Glycoproteins in the local mechanism of calcification.

Glycoproteins in calcifiable matrices appear to play two different roles. As structural components, glycoproteins are associated with proteoglycans of which they share many properties and functions. In particular they are removed before the onset of calcification, therefore unmasking sites of calcium and phosphate deposition. As calcium and/or phosphate binding sites, glycoproteins appear to be associated with the mechanism of calcification. Glycoproteins may aid in transporting calcium and phosphate from points of initial deposition into the relatively inaccessible sites of mineralization. It is possible to assume also a nucleating effect and it cannot be excluded that Ca-binding glycoproteins are components of the "crystal ghosts." Glycoproteins also modulate calcium dependent phosphatase and ATPase activities. The recent discovery of an ATP-driven calcium pump associated with the plasma membrane of chondrocytes and the previous finding of an ATPase activity associated with the cartilage calcium-binding glycoprotein suggest that reduction in ATP content in matrix vesicles may be responsible for the initial deposits of mineral.