NITRIC-OXIDE - A PROTECTIVE OR PATHOGENIC MOLECULE

Nitric oxide is a labile molecule involved in a diverse range of biolo gical functions including host defence against infection by pathogens. Oxidative deamination of the amino acid L-arginine, catalysed by a gr oup of enzymes called nitric oxide synthases, results in the formation of L-citrulline and nitric oxide. A large number of pathogens, includ ing bacteria, yeasts and multicellular parasites are susceptible to ni tric oxide-mediated toxicity in vitro suggesting that nitric oxide is potentially an important molecule in host defence. Recent studies in v ivo have demonstrated that infection of a host by micro-organisms can result in the induction of nitric oxide synthase, release of high leve ls of nitric oxide by inflammatory cells and resolution of the infecti on. However, the persistent production of high levels of nitric oxide in chronic disease states, can result in immune suppression, pathologi cal changes and even host death. Whether nitric oxide is predominantly an immunoprotective or host-damaging agent, following infection, may depend on the host-pathogen equilibrium. This review discusses the rol e of nitric oxide both as a protective and pathogenic molecule in dise ase. (C) 1998 Lippincott Williams & Wilkins.