An insoluble preparation of rat dentin matrix was shown to possess bone morphogenetic protein (BMP) activity, i.e. the capacity to induce the formation of cartilage and bone when implanted intramuscularly. Since BMP activity was previously attributed to noncollagenous proteins (NCP) of bone and dentin, the nature of NCP of the rat dentin was examined. After treatment of the matrix with purified bacterial collagenase, three NCP were solubilized concomitantly with digestion of the dentin collagen to smaller peptides. The three proteins were separated by anion-exchange chromatography on DEAE-cellulose. Two of the NCP were rich in aspartate, glutamate, glycine, serine, and alanine, and thus displayed compositions similar to acidic proteins of other connective tissues. The third NCP was shown by amino acid composition to be the aspartate, serine-rich phosphoprotein, which occurs mostly in a soluble form in rat dentin. This observation supports the view that a portion of dentin phosphoprotein is firmly bound.
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