Liver Enzymes
The assay of serum enzymes is very useful for the differential diagnosis and monitoring of various liver disorders. Liver enzymes acts as marker of hepatocellular damage, cholestasis and disturbances in the hepatocellular synthesis.
Markers of Hepatocellular Damage
In case of hepatocellular damage, the enzymes which are normally present inside the hepatocytes are released into the blood. Aminotransaminases such as aspartate transaminase (AST) and alanine transaminase (ALT) are routinely used in diagnosis of hepatocellular damages. Transaminases are enzymes involved in the transfer of an amino group from a 2-amino- to a 2-oxoacid. The 2- oxoglutarate acts as amino group acceptor and the L-glutamate serves as donor in all amino-transfer reactions. The specificity of the individual enzymes derives from the particular amino acid that serves as the other donor of an amino group.
Aspartate transaminase (AST)
Aspartate transaminase is present in high concentrations in cells of cardiac and skeletal muscle, liver, kidney and erythrocytes. Damage to any of these tissues may increase plasma AST levels. The normal value of AST for male is <35 U/
L and for female it is <31 U/L. Marked increase of AST activity in the range of 10 to 100 times the upper adult reference limit indicates myocardial infarction or acute viral or toxic hepatitis.
Alanine transaminase (ALT)
Alanine transaminase is present at high concentrations in liver and to a lesser extent, in skeletal muscle, kidney and heart. Thus in case of liver damage increase in both AST and ALT were noticed. While in myocardial infarction AST
is increased with little or no increase in ALT. The normal value of ALT is <45 U/L and <34 U/L for male and female respectively. In acute viral hepatitis there is a 100-1000 times increase in both ALT and AST but ALT level is increased more than that of AST.
Markers of cholestasis
Enzymatic markers of cholestasis are membrane bound enzymes. Markers of cholestasis includes alkaline phosphatases, gamma-glutamyltransferase and glutamate dehydrogenase.
Alkaline phosphatases
Alkaline phosphatases are a group of enzymes that hydrolyse organic phosphates at high pH. They are present in osteoblasts of bone, the cells of the hepatobiliary tract, intestinal wall, renal tubules and placenta.
Gamma-glutamyl-transferase (GGT)
Gamma-glutamyl-transferase catalyzes the transfere of the ?–glutamyl group from peptides. The activity of GGT is higher in men than in women. In male the normal value of GGT activity is <55 U/L and for female it is <38 U/L. Rise in plasma GGT activity is due to infectious hepatitis and induction of enzyme synthesis, without cell damage, by drugs or alcohol.
Glutamate dehydrogenase (GLD)
Glutamate dehydrogenase is a mitochondrial enzyme found in liver, heart muscle and kidneys. Small amounts of GLD are even observed in brain, skeletal muscle tissue and leukocytes. GLD is increased in serum of patients with hepatocellular damage offering differential diagnostic potential in the investigation of liver disease. GLD is released from necrotic cells and is of value in estimation of the severity of liver cell damage. The GLD upper reference limits are 6U/L (women) and 8U/ (men).
