A membrane-bound glycoprotein enzyme that catalyses the hydrolysis of phosphoric acid esters under conditions of alkaline pH. In humans the blood levels of alkaline phosphatase are measured as part of the assessment of liver function; in the blood the enzyme also breaks down phosphates required for mineralization of bone. There are four distinct forms of the enzyme (see isozyme) in humans—intestinal, placental, germ cell, and liver/bone/kidney alkaline phosphatases—each encoded by different genes.