Alkaline phosphatases

Alkaline phosphatases are enzymes which release inorganic phosphate from a variety of orthophosphates. They are present in nearly all tissues, particularly the liver, biliary system, kidney, bone, intestine, placenta, and tumors. In the liver, alkaline phosphatase is located in the microvilli of the bile canaliculus and on the sinusoidal surface of the hepatocytes. Serum alkaline phosphatase levels vary depending on age. Levels are raised until about 20 years of age and then again in old age, with increased contributions of liver alkaline phosphatase in males and bone alkaline phosphatase in postmenopausal females. Serum levels of alkaline phosphatase enzymes rise in many types of liver disease, particularly where there is either intra- or extrahepatic obstruction to the flow of bile and in the presence of intrahepatic space-occupying lesions such as tumors. The raised levels of the enzyme seem to be due to increased synthesis in the canalicular membrane rather than failure of excretion. Specific assays are available to differentiate between the different isoenzymes of alkaline phosphatase, which makes specific diagnosis simpler and more accurate. When these specific isoenzyme assays are not available, other markers of biliary obstruction such as g-glutamyl transferase should be monitored.

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