Alanine aminotransferase (Alt, L-alanine:2-oxoglutalate aminotransferase) is a pyridoxal enzyme which catalyses the reversible interconversion of L-alanine and 2-oxoglutalate to pyruvate and L-glutamate. The enzyme is widely distributed in various tissues from animals and even in some kind of plants. Isoenzymes of human ALT localize in the cytosol (c-ALT) and mitochondria (m-ALT) of tissues such as liver, kidney, skeletal and cardiac muscles. Amino acid sequence of c-ALT from rat and human liver has been wholly determined by Ishiguro et al. It is suggested that c-ALT is associated to the utilization of pyruvate in glycolysis and m-ALT is involved in the conversion of alanine to pyruvate for gluconeogenesis.