Search

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).

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 ).

Objective— To determine whether serum contains an activity that induces artery calcification. Methods and Results— The elastic lamellae of devitalized rat aortas calcify rapidly in rat or bovine serum, or in human serum provided [Pi] ≥2 mmol/L. This calcification is attributable to a potent serum calcification factor (SCF), one that causes devitalized aortas to calcify when incubated in DMEM containing as little as 1.5% serum but not in DMEM alone. The SCF that initiates medial elastin calcification has the same 50- to 150-kDa size and protease sensitivity as the SCF shown previously to initiate calcification of type I collagen. Our working hypothesis is that the same SCF initiates calcification of collagen and elastin, and that this SCF arises from sites of normal bone mineralization and, like alkaline phosphatase, is released into general circulation. The SCF does not initiate medial elastin calcification in living arteries, which suggests that vascular cells may prevent this calcification. This hypothesis is supported by the observations that living arteries secrete the calcification inhibitor matrix Gla protein (MGP); that inactivation of MGP with warfarin causes living arteries to calcify; and that addition of MGP to medium containing warfarin prevents this calcification. Conclusion— The elastic lamellae of devitalized aortas calcify rapidly in serum.