Differences between enamel-related and cementum-related dentin in the rat incisor with special emphasis on the phosphoproteins.

In order to determine whether qualitative and quantitative differences exist between the non-collagenous proteins of crown and root dentin, rat incisors were separated into their enamel- and cementum-related dentin portions (ERD and CRD, respectively). Isolation of the mineral-bound proteins was performed under nondegradative conditions. Analytical procedures included DEAE-chromatography on high pressure liquid chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and determination of phosphate, protein, and hydroxyproline. The results have shown that considerable differences exist among the two dentins with respect to the quantity of the various phosphoproteins. For this group of proteins as a whole, the ERD contains about 2 times the amount of organic phosphate found in the CRD and about 1.4 times the amount of protein. The content of higher phosphorylated phosphoproteins was about 4 times higher in the ERD than in the CRD, whereas the reverse was shown for the lower phosphorylated phosphoproteins. All differences were found to be statistically significant. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that while the ERD contains phosphoproteins with an apparent molecular mass of 98 kDa, the CRD contains two classes of phosphoproteins one of 98 and one of 88 kDa. The relevance of the observed differences in phosphoprotein distribution is discussed in relation to their possible role in mineralization.