Under physiological conditions, the intracellular NADH concentration is very low and the enzyme activity is almost completely lost; however, when the enzyme activity is measured in a test tube, the enzyme activity can be measured completely by adding NADH equivalent to tens or even hundreds of times of the physiological concentration. If the enzyme activity is measured at low substrate concentrations, its instantaneous initial velocity must be recorded with a time scan, otherwise the error is too large. Other inherited enzyme deficiency diseases have similar problems. NADH-MetHb reductase activity was measured using cyanogenic methemoglobin as a substrate, or dichlorophenol indophenol as a substrate for yellow delivery enzyme activity, or cytochrome b5 as a substrate for b5R activity. It should be emphasized that because the mechanism of action of different genetic variants is different, the results of the assay in a test tube (at high substrate concentration) do not exactly reflect the degree of reduced catalytic efficiency in living cells (at low substrate concentration). In acute pancreatitis, serum and urinary amylase activity is significantly elevated; in hepatitis and other causes of liver damage, hepatocyte necrosis or increased permeability, a large amount of transaminase is released into the blood, raising serum transaminase; in myocardial infarction, serum lactate dehydrogenase and phosphocreatine kinase are significantly elevated; when organophosphorus pesticide poisoning is present, cholinesterase activity is inhibited and serum cholinesterase activity is decreased; in certain hepatobiliary diseases, especially In some hepatobiliary diseases, especially biliary obstruction, serum r-glutamyl transferase is increased, etc. In cellular aging, there are a series of changes such as water loss, accumulation of age pigment, lipofuscin, reduced enzyme activity, and slower metabolic rate.