What is Alkaline Phosphatase?

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Alkaline phosphatase is an enzyme found in all body tissues, with the highest concentrations found in growing bone and bile, two parts of the body responsible for enzyme production. Alkaline phosphatase is important in a wide variety of biological functions, including its ability to help diagnose diseases and conditions.

Structure

Like many enzymes, alkaline phosphatase is a glycoprotein. The structure of alkaline phosphatase differs depending on where that particular enzyme originated. Alkaline phosphatase from bone looks different from alkaline phosphatase from the liver. These different structures are called isozymes.

In general, alkaline phosphatase is a dimer containing nearly identical subunits which each have two molecules of zinc. One molecule of zinc is tightly bound, giving the structure stability and the other is loosely bound which provides for the catalytic activity.

Function

Alkaline phosphatase is an enzyme that is responsible for the removal of phosphate groups from various molecules, such as alkaloids, nucleotides and proteins. Although the function of alkaline phosphatase is widely studied and used extensively in laboratories, the magnitude of the biological significance remains somewhat of a mystery and an area of study.

Alkaline phosphatase plays a role in the calcification of cartilage and bone. The alkaline phosphatase catalyzes the hydrolysis of the phosphoric acid ester by causing a supersaturation of phosphate ions. It is also believed that alkaline phosphatase also plays a role in bone resorption by removing a layer of phosphate that is present on the surface of bones.

Alkaline phosphatase also plays a role in the synthesis of proteins within cells. It is also a player in the synthesis of DNA as it is capable of hydrolyzing both DNA and RNA.

Testing

One of the most important functions of alkaline phosphatase is as an indicator for disease. High levels of alkaline phosphatase in the blood can indicate diseases of the bones, liver, bile system or malignancies. Abnormally low levels are symptomatic of a genetic disorder known as hypophosphatasia, which can result in bone deformities, and even eventually lead to death if left untreated.

Why Perform the Alkaline Phosphatase Test?

Performing an alkaline phosphatase test aids in the diagnosis of the following diseases/conditions: liver disease, jaundice, Vitamin D deficiency, bone disease, parathyroid disease, gastric pain and chronic mylogenous leukemia.

The alkaline phosphatase test is also used by doctors to monitor the effect of prescription drugs on liver function. Many drugs including antidepressants, birth control pills, antibiotics and anti-inflammatory drugs to name a few, are broken down in the liver, thereby affecting liver functions. Monitoring the alkaline phosphatase levels in the blood helps doctors to determine dosage levels and can help to avoid complications that can arise from continued prescription drug use.

Activators and Inhibitors

Alkaline phosphatase has several activators, which are molecules that put it to work. Divalent metal ions, including Mg2+, Co2+ and Mn2+, and amino alcohols all can bind to the active site thereby kick starting the reactions utilizing alkaline phosphatase. There are also conditions which inhibit the ability of alkaline phosphatase to work. They include the presence of neutral or acidic pH, chelators of the metal ions mentioned above, inorganic phosphates, urea and high levels of Zn2+.

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