In the presence of adenosine diphosphate (ADP), pyruvate kinase (PK) catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, Lactate dehydrogenase catalyzes the reaction of pyruvic acid with NADH to produce lactic acid and NAD+. The activity of pyruvate kinase can be calculated by detection the absorbance changes at 340 nm.