HO 2 •: The Forgotten Radical

Abstract

HO₂•, usually termed either hydroperoxyl radical or perhydroxyl radical, is the protonated form of superoxide; the protonation/deprotonation equilibrium exhibits a pK_a of around 4.8. Consequently, about 0.3% of any superoxide present in the cytosol of a typical cell is in the protonated form. This ratio is rather accurately reflected by the published literature on the two species, as identified by a PubMed search; at the time of writing only 28 articles mention "HO2," "hydroperoxyl" or "perhydroxyl" in their titles, as against 9228 mentioning superoxide. Here it is argued that this correlation is not justifiable: that HO₂•'s biological and biomedical importance far exceeds the attention it has received. Several key observations of recent years are reviewed that can be explained much more economically when the participation of HO₂• is postulated. It is suggested that a more widespread appreciation of the possible role of HO₂• in biological systems would be of considerable benefit to biomedical research.