Mineral induction in vivo by dentine proteins

The mineral-inductive capacity of polyanionic noncollagenous proteins, isolated from calcified tissues, has earlier been demonstrated in vitro. These macromolecules possess this capacity, provided that they are immobilized on a solid support, whereas in solution they inhibit crystal mineral induction and growth. The present study demonstrates this capacity also in vivo. Phosphoprotein, proteoglycan and gamma-carboxyglutamate-containing protein (Gla-protein) of the osteocalcin type were covalently coupled to AH-Sepharose beads, and these were implanted subcutaneously in minipigs for periods between 3 and 24 weeks. Similarly, demineralized rat incisor dentine was implanted for 3 weeks, as was predentine dissected from dentinogenically active bovine incisors. The results were analyzed by scanning electron microscopy and energy dispersive X-ray analysis. Dentine proteoglycan and phosphoprotein coupled to beads were found to be inducers of calcium/phosphate-containing mineral, whereas the efficacy of Gla-protein was negligible. Dissected predentine did not induce any mineral, but demineralized dentine preparations, still containing some noncollagenous anionic components, were found to induce mineral. The results indicate that macromolecules such as proteoglycan and phosphoprotein may be responsible for mineral induction during mineralization of calcified tissues. The results further imply that such molecules may be of importance for the remineralization of dentine in conjunction with the caries process.