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Nitric Oxide Determinations: Much Ado About NO^•-Thing?

Groningen Institute for Drug Studies, Department of Medicine, Division of Gastroenterology and Hepatology, University Hospital Groningen, PO Box 30.001, NL-9700 RB Groningen, The Netherlands, Fax +31-50-3614756, E-mail h.moshage@med.rug.nl

The nitric oxide radical (NO^•) is an important mediator of both physiological and pathophysiological processes. NO^• is produced by nitric oxide synthase (NOS; EC 1.14.13.39), an enzyme that exists in three isoforms encoded by distinct genes (1)(2). All isoforms of NOS catalyze the conversion of L-arginine into citrulline and NO^•. In this reaction, which requires oxygen and NADPH, a guanidino-nitrogen atom of L-arginine is incorporated into NO^•. Neuronal NOS (type I, nNOS) and endothelial (type III, eNOS) are Ca²⁺- and calmodulin-dependent constitutive NOS isoforms. nNOS has a function in neurotransmission. NO^• produced by eNOS is identical to endothelium-derived relaxing factor and is the principal signal for relaxation of vascular smooth muscle cells. In addition, NO^• produced by the endothelium has antithrombotic actions. Thus, eNOS and nNOS isoforms have important functions under normal conditions. They are present intracellularly, are rapidly activated by intracellular Ca²⁺ fluxes, and produce small quantities of NO^•.

Inducible NOS (iNOS, type II) is not expressed under normal conditions. iNOS is induced by cytokines and (or) endotoxin during inflammatory and infectious processes and produces abundant amounts of NO^• for extended periods. iNOS can be induced in many cell types, including hepatocytes, macrophages, neutrophils, smooth muscle cells, and chondrocytes. Induction of iNOS requires de novo protein synthesis. NO^• produced by iNOS has antimicrobial activity and may be involved in killing tumor cells. As such, it is part of the nonspecific host defense system. Increased expression of iNOS has been demonstrated in a wide range of disorders, including sepsis, asthma, rheumatoid arthritis, atherosclerotic lesions, tuberculosis, inflammatory bowel disease, Helicobacter pylori-induced gastritis, allograft rejection, Alzheimer disease, and multiple sclerosis (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(. . . [Full Text of this Article]

methods for determining no^• and metabolites

"Real-time" NO^•determination.
Determination of stable end-products nitrite/nitrate.
Incorporation of stable heavy nitrogen isotopes into nitrite and nitrate.
Determination of NOS activity.
Spectroscopy: electron paramagnetic resonance (EPR) and near-infrared.

meaning and value of nitrite and nitrate determinations

beyond no^•

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