Alanine transaminase (ALT) is a transaminase enzyme (). It is also called alanine aminotransferase (ALT or ALAT) and was formerly called serum glutamate-pyruvate transaminase or serum glutamic-pyruvic transaminase (SGPT) and was first characterized in the mid-1950s by Arthur Karmen and colleagues. ALT is found in plasma and in various body tissues but is most common in the liver. It catalyzes the two parts of the alanine cycle. Serum ALT level, serum AST (aspartate transaminase) level, and their ratio (AST/ALT ratio) are commonly measured clinically as biomarkers for liver health. The tests are part of blood panels. The half-life of ALT in the circulation approximates 47 hours. Aminotransferase is cleared by sinusoidal cells in the liver. Function ALT catalyzes the transfer of an amino group from L-alanine to α-ketoglutarate, the products of this reversible transamination reaction being pyruvate and L-glutamate. L-alanine + α-ketoglutarate ⇌ pyruvate + L-glutamate ALT (and all aminotransferases) require the coenzyme pyridoxal phosphate, which is converted into pyridoxamine in the first phase of the reaction, when an amino acid is converted into a keto acid. Clinical significance ALT is commonly measured clinically as part of liver function tests and is a component of the AST/ALT ratio. When used in diagnostics, it is almost always measured in international units/liter (IU/L) or µkat. While sources vary on specific reference range values for patients, 0-40 IU/L is the standard reference range for experimental studies. Elevated levels Test results should always be interpreted using the reference range from the laboratory that produced the result. However typical reference intervals for ALT are: Significantly elevated levels of ALT (SGPT) often suggest the existence of other medical problems such as viral hepatitis, diabetes, congestive heart failure, liver damage, bile duct problems, infectious mononucleosis, or myopathy, so ALT is commonly used as a way of screening for liver problems. Elevated ALT may also be caused by dietary choline deficiency. However, elevated levels of ALT do not automatically mean that medical problems exist. Fluctuation of ALT levels is normal over the course of the day, and they can also increase in response to strenuous physical exercise. When elevated ALT levels are found in the blood, the possible underlying causes can be further narrowed down by measuring other enzymes. For example, elevated ALT levels due to hepatocyte damage can be distinguished from bile duct problems by measuring alkaline phosphatase. Also, myopathy-related elevations in ALT should be suspected when