Reactivity of hypotaurine and cysteine sulfinic acid towards carbonate radical anion and nitrogen dioxide as explored by the peroxidise activity of Cu,Zn-superoxide dismutase and by pulse radiolysis

oxidative agents that may be present in biological systems. In this work, the relevance of both the carbonate anion and nitrogen dioxide radicalsin the oxidation of hypotaurine and cysteine sulfinic acid has been explored by the peroxidase activity of Cu,Zn superoxide dismutase(SOD) and by pulse radiolysis. The extent of sulfinate oxidation induced by the system SOD/H2O2 in the presence of bicarbonate (CO3o?generation), or nitrite (oNO2 generation) has been evaluated. Hypotaurine is efficiently oxidized by the carbonate radical anion generated bythe peroxidase activity of Cu,Zn SOD. Pulse radiolysis studies have shown that the carbonate radical anion reacts with hypotaurine morerapidly (k ? 1.1 ? 109 M?1s?1) than nitrogen dioxide (k ? 1.6 ? 107 M?1s?1). Regarding cysteine sulfinic acid, it is less reactive with thecarbonate radical anion (k ? 5.5 ? 107 M?1s?1) than hypotaurine. It has also been observed that the one-electron transfer oxidation of bothsulfinates by the radicals is accompanied by the generation of transient sulfonyl radicals (RSO2o). Considering that the carbonate radicalanion could be formed in vivo at high level from bicarbonate, this radical can be included in the oxidants capable of performing the lastmetabolic step of taurine biosynthesis. Moreover, the protective effect exerted by hypotaurine and cysteine sulfinate on the carbonate radicalanion-mediated tyrosine dimerization indicates that both sulfinates have scavenging activity towards the carbonate radical anion. However,the formation of transient reactive intermediates during sulfinate oxidation by carbonate anion and nitrogen dioxide radical may at the sametime promote oxidative reactions.