Efficient depletion of ascorbate by amino acid and protein radicals under oxidative stress

Ascorbate levels decrease in organisms subjected to oxidative stress, but the responsible reactions have not been identified. Our earlier studies have shown that protein C-centered radicals react rapidly with ascorbate. In aerobes, these radicals can react with oxygen to form peroxyl radicals. To estimate the relative probabilities of the reactions of ascorbate with protein C- and O-centered radicals, we measured by pulse radiolysis the rate constants of the reactions of C-centered radicals in Gly, Ala, and Pro with O2 and of the resultant peroxyl radicals with ascorbate. Calculations based on the concentrations of ascorbate and oxygen in human tissues show that the relative probabilities of reactions of the C-centered amino acid radicals with O2 and ascorbate vary between 1:2.6 for the pituitary gland and 1:0.02 for plasma, with intermediate ratios for other tissues. The high frequency of occurrence of Gly, Ala, and Pro in proteins and the similar reaction rate constants of their C-centered radicals with O2 and their peroxo-radicals with ascorbate suggest that our results are also valid for proteins. Thus, the formation of protein C- or O-centered radicals in vivo can account for the loss of ascorbate in organisms under oxidative stress.

Graphical AbstractFormation and reactions of protein radicals in vivo. Boxes marked ⁎ show protein repair, and those marked ⁎⁎ show protein damageFigure optionsDownload full-size imageDownload high-quality image (128 K)Download as PowerPoint slideHighlights► Ascorbate levels are reduced by oxidative stress. ► The mechanism of the loss is not understood. ► Ascorbate is rapidly oxidized by amino acid peroxyl radicals. ► Amino acid and protein peroxyl radicals may be the oxidants of ascorbate in vivo.