Transaminases in assessing liver function :
Liver cells contain several enzymes which may be released into the circulation in liver damage. Measurement of selected enzymes in serum is often used to assess the liver function.
It must, however, be noted that there is no single enzyme that is absolutely specific to liver alone. Despite this fact, serum enzymes provide valuable information for LFT.
The activities of two enzymes -
serum glutamate pyruvate transaminase (SGPT; now called alanine transaminase - ALT)
serum glutamate oxaloacetate transaminase (SGOT; now called aspartate transaminase - AST) -
It is widely used to assess the liver function by the Reitman and Frankel method.
Transamination is the term given to the process in which an amino group is transferred from an a amino acid to an α-keto acid.
As a result, different a amino acids and α-keto acids are formed.
All naturally occurring a amino acids can take part in such reactions, each with their specific enzymes.
The enzymes that carry out transamination are called transaminases and their function is to retain amino groups during amino-acid breakdown by transferring the amino group from the amino acid to a keto acid.
They are found in many tissues including heart, liver, kidney, and muscle. Two such important enzymes are glutamate-oxaloacetate transaminase and glutamate-pyruvate transaminase.
SGPT - ALT
SGOT - AST
ALT is a cytoplasmic enzyme while AST is found in both cytoplasm and mitochondria.
The activity of these enzymes is low in normal serum.
Serum ALT and AST are increased in liver damage.
However, alanine trans-aminase is more sensitive and reliable for the assessment of LFT.
The normal AST/ALT ratio is around 0.8.
This ratio is increased (>2) in myocardial infarction, alcoholic hepatitis, and cirrhosis.
AST/ALT ratio is decreased (i.e. ALT higher) in acute hepatocellular damage and cholestasis.