Alanine aminotransferase (ALT), also known as glutamate pyruvate transaminase (Gpt), is a pyridoxal enzyme which catalyses the reversible interconversion of L-alanine and 2-oxoglutalate to pyruvate and L-glutamate, and plays a key role in the intermediary metabolism of glucose and amino acids. As a key enzyme for gluconeogenesis, Gpt is a widely-used serum marker for liver injury. Two ALT isoenzymes have been identified, ALT1 and ALT2 (GPT1 and GPT2), which are encoded by separate genes and share significant sequence homology, but differ in their expression patterns. Gpt1 is widely distributed and mainly expressed in intestine, liver, fat tissues, colon, muscle, and heart, in the order of high to low expression level, whereas Gpt2 expression is more restricted, mainly in liver, muscle, brain, and white adipose tissue. It has been reported that hepatic ALT2 protein is approximately four times higher in male rats than in female rats.