Heterotopic bone formation and alkaline phosphatase.

Heterotopic bone formation is osteogenesis that occurs in a part of the body that does not normally form bone. Alkaline phosphatase (orthophosphoric monoester phosphorylase) has a wide substrate specificity for catalyzing the hydrolysis of phosphate esters at an alkaline pH. The various isoenzymes are found to originate in the liver, bones, intestinal mucosa, and placenta. Numerous disorders are associated with elevations of the individual isoenzymes. The physiologic function of alkaline phosphatase is as yet unknown. The bone isoenzyme (heat labile) is found elevated in cases of heterotopic bone formation and fracture repair. Osteoblasts (bone forming cells) are unusually rich in alkaline phosphatase but the relationship of this enzyme to the process of calcification and bone formation is not clear. Alkaline phosphatase appears to be involved with the elaboration of matrix rather than with the process of mineralization in osteogenesis.