Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are enzymes required for the mutual transformation of sugars and proteins in the body and are widely distributed in the body The distribution of ALT is highest in the liver, followed by the kidney, heart, skeletal muscle, and spleen, etc. The distribution of AST is highest in the heart muscle, followed by the liver, skeletal muscle, and kidney, etc. ALT is mainly found in hepatocyte plasma and AST is found in hepatocyte plasma and mitochondria. Normal cells do not release ALT and AST into the blood due to the encapsulation of the cell membrane. When hepatocytes are damaged, the cells become degenerated and necrotic, the cell membrane is broken or the permeability of the cell membrane increases, and the ALT and AST contained in hepatocytes are released into the blood, causing an increase in ALT and AST activity in the blood. The total activity of intrahepatic ALT is more than one thousand times that of serum, and as long as 1% of hepatocytes are damaged, ALT in blood can be significantly increased, which is responsive to liver damage and is a commonly used clinical indicator at present. The normal value of serum alanine aminotransferase is: 5-40 units/liter. The normal value of serum aspartate aminotransferase enzyme is: 5-40 units/liter. Hepatocellular damage from any cause can cause elevated serum aminotransferases. Acute viral hepatitis has a significant increase in ALT before clinical symptoms appear at the end of latent phase. It can be used as one of the tools for early diagnosis of acute viral hepatitis. In acute non-jaundiced hepatitis, serum aminotransferases may be increased from double to several dozen times normal. In most patients, serum aminotransferases return to normal about one month after the onset of the disease, but in a few patients they may persist for a longer period of time, but usually not more than six months. If the enzyme activity decreases for a long time, it indicates that the lesion is still active and tends to be chronic. In chronic hepatitis, normal enzyme activity depends on the degree of activity of the lesion. When the lesion is quiescent, the enzyme activity is normal; when it is mildly active, the enzyme activity increases singly, with intermittent transient fluctuations; when it is significantly active, the enzyme activity increases more significantly and lasts longer. It is often accompanied by changes in protein metabolism. In cirrhosis, the enzyme activity is similar to that of chronic hepatitis, with normal enzyme activity at rest and elevated activity. The ratio of AST/ALT is often used to reflect the damage to the liver cells; ALT is located in the hepatocyte plasma, while AST is located in the hepatocyte plasma and mitochondria, and the AST/ALT ratio in normal individuals is about 1.15, which means that AST is slightly higher than ALT. When the hepatocytes are mildly diseased, only the enzyme in the hepatocyte plasma is released, and the rise of A LT is greater than that of AST. For example, in the early stage of acute hepatitis, the AST/ALT ratio can fall to about 0.56, and in the recovery period of hepatitis, the ratio gradually rises to normal. When hepatocytes are severely damaged, enzymes in the cytoplasm and mitochondria are released into the blood, resulting in a greater increase in serum AST than ALT. For example, in cirrhosis, the ratio can increase to 1.44, and in chronic active hepatitis, the ratio is often higher than normal. In conclusion, an AST/ALT ratio of <1 in liver disease often indicates mild liver damage, while an AST/ALT ratio of >1 indicates severe liver damage. However, alcohol has a specific damage to mitochondria, and more than 90% of acute alcoholic hepatitis cases have an AST/ALT ratio >1, which is an exception.