Stereochemically altered noncollagenous protein from human dentin

Highly phosphorylated noncollagenous proteins (NCP) with molecular weights of approximately 70-100,000 daltons have been purified from rat and bovine dentin. Efforts to isolate phosphoprotein from human teeth have not yielded consistent results, and failures have been attributed to proteolysis due to preparative techniques. Diagenetic reactions affecting metabolically stable proteins in vivo also can interfere in protein purification. Racemization is one of the reactions known to take place in human dentin. EDTA extraction of dentin from an age-graded series of human teeth has yielded an EDTA-soluble NCP fraction having an aspartic acid racemization rate 3 X that in unfractionated dentin and 8 X the rate in EDTA-insoluble protein. D-Aspartic acid is accumulating in EDTA-S protein at a rate of 0.22% yr-1. For humans, more than 13% of the aspartyl residues in NCP will be the D-enantiomer by 60 years of age. While racemization presents no problem for shorter lived mammals, such as rats, it could be partly responsible for purification difficulties with human dentin.