GPT/ALT

Alanine aminotransferase (ALT), also known as glutamate-pyruvate transaminase (GPT), is a pyridoxal phosphate-dependent aminotransferase that catalyzes the reversible transfer of an amino group between L-alanine and α-ketoglutarate to generate pyruvate and glutamate, thereby linking amino acid metabolism with glucose metabolism^[1][2]. ALT plays a central role in intermediary metabolism and hepatic gluconeogenesis, making it an important regulator of carbon and nitrogen flux between tissues^[1][2]. Two distinct isoforms, ALT1 (GPT1) and ALT2 (GPT2), are encoded by separate genes and exhibit different cellular and tissue distributions despite catalyzing the same transamination reaction^[1][3][2]. Mechanistically, ALT1 is predominantly localized in the cytosol, whereas ALT2 is mainly associated with mitochondria, indicating nonredundant metabolic functions in cellular amino acid handling and energy metabolism^[1][3][2]. In disease settings, circulating ALT activity is widely used as a biomarker of hepatocellular injury, liver disease, and hepatotoxicity because liver tissue contains abundant ALT expression^[3]. Compared with ALT1, ALT2 has been linked more closely to metabolic adaptations associated with obesity and altered gluconeogenic amino acid utilization, and hepatic GPT2 expression decreases following substantial weight-loss-induced metabolic improvement^[4]. Therefore, isoform-specific analysis of ALT1 and ALT2 provides a more refined framework for investigating liver metabolism, metabolic disease mechanisms, and tissue-specific sources of aminotransferase activity^[3][4].