Dentin sialoprotein (DSP) has limited effects on in vitro apatite formation and growth

Sialoproteins such as bone sialoprotein (BSP) and dentin sialoprotein (DSP) accumulate at the mineralization fronts in bone and dentin, respectively, suggesting they have some function in the mineralization process. BSP, a hi ghly phosphorylated protein rich in polyglutamate repeats, is an effective nucleator of hydroxyapatite (HA) formation in vitro. The present study exam ines the effect of DSP, a low phosphorylated but related sialoprotein, on t he formation and growth of HA. In vitro, in a gelatin gel diffusion system, DSP at low concentrations (<25 <mu>g/ml) slightly increased the yield of H A formed at 3.5 and 5 days, while at higher concentrations (50-100 mug/ml) it slightly inhibited accumulation. Fewer mineral crystals were formed in t he presence of high concentrations of DSP but they tended to aggregate (mak ing them appear larger by electron microscopic analysis) than those formed in DSP-free gels. X-ray diffraction line broadening analysis failed to show significant changes in c-axis crystal dimensions with increasing DSP conce ntration. When HA-seed crystals were coated with DSP before inclusion in th e gelatin gel there was a reduction in mineral accumulation relative to HA- seeds which had not been coated with DSP, but the extent of inhibition was significantly less than that seen in this system with other mineralized tis sue matrix sialoproteins, such as osteopontin or BSP. The low affinity of D SP for well-characterized seed crystals and the limited effect of this prot ein on HA formation and growth suggest that the role of DSP in dentin is no t primarily that of a mineralization regulator.