– Alanine aminotransferase (ALT), also known as glutamate pyruvate transaminase (GPT), is a pyridoxal enzyme that belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family, Alanine aminotransferase subfamily. Gpt / Gpt1 / ALT catalyzes 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. Gpt / Gpt1 / ALT is expressed in the liver, kidney, heart, and skeletal muscles and it expresses at moderate levels in the adipose tissue. 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/GPT is widely distributed and mainly expressed in the intestine, liver, fat tissues, colon, muscle, and heart, in the order of high to low expression level. Serum activity levels of this enzyme are routinely used as a biomarker of liver injury caused by drug toxicity, infection, alcohol, and steatosis.