The muscle enzyme lactate dehydrogenase catalyzes the reaction

NADH and NAD⁺ are the reduced and oxidized forms, respectively, of the coenzyme NAD. Solutions of NADH, but not NAD⁺ , absorb light at 340 nm. This property is used to determine the concentration of NADH in solution by measuring spectrophotometrically the amount of light absorbed at 340 nm by the solution. Explain how these properties of NADH can be used to design a quantitative assay for lactate dehydrogenase