Various nonenzymatic reactions can produce the compositional and molecular weight changes observed in maturing bovine and human dentin phosphoprotein. Beta-Elimination of phosphoserine will generate dehydroalanyl residues that can (1) form crosslinks with other nucleophilic side chains or (2) hydrolyze the peptide bond forming pyruvate and ammonia. The resulting changes in molecular weight and composition would affect phosphoprotein particularly in secondary teeth. The crosslinked amino acids lysinolanine and histidinoalanine recently have been detected in human dentin, although localization to collagenous protein is preliminary. Racemization of aspartyl residues, another non-enzymatic reaction, also has been observed in human dentin, predominantly in non-collagenous protein. Experiments are proposed here to investigate the kinetics of the Beta-elimination and racemization reactions in an age-graded series of human dentin. Phosphoprotein, collagen, and phosphoprotein-collagen conjugates can be isolated. The rates of formation of crosslinked amino acids and pyruvate, decrease in phosphate content, and aspartic acid racemization will be examined. Analyses of molecular weight species of phosphoprotein and other fractions derived from the isolation steps will enable localization of the crosslinks and D-aspartic acid residues and resolve the following questions: does D-aspartic acid coincide with phosphoprotein exclusively and thus label the diagenetic products of phosphoprotein; do lysinoalanine and histidinoalanine occur in higher molecular weight soluble proteins; and are the dehydroalanyl crosslinks found in phosphoprotein-collagen conjugates. Elucidation of the kinetics of these reactions would show if nonenzymatic mechanisms are sufficiently rapid to influence phosphoprotein during mineralization or subsequent processing. In addition, the use of protease inhibitors and dissociative solvents during the purification of phosphoprotein from the youngest series of teeth, unerupted third molars, may permit characterization of protein comparable to the highly phosphorylated, heavy molecular weight phosphoprotein described in rat incisors and fetal bovine teeth.
| Masters, P M (1986) Age at death determinations for autopsied remains based on aspartic acid racemization in tooth dentin: importance of postmortem conditions. Forensic Sci Int 32:179-84 |
| Masters, P M (1985) In vivo decomposition of phosphoserine and serine in noncollagenous protein from human dentin. Calcif Tissue Int 37:236-41 |
