Induction and inhibition of hydroxyapatite formation by rat dentine phosphoprotein in vitro

Highly phosphorylated rat incisor phosphoprotein (PP-H) was purified and covalently attached to agarose beads. The beads were incubated for 24 h in solutions having an ionic strength of 0.165, a molar Ca/P ratio of 1.67, and a pH of 7.4. The calcium-phosphate concentration products [( Ca][P]) in the stable incubation solutions ranged from 1.0 to 1.8 mM2, from which no spontaneous precipitation occurred. In a timed series mineral formation was monitored by SEM and X-ray diffraction. The inhibitory capacity of PP-H, free solution, was also studied. The first mineral appeared after 10 min at a [Ca][P] product as low as 1.2 mM2; X-ray diffraction showed that the mineral was (hydroxy)apatite. Thus small amounts of PP-H attached to a surface are capable of inducing mineral formation in vitro at comparatively low supersaturation, whereas PP-H is a mineral inhibitor when free in solution.