Comparison of the redox chemistry of sulfur- and selenium-containing analogs of uracil.

Selenium is present in proteins in the form of selenocysteine, where this amino acid serves catalytic oxidoreductase functions. The use of selenocysteine in nature is strongly associated with redox catalysis. However, selenium is also found in a 2-selenouridine moiety at the wobble position of tRNAGlu , tRNAGln and tRNALys . It is thought that the modifications of the wobble position of the tRNA improves the selectivity of the codon-anticodon pair as a result of the physico-chemical changes that result from substitution of sulfur and selenium for oxygen. Both selenocysteine and 2-selenouridine have widespread analogs, cysteine and thiouridine, where sulfur is used instead. To examine the role of selenium in 2-selenouridine, we comparatively analyzed the oxidation reactions of sulfur-containing 2-thiouracil-5-carboxylic acid (s2 c5 Ura) and its selenium analog 2-selenouracil-5-carboxylic acid (se2 c5 Ura) using1 H-NMR spectroscopy,77 Se-NMR spectroscopy, and liquid chromatography-mass spectrometry. Treatment of s2 c5 Ura with hydrogen peroxide led to oxidized intermediates, followed by irreversible desulfurization to form uracil-5-carboxylic acid (c5 Ura). In contrast, se2 c5 Ura oxidation resulted in a diselenide intermediate, followed by conversion to the seleninic acid, both of which could be readily reduced by ascorbate and glutathione. Glutathione and ascorbate only minimally prevented desulfurization of s2 c5 Ura, whereas very little deselenization of se2 c5 Ura occurred in the presence of the same antioxidants. In addition, se2 c5 Ura but not s2 c5 Ura showed glutathione peroxidase activity, further suggesting that oxidation of se2 c5 Ura is readily reversible, while oxidation of s2 c5 Ura is not. The results of the study of these model nucleobases suggest that the use of 2-selenouridine is related to resistance to oxidative inactivation that otherwise characterizes 2-thiouridine. As the use of selenocysteine in proteins also confers resistance to oxidation, our findings suggest a common mechanism for the use of selenium in biology.