Self-association of calcium and magnesium complexes of dentin phosphophoryn.
Self-association of rat dentin phosphophoryn in the presence of calcium and magnesium ions was examined by chemical cross-linking and electron microscopy. Highly phosphorylated phosphophoryn (HP) binds a maximum of 1.33 calcium ions or 1.07 magnesium ions per organic phosphate residue at pH 7.4-8.0. The Ca-HP complexes are predominantly linear when the calcium content of the complex is less than about 65% of the saturation level. At higher calcium levels, the protein has a folded conformation, and transient protein-protein interactions occur. The equilibrium mixture of monomers and oligomers is predominantly monomeric unless the protein is saturated with calcium. The saturated Ca-HP complex forms discrete high molecular weight particles about 25 nm in diameter. The particles are electrically neutral and generally occur in clusters. Mg-HP complexes appear predominantly linear by electron microscopy at all concentrations of bound magnesium up to about 99% of the saturation level; however, protein-protein interaction is measurable when the magnesium content is as little as 65% of the saturation level. At saturation, Mg-HP complexes form high molecular weight particles which are negatively charged. Because of the negative charge, these particles form a stable colloidal suspension and have a rather stellate configuration.
Marsh
M E
ME
Dental Science Institute, University of Texas Health Science Center, Houston 77225.
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism / Q000648:ultrastructure; D003432:Cross-Linking Reagents; D003804:Dentin / Q000378:metabolism*; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism; D008854:Microscopy, Electron; D008970:Molecular Weight; D010750:Phosphoproteins / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats
D002135:Calcium-Binding Proteins; D003432:Cross-Linking Reagents; D010750:Phosphoproteins; C017630:phosphophoryn; D008274:Magnesium; D002118:Calcium