Mechanisms of calcification: role of collagen, polyphosphates, and phosphatase

Abstract

The nucleating properties of various collagens have been tested by determining quantitatively the minimum ion product, Ca times P, required to form calcium phosphate crystals. While most collagens require a product of 35 (mg%)2 or greater for calcium phosphate to precipitate, some collagens extracted from tendon induce mineralization at a product of 16 (mg%)2. Plasma ultrafiltrate contains one or more substances which increase this minimum ion product, and therefore inhibit calcium phosphate nucleation. These substances are inactivated by alkaline phosphatase. Finally, polyphosphates, both organic and inorganic, are highly inhibitory to calcium phosphate nucleation and precipitation. It is therefore suggested that at least two mechanisms may be necessary for a tissue to mineralize: 1) the formation of a nucleating collagen and 2) the presence of phosphatase for the local destruction of the inhibitor(s) present in plasma.