Tissue-nonspecific alkaline phosphatase is required for the calcification of collagen in serum: a possible mechanism for biomineralization.

Previous studies have shown that the type I collagen of tendon and demineralized bone both calcify rapidly in serum. The speed, collagen matrix-type specificity, and extent of the re-calcification of demineralized bone in serum suggest that the serum calcification activity identified in these studies may participate in normal biomineralization. Because of its presence in serum and its long history of association with the normal mineralization of the collagen matrix of bone, tissue-nonspecific alkaline phosphatase (TNAP) is an obvious candidate for a protein that could be a component of serum calcification activity, and experiments were therefore carried out to test this possibility. These experiments show that the inactivation of TNAP in serum prevents collagen calcification, and that the addition of physiological levels of purified TNAP restores the ability of TNAP-deficient serum to calcify collagen. Additional experiments show that the role of TNAP in collagen calcification is to activate a serum nucleator of apatite crystal formation. Based on these and earlier studies, the mechanism of collagen calcification in serum requires at least four elements as follows. 1) A matrix (collagen fibrils) that is accessible to small apatite crystals but not large molecules ( Toroian, D., Lim, J. E., and Price, P. A. (2007) J. Biol. Chem. 282, 22437-22447 ). 2) A large serum nucleator that generates small crystals, some of which diffuse into the fibrils. 3) A source of TNAP to activate the serum nucleator. 4) A large protein (fetuin) that selectively inhibits growth of crystals remaining in solution, thereby ensuring that only crystals within fibrils grow ( Toroian, D. T., and Price, P. A. (2008) Calcif. Tissue Int. 82, 116-126 ).