Nitrate reductase (NR) catalyzes the reduction of nitrate to nitrite according to the following reaction:
NO₃⁻ + NADH + H⁺ → NO₂⁻ + NAD⁺ + H₂O
NADH exhibits a characteristic absorption peak at 340 nm. Therefore, the activity of nitrate reductase can be quantitatively determined by measuring the change in absorbance at this wavelength.