Circulating and excretory nitrite and nitrate as indicators of nitric oxide synthesis in humans: methods of analysis

Abstract

Nitric oxide (NO) is produced in various cells from l-arginine by the catalytical action of NO synthases (NOS). The main metabolic fate of NO includes oxidation to nitrate by oxyhemoglobin in red blood cells and autoxidation to nitrite. Nitrate and nitrite circulate in blood and are excreted in urine. The concentration of these NO metabolites in plasma, serum, and urine can be used to assess NO synthesis in humans. Circulating nitrite reflects constitutive endothelial NOS activity. Excretory nitrate indicates systemic NO production. Nitrite and nitrate can be measured in plasma, serum, and urine of humans by various analytical methods which are based on different analytical principles. These methods include colorimetry, spectrophotometry, fluorescence, chemiluminescence, gas and liquid chromatography, electrophoresis, and mass spectrometry. The present article gives an overview of the most significant currently used quantitative methods of analysis of nitrite and nitrate in human plasma, serum, and urine in the framework of clinical studies and discusses their significance.