What is Alkaline Phosphatase?
Alkaline phosphatase is a membrane-bound enzyme that hydrolyses phosphate esters at alkaline pH. It is expressed in multiple tissues, with the highest concentrations in liver (canalicular membrane of hepatocytes and bile-duct epithelium), bone (osteoblasts), intestine, and placenta. The circulating enzyme reflects a mixture of isoenzymes, predominantly hepatic and bone in most clinical contexts. Because ALP is excreted via bile, any obstruction or injury to the biliary tree causes retention and elevation in serum. In bone, ALP is released by osteoblasts during active mineralisation, so growing children and conditions with increased bone turnover show marked elevations.