Abstract
In dentin, phosphophoryn is a major non-collagenous protein for mineralization. The most part of phosphophoryn is electrostatically bound to type I collagen and hydroxyapatite, and the remaining small part of phosphophoryn is covalently bound to the collagen fibrils. Previously we reported that immobilized phosphophoryn induced mineral formation. Free-type phosphophoryn is known to inhibit spontaneous precipitation from the mineralization medium having high solution saturation. In this study we examined the interaction of the role between bound-type and free-type phosphophoryn in mineralization in vitro using the classical nucleation theory. The concentrations of 0.8-800 μg/3ml of free-type phosphophoryn were incubated with 0.1 mg of insoluble dentin collagen (2.64 μg of bound-type phosphophoryn) in the mineralizing solution that no spontaneous precipitation occurred. The interfacial tensions on hydroxyapatite nucleation for mixture of 2.64 μg of collagen-bound phosphophoryn with 0.8 μg, 8.0 μg, 80 μg, and 800 μ of free type phosphophoryn were calculated using the data on mineral induction time (the time lag before mineral induction occurs) and solution saturation. It was 91.5 ergs/cm2 for 0.1 mg of insoluble dentin collagen having 2.64 mg of collagen-bound phosphophoryn. For 0.8 μg of free phosphophoryn added, it was 90.2 ergs/cm2, 91.2 ergs/cm2 for 8.0 μg of free phosphophoryn added, 91.7 ergs/cm2 for 80 μg of free phosphophoryn added, and 99.2 ergs/cm2 for 800 μg of free phosphophoryn added. Electron microscopic observation showed a difference in the shape of crystals induced by immobilized phosphophoryn between with and without free phosphophoryn. These indicated that inhibition of the apatite induced by immobilized phosphophoryn required a much greater amount of phosphophoryn free in solution. Therefore, free phosphophoryn appears to regulate the crystal growth that induced by immobilized phosphophoryn.
