The activity (reaction rate) of alanine transferase is usually measured by including an excess of pure lactate dehydrogenase and NADH in the reaction system. The rate of alanine disappearance is equal to the rate of NADH disappearance, which can be measured spectrophotometrically ("color change assay"). Explain how this assay works; i.e. why lactate dehydrogenase and NADH are added in excess and why the rate of alanine disappearance is equal to the rate of NADH disappearance.