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What Is Alkaline Phosphatase?

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  • Written By: Helga George
  • Edited By: Michelle Arevalo
  • Last Modified Date: 13 April 2014
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Alkaline phosphatases are a group of enzymes that are used medically to diagnose certain conditions. Enzymes are proteins that catalyze reactions and help them to happen much more quickly. Alkaline phosphatase activity can also be the result of normal activity, such as pregnancy or bone growth. Although present in all tissues in the body, it is most prevalent in the bone and liver. High levels are found in the placenta during pregnancy.

An alkaline phosphatase test is frequently part of a routine blood test for liver function. Elevated levels are normal in children, since they undergo rapid growth of their bones. With non-pregnant adults, it is a cause for concern and can indicate a number of pathological conditions.

If there are high alkaline phosphatase levels, further tests may be done to determine if there is a bone or liver problem. A number of liver conditions can cause high levels, including cirrhosis, tumors, hepatitis, and drug intoxication. Fractured bones and the spread of cancer to the bone can also result in elevated levels. Paget’s disease is a chronic bone disorder that is typically diagnosed by high amounts of alkaline phosphatase. Many other conditions can also cause elevated enzyme levels.

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If the source of the elevated levels is not clear, there are biochemical ways to determine which type of alkaline phosphatase is responsible for the activity. The bone and liver forms differ in their heat stability. One can also perform electrophoresis, and run the samples on a gel with an electrical current to distinguish the different forms. There will be visible differences in the location of the different enzymes on the gel.

Low alkaline phosphatase levels are observed less frequently than high ones. Low levels can indicate several types of severe anemia or chronic myelogenous leukemia. Post-menopausal women taking estrogen for osteoporosis can also have lower levels, as can men who have had heart surgery recently or suffer from malnutrition or hypothryoidism.

The mode of action of alkaline phosphatases is to remove phosphate groups, PO4, from certain molecules. This process is called dephosphorylation. These enzymes can act on many types of compounds, including nucleotides — the building blocks of DNA. Proteins and DNA can also be dephosphorylated. Alkaline phosphatases function best in an alkaline, or high pH, environment.

There are practical, non-medical uses for alkaline phosphatases. These are used in molecular biology to cleave the phosphate group off the 5', or five prime, end of DNA. This prevents the DNA from forming a circle, and leaves it linear for the next experimental step. Such processes are also used in the pasteurization of milk, as a marker to make sure that the milk has been pasteurized. The enzyme loses activity at high temperatures and stops reacting with a dye when the pasteurization process is complete.

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