The liver performs thousands of biochemical functions. In order to perform these functions, the liver contains thousands of enzymes. Enzymes are unique proteins that act as catalysts to speed the rate of particular chemical or metabolic reactions. Liver enzymes, therefore, help the liver to do its job. Enzymes utilized by the liver primarily exist in the cells of the liver. Under normal circumstances, these enzymes are also present in the bloodstream in low concentrations. However, when the liver is not working as it should, these enzymes spill over into the bloodstream. Liver function tests are blood tests that are used to detect the presence of enzymes in the bloodstream to determine liver disease.
Liver enzymes perform functions special for the liver though they are present in other organs of the human body. One category of liver enzymes is the aminotransferases or transaminases. These enzymes are used to detect the presence of liver damage or disease. Another category of enzymes include alkaline phosphatase (ALP) and gamma glutamyl transpeptidase (GGT). These enzymes are used to detect the presence of malfunction of the biliary tract.
The aminotransferase category includes the enzymes alanine aminotransferase (ALT), sometimes referred to as serum glutamic pyruvic transaminase (SGPT), and aspartate aminotransferase (AST), also known as serum glutamic oxaloacetic transaminase (SGOT). In particular, the aminotransferases act as a catalyst in the transference of amino group donor molecules to recipient molecules. In other words, these enzymes take out pieces from one structure to assist with production of another structure. The aminotransferases are the most sensitive and widely used of the liver enzymes and are normally present in the bloodstream in low concentrations. An increase in the bloodstream of either of these may indicate hepatocyte (liver cell) damage.
Although ALT is found primarily in the liver, lesser amounts can also be found in other organs, such as the kidneys, heart and muscles. When an increase of ALT levels has been detected in the bloodstream, it is usually due to liver damage, such as inflammation or cell death. An elevation in AST levels may be due to diseases of other organs, such as the heart. For this reason, clinicians determine the ratio of AST to ALT to determine if indeed the liver has been damaged.
ALP and GGT enzymes also participate in the movement of amino acids across the cellular membrane as well as metabolism. Like the aminotransferases, these enyzmes expand into the bloodstream when the biliary tract is obstructed, either within the liver or outside the liver in the bile channels. Since ALP is also found in organs other than the liver, such as bone, placenta and intestine, GGT levels are screened when there is an elevation in ALP levels. Since GGT does not increase in the presence of these other diseases, clinicians use the results from both tests to determine the cause of ALP elevation.
Clinicians use these tests together to determine the presence of liver disease or damage. Alone, these tests cannot allow a definite diagnosis but together they provide a better overall picture of liver function.
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