source：A.Sugiyama et al. J Vet Med Sci. 2009 Jul;71(7):885-90.
The liver is an important organ, comparable to a chemical factory, that plays various roles, such as energy production and detoxification of harmful substances. Owing to its special attributes, the liver often produces harmful active oxygen. The liver protect itself from active oxygen through the immune system and enzymes in the body, but this defensive function declines with deterioration of liver function, due to fatty liver, aging, excessive eating and drinking, and stress. AST and ALT are hepatocyte enzymes, and their blood levels increase when hepatocytes are destroyed, thus serving as indicators of liver damage.