HO₂•: 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.

Paraquat-Induced Hair Cell Damage and Protection with the Superoxide Dismutase Mimetic M40403

“…A more recent explanation for this differential vulnerability is that more vulnerable hair cells (base vs. apex, OHC vs. IHC) have less glutathione, an antioxidant capable of preventing the formation of the toxic hydroxyl radical [Sha et al, 2001]. Since paraquat did not preferentially damage OHC over IHC, the paraquat-induced damage observed here may arise from the transformation of superoxide into other ROS such as peroxynitrite or hydroperoxyl [De Grey, 2002;McFadden et al, 2001;Mollace et al, 2003].…”

Free Radicals: Properties, Sources, Targets, and Their Implication in Various Diseases

“…The simplest form of peroxyl radical is perhydroxyl radical (HOO • ) which is formed by the protonation of superoxide [30]. About 0.3 % of the total O 2 •-in the cytosol of a typical cell is in the protonated form.…”

Diabetes Mellitus: Role of Free Radicals and Oxidative Stress

“…[76] Other reactive radicals which are produced by oxygen in living system is peroxyl radicals and HOO* is the most simple form of peroxyl radical, called hydroperoxyl radical. [77,78] …”