NITRATE DETERMINATION IN BIOLOGICAL-FLUIDS BY AN ENZYMATIC ONE-STEP ASSAY WITH NITRATE REDUCTASE

We report a simple, enzymatic, end point method for determining nitrat e in serum and urine with use of nitrate reductase from Aspergillus sp . (EC 1.6.6.2). The decrease in absorbance at 340 nm as a result of th e oxidation of beta-NADPH is measured. We used FAD as a supplementary electron carrier and added an internal standard to avoid interference from possible inhibitors of the enzymatic reaction. The method was lin ear from 5 to 200 mu mol/L nitrate in serum. Within-run (n = 12) and t otal (n = 14 days) imprecisions (CV) were 5.1-7.7% and 6.2-9.8%, respe ctively, at 20-90 mu mol/L nitrate in serum. Recoveries of added nitra te were 80-106%. The median (range) concentration in serum of 20 healt hy subjects was 16(0-42) mu mol/L.