Enzymatic nitrate assay by a kinetic method employing Escherichia coli nitrate reductase

An enzymatic assay system for nitrate employing the membrane-bound nitrate reductase (EC 1.7.99.4) of E. coli is described. Contrary to previous enzymatic assay systems, the present method is a kinetic one, i.e. the substrate, nitrate, is assayed by measuring the reaction rate of the nitrate reductase-catalyzed reaction. Based on the observation that the nitrate reductase-catalyzed reaction obeys pseudo-first order kinetics, a test system is described allowing the assay of nitrate at a concentration as low as 1 ppm. The relatively high Michaelis-Menten constant for nitrate (0.3 mM) of the E. coli nitrate reductase favours nitrate assay by the kinetic method.