Purpose of review: Alkaline phosphatase is an important component in hard tissue formation, highly expressed in mineralized tissue cells. It is appropriate to review the current status of this important enzyme.

Recent findings: The mechanism with which this enzyme carries out its function is not completely understood, but it appears to act both to increase the local concentration of inorganic phosphate, a mineralization promoter, and to decrease the concentration of extracellular pyrophosphate, an inhibitor of mineral formation. The enzyme is localized to the outside of the plasma membrane of cells, and of the membrane of matrix vesicles. It is attached to the membrane by a glycophosphatidylinositol anchor, and is found in membrane microdomains known as rafts. Alkaline phosphatase has also been implicated in cardiovascular calcification which appears to proceed by an osteogenic mechanism. Significant interest in alkaline phosphatase expression has also come from tissue engineering experiments, where enzyme expression is a good predictor of neotissue mineralization.

Summary: The high level of activity in this field is sure to provide new and important information into the fundamental mechanisms of hard tissue formation, provide therapeutic opportunities for treatment of bone diseases, and enhance our ability to create useful bone biomaterials.