Liver enzymes are enzymes typically found in liver cells (hepatocytes). In clinical parlance, they are often called liver enzymes. An increase in certain enzymes is an indication of liver damage, whereas other enzymes occur at lower levels in liver disease.
What are liver enzymes?
In liver disease, liver enzymes often provide important clues as to what type of disease is involved. In general, the body needs enzymes to maintain metabolic performance. If the liver cells are damaged, the liver enzymes in the blood serum are elevated. Depending on the elevated enzyme, the type of disease can then be concluded. The causes of cell damage can be alcohol, viral infections, tumors or poisoning. Liver enzymes that are commonly measured include:
- Gamma-glutamyltransferase (gamma-GT).
- Glutamate dehydrogenase (GLDH)
- Aspartate aminotransferase (AST, ASAT)
- Alanine aminotransferase (ALT, ALAT)
- Alkaline phosphatase
Function, action and tasks
The liver, which is located in the right upper abdomen, is involved in many breakdown and construction processes of the body. For example, important proteins are produced here, and the breakdown of hormones and the breakdown of red blood pigment also take place in the liver. The red blood pigment is then turned into bile, which, together with other substances, forms bile. This is secreted into the small intestine, where it plays an essential role in fat digestion. In addition, the liver also stores glycogen, copper or iron, and breaks down food components that can then be used by the body. All these processes require enzymes that mediate chemical reactions. However, they themselves are not consumed in the process. For this reason, they are also called catalysts. Such enzymes include, for example, transaminases such as glutamate pyronate transaminase or glutamate oxaloacetal transaminase. They occur in the liver in very large quantities and are released when liver cells are damaged. Aspartate aminotransferase is important for the respiratory chain or the malate-aspartate shuttle and ensures that a transfer of the L-amino group to an a-keto acid takes place. ALT plays an important role in the glucose–alanine cycle and catalyzes the reaction L-alanine + a-ketoglutarate = pyruvate + L-glutamate. Gamma-glutamyl transferase transfers the glutamyl residue of glutathione (GSH) to peptides or water, which degrades glutathione. Cysteine occurs in the glutathione, which is then transported into the cells. Here, glutathione is then rebuilt. The role of alkaline phosphatases, which act as markers for various skeletal diseases and liver diseases, has not yet been fully clarified. If liver disease is present, the enzymes are determined, giving the physician indications of the extent or nature of the disease. Here, the level of increase in the particular enzyme indicates the extent of damage.
Formation, occurrence, properties, and optimal levels
The production of liver enzymes takes place in liver cells. In this process, the various enzymes contribute to the acceleration of metabolic processes that take place in the liver cells. If damage to the liver cells occurs, the enzymes are released and enter the blood. One of the most important liver enzymes is glutamate oxaloacetate transaminase, which is found in the liver, skeletal muscle and heart muscle and is now also known as aspartate aminotransferase (AST). In the cytoplasm of liver cells, the enzyme glutamate pyruvate transaminase or alanine aminotransferase (ALT) can be found. Pyruvate is formed from alanine aminotransferase, alanine from excess nitrogen. A so-called membrane-bound enzyme is gamma-glutamyl transferase (y-GT), which is found in the liver, but also in the kidneys, small intestine, spleen and pancreas. Alkaline phosphatases are enzymes that can cleave phosphoric acid monoesters and are found in the liver, bones, kidneys, or small intestine.
Diseases and disorders
Liver enzymes are determined in the diagnosis of liver disease. To do this, the doctor takes blood from the patient, which is then tested in the laboratory. Important syndromes that can lead to liver damage include autoimmune inflammatory diseases, hepatocellular insufficiency, cholestasis, and cytolysis.The cause may be neoplastic, autoimmune, traumatic, toxic or infectious. In cytolysis syndrome, liver cells are dissolved and the cell contents enter the blood. The enzyme that predominates in cytolysis is ALAT. In cirrhosis stage disease or alcohol-induced hepatitis, ASAT predominates. If the ASAT is moderately elevated, this may indicate muscle cell damage, which can be confirmed by subsequent determination of the so-called creatine kinase. Cholestasis syndrome indicates a disturbance in bile excretion or synthesis. A distinction can be made between obstructive and non-obstructive cholestasis. In obstructive cholestasis, the bile ducts are obstructed by gallstones, for example, while in non-obstructive cholestasis the epithelial cells in the bile ducts are damaged, resulting in reduced secretion of bile acid. In cholestasis, there is an increase in the enzymes GT and ALP. If the level of ALP is normal and only the enzyme GT is elevated, chronic alcoholism is usually present. If only the AlP value is elevated, this indicates bone disease. In hepatocellular insufficiency, liver function is damaged, resulting in decreased albumin synthesis and slowed protein metabolism or conversion of fats and sugars. In the autoimmune-inflammatory syndrome, immunoglobulins increase, and elevated IgA indicates alcohol-induced cirrhosis.
