Aspartate transaminase is a liver enzyme used by the body to metabolize amino acids. Unlike the other major liver enzyme, ALT, AST is found outside the liver in the heart, kidneys, brain, red blood cells and muscles in significant quantities. As such, testing for it in other parts of the body outside the liver is a good indicator of liver health. As such, comparing the ALT and AST levels of a normal patient to a sick patient will give the physican an indication as to whether a disorder is limited to the liver or is affecting multiple organs in addition to the liver.