Liver Enzymes

Liver Enzymes

  Definition

A liver enzyme is a protein that helps to speed up a chemical reaction in the   liver. Liver function tests are blood tests that are used to evaluate various functions of the liver - for example, metabolism, storage, filtration and excretion, which are often performed by liver enzymes. However, not all liver function tests are measures of enzyme function.

• ALT - (alanine aminotransferase) 

- was previously called SGPT is more specific for liver damage. The ALT is an enzyme that is produced in the liver cells (hepatocytes) therefore it is more specific for liver disease than some of the other enzymes. It is generally increased in situations where there is damage to the liver cell membranes. All types of liver inflammation can cause raised ALT. 

 

• AST - (aspartate aminotransferase) which was previously called SGOT.

This is a mitochondrial enzyme that is also present in heart, muscle, kidney and brain therefore it is less specific for liver disease. In many cases of liver inflammation, the ALT and AST activities are elevated roughly in a 1:1 ratio. 

 

• AP - (alkaline phosphatase) 

is elevated in many types of liver disease but also in non-liver related diseases. 

Alkaline phosphatase is an enzyme, or more precisely a family of related enzymes, that is produced in the bile ducts and sinusoidal membranes of the liver but is also present in many other tissues.

 

 An elevation in the level of serum alkaline phosphatase is raised in bile duct blockage from any cause. Therefore raised AP in isolation will generally lead a physician to further investigate this area. 

Conditions such as Primary Biliary Cirrhosis and Sclerosing Cholangitis will generally show a raised AP. Raised levels may also occur in cirrhosis and liver cancer. Alkaline phosphatase is also produced in bone and blood activity can also be increased in some bone disorders.

• GT - (gamma glutamyl transpeptidase) 

 

is often elevated in those who use
alcohol or other liver toxic substances to excess. An enzyme produced in many tissues as well as the liver. Like alkaline phosphatase, it may be elevated in the serum of patients with bile duct diseases. Elevations in serum GGT, especially along with elevations in alkaline phosphatase, suggest bile duct disease. Measurement of GGT is an extremely sensitive test, however, and it may be elevated in virtually any liver disease and even sometimes in normal individuals. GGT is also induced by many drugs, including alcohol, therefore often when the AP is normal a raised GGT can often (but not always) indicate alcohol use. Raised GGT can often be seen in cases of fatty liver and also where the patient consumes large amounts of Aspartame (artificial Sweetener) in diet drinks for example.

• Bilirubin 

 

is the major breakdown product that results from the destruction of old red blood cells (as well as some other sources). It is removed from the blood by the liver, chemically modified by a process call conjugation, secreted into the bile, passed into the intestine and to some extent reabsorbed from the intestine. It is basically the pigment that gives faeces its brown colour.

• Bilirubin concentrations are elevated in the blood either by increased production, decreased uptake by the liver, decreased conjugation, decreased secretion from the liver or blockage of the bile ducts. 

• In cases of increased production, decreased liver uptake or decreased conjugation, the unconjugated or so-called indirect bilirubin will be primarily elevated. 

• In cases of decreased secretion from the liver or bile duct obstruction, the conjugated or so-called direct bilirubin will be primarily elevated. 

 

Many different liver diseases, as well as conditions other than liver diseases, can cause the serum bilirubin concentration to be elevated. Most adult acquired liver diseases cause impairment in bilirubin secretion from liver cells that cause the direct bilirubin to be elevated in the blood. In chronic, acquired liver diseases, the serum bilirubin concentration is usually normal until a significant amount of liver damage has occurred and cirrhosis is present. In acute liver disease, the bilirubin is usually increased relative to the severity of the acute process. In bile duct obstruction, or diseases of the bile ducts such as primary biliary cirrhosis or sclerosing cholangitis, the alkaline phosphatase and GGT activities are often elevated along with the direct bilirubin concentration. (See Gilberts Syndrome)

 

• Albumin 

- Albumin is the major protein that circulates in the bloodstream. As it is made by the liver and secreted into the blood it is a sensitive marker and a valuable guide to the severity of liver disease.

Low serum albumin concentrations indicate the liver is not synthesizing the protein and is therefore not functioning properly. The serum albumin concentration is  usually normal in chronic liver diseases until cirrhosis and significant liver damage is present. 

 

• Platelet count - Platelets are cells that form the primary mechanism in blood clots. They're also the smallest of blood cells. They derived from the bone marrow from the larger cells known as megakaryocytes. Individuals with liver disease develop a large spleen. While the trapping of platelets is a normal function for the spleen, in liver disease it becomes exaggerated because of the enlarged spleen (splenomegaly). Subsequently, the platelet count may become diminished.

 

• Prothrombin time (Clotting Studies) the prothrombin time is tested to evaluate disorders of blood clotting, usually bleeding. It is a broad screening test for many types of bleeding disorders. When the liver is damaged it may fail to produce blood clotting factors.