AST is widely distributed throughout the tissue with significant amounts being in the heart and liver. Lesser amounts are also found in skeletal muscles, kidneys, pancreas, spleen, lungs and brain. Injury to these tissues results in the release of the AST enzyme to the general circulation. In myocardial infarction, serum AST may begin to rise within 6-8 hours after onset, peak within two days and return to normal by the fourth or fifth day post infarction. An increase in serum AST is also found with hepatitis, liver necrosis, cirrhosis and liver metastasis.