍ con centration, GPx in the presence of GSH, but neither GPx nor GSH alone, effectively inhib ited the hydroxylation of benzoate by peroxynitrite. Under these steady-state conditions peroxynitrite did not cause loss of ‘classical’ GPx activity. GPx, like selenomethionine, pro tected against protein 3-nitrotyrosine formation in human fibroblast lysates, shown in Western blots. The formation of nitrite rather than nitrate from peroxynitrite was enhanced by GPx , ebselen or selenomethionine. The selenoxides can be effectively reduced by glutathione, establishing a biological line of defense against peroxynitrite. The novel function of GPx as a peroxynitrite reductase may extend to other selenoproteins containing selenocysteine or selenomethionine. Recent work on organotellurium compounds revealed peroxynitrite reductase activity as well. Inhibition of dihydrorhodamine 123 oxidation correlated well with the GPx-like activity of a variety of diaryl tellurides.