ABSTRACT
Peroxides are chemical compounds in which two oxygen atoms are linked together by a single covalent bond. Hydrogen peroxide (H2O2), the simplest peroxide, is formed through different routes in biological systems: superoxide radical (O2•-) dismutation, which can be spontaneous, or, depending on the organism and cellular compartment, catalyzed by different superoxide dismutases (SODs); one-electron O2•- reduction, such as during aconitase oxidation; or direct two-electron reduction of oxygen, which can be catalyzed by the divalent activity of several oxidases, including xanthine oxidase, Ero1, aldehyde oxidase, and monoamine oxidase. Depending on the cell type, those sites could be mitochondria, phagosomes of inflammatory cells, as well as the extracellular space. In biological systems, thiol functional groups can be found as part of low-molecular-weight (LMW) compounds and in protein Cys residues. Most living organisms contain millimolar concentrations of LMW thiols that play biological functions, such as to keep an intracellular reducing environment, to provide electrons for redox enzymes, and to react with electrophilic compounds.
