Determination of Nitrite and Nitrate in Stored Urine

Nitric oxide (NO·) plays an important role both in maintaining normal homeostasis and in the pathogenesis of various disorders (1). NO· has a short biological half-life and is rapidly converted into its stable metabolites, nitrite and nitrate (1)(2). In plasma, nitrite is rapidly oxidized to nitrate (2). Determination of nitrite and nitrate (NOx) in body fluids like plasma and urine is widely used as a marker of NO· production (3). However, bacteria in urine are known to produce nitrite, and leukocytes in urine sediments contain NO synthase activity (4). Therefore, it has been suggested that NOx determinations in urine are reliable only when precautions have been taken to prevent bacterial growth in the urine specimens. Indeed, Smith et al. (4) have shown that incubation of urine samples that contain bacteria leads to increased nitrite concentrations in these samples. Therefore, urine is often collected on ice and/or in the presence of antibiotics or organic solvents (5) …