Age-related de-phosphorylation of proteins in dentin: a biological tool for assessment of protein age

Mature tooth dentin has essentially no metabolic activity, and thus post-translational modifications accumulate with aging in this tissue. In the present paper, we have studied age-related covalent changes of human dentin proteins. Dentin phosphorproteins (PP) were extracted and purified using ion exchange chromatography. Collagen was purified by CNBr cleavage and acetic acid extraction. The amino acid composition of the resultant protein preparations was determined by HPLC after post-column derivatization. Likewise the extent of aspartic acid (Asp) racemization was determined in total dentin, dentin collagen and PP. Collagen only displayed small, insignificant changes in amino acid composition and racemization with age. In contrast, PP exhibited significant age-related changes in amino acid composition, cross-linking and racemization. Thus the rate of Asp racemization in PP was 500-fold that found in collagen. Moreover, the phosphoserine (Ser(P)) content in human PP decreases dramatically with age, resulting in almost complete dephosphorylation over a life span. The loss of Ser(P) was accompanied by an increased content of the bifunctional cross-link histidinoalanine consistent with a beta-elimination pathway. The relative Ser(P) content was highly correlated with dentin age (r2 = 0.96). The Ser(P) contents may thus potentially be applied in forensic investigations to deduce human age. The possible role of covalent modifications of the protein matrix in the degradation of mineralized tissue and its implications for the age-related decline of tissue functionality is discussed.