Alkaline phosphatase is an enzyme which catalyzes the hydrolysis of phosphate monoesters. It is membrane-bound and widely found:
- liver - 55%; in hepatocytes next to biliary canaliculi
- bone - 45%; in osteoblasts
- gut - 5%
The liver, bone and kidney enzymes are different forms of the same gene product; the gut and placental forms are isoenzymes.
Total serum alkaline phosphatase is normally measured but liver and bone isoenzymes may be distinguished if required. They are normally present in roughly equal amounts within plasma. Gut ALP rises after meals, especially in those of blood group B or O who are secretors of blood group substances. Bone ALP increases at the time of physiological growth spurts. Placental ALP normally increases at the end of the third trimester.