Free radicals in viral pathogenesis: Molecular mechanisms involving superoxide and NO

The importance of free radical molecular species in the pathogenesis of various viral diseases has been increasingly recognized in recent years. Oxygen radicals such as superoxide (O₂^-) and hydroxyl radical (·OH) have been implicated as possible pathogenic molecules in viral disease pathogenesis. Much attention hies been given to another simple inorganic radical [nitric oxide (NO)] in the host's defense mechanism and pathogenesis of virus infection. The NO synthesis pathway, in particular, the inducible isoform of NO synthase (iNOS), is expressed in different viral diseases via induction of proinflammatory cytokines such as interferon-γ. INOS produces an excessive amount of NO for a long time compared with other constitutive isoforms of NOS (i.e., neuronal NOS and endothelial Nos). Recent studies indicate that NO and O₂^- are produced in excess during the host's defense responses against various intruding microbes. Reactive nitrogen oxide species such as peroxynitrite (ONOO^-) and NOx (NO₂ and N₂O₃) are produced in biological systems through the reaction of NO with either O₂^- or O₂. Among these reactive nitrogen species, ONOO^and its biological actions are of considerable interest in that ONOO^- causes oxidation and nitration of amino acid residues of proteins and guanine of DNA, lipid peroxidation, and DNA cleavage. Because the ONOO^- is formed via a diffusion-limited fast reaction of NO and O₂^-, it may be a dominant nitrogen oxide species during the host's defense reactions, when both NO and O₂^- are produced in excess. Thus, understanding the role of NO and oxygen radical generation in virus infections will provide insight into not only viral pathogenesis but also the host-pathogen interaction in microbial infections at a molecular level.