Spiroiminodihydantoin and Guanidinohydantoin Are the Dominant Products of 8-Oxoguanosine Oxidation at Low Fluxes of Peroxynitrite: Mechanistic Studies with 18O

Abstract

Peroxynitrite-mediated oxidation of 8-oxoguanosine results in the formation of two product classes distinguished by the source of their incorporated oxygen atoms. The first product class consists of dehydroguanidinohydantoin (DGh), N-nitro-dehydroguanidinohydantoin (NO2-DGh), and 2,4,6-trioxo[1,3,5]triazinane-1-carboxamidine (CAC) with peroxynitrite as the exogenous O atom source, and the second includes spiroiminodihydantoin (Sp), guanidinohydantoin (Gh), and 4-hydroxy-2,5-dioxo-imidazolidine-4-carboxylic acid (HICA), with water serving as the exogenous O atom source. The first product class forms exclusively at high peroxynitrite fluxes, while the second forms exclusively at limiting peroxynitrite fluxes. At intermediate peroxynitrite fluxes, both sets of products are formed. At high fluxes, DGh was the major reaction product, and after several of the peroxynitrite-derived radicals were eliminated as the exogenous O atom source, the peroxynitrite anion emerged as the most likely candidate. On the other hand, at lower fluxes, either Gh or Sp was the major product, depending on the pH of the reaction mixture. At low and high pH, respectively, Gh and Sp were the major products, and the plot of pH vs ratio of Sp/(Sp+Gh) had an inflection at pH 5.8. Interestingly, the pH dependence for oxidation of 8-oxoGuo with CoCl2 and KHSO5 was identical to that for oxidation by peroxynitrite, indicating that the phenomenon arises due to characteristics of an 8-oxoGuo-derived rather than an oxidant-derived intermediate, since these two systems generate different reactive species. On the basis of these findings, a model in which 8-oxoGuo is oxidized to the bisimine intermediate, 1 is proposed. At high peroxynitrite fluxes, the reaction of 1 with ONOO- predominates over the reaction with H2O, leading exclusively to DGh, NO2-DGh, and CAC, while at limiting peroxynitrite concentrations, the reaction with H2O dominates, and Gh and Sp are formed exclusively. At intermediate peroxynitrite fluxes, the relative kinetics of the reaction between 1 and ONOO- or H2O are such that both product classes are formed. To explain the pH-dependent Gh and Sp yields, we propose that 5 has a pKa approximately 5.8 and that the differential reactivity of the protonated and deprotonated form of 5 leads to its partitioning into Gh and Sp, respectively.