In vivo decomposition of phosphoserine and serine in noncollagenous protein from human dentin.

HCl-soluble proteins in human dentin ranging in age from 3 to 45 years exhibit amino acid compositional changes consistent with beta-elimination and hydrolysis of phosphoserine as well as dehydration and aldol cleavage of serine. This is the first evidence of nonenzymatic mechanisms for in vivo degradation of hydroxy and substituted hydroxy amino acids in dentin. Decomposition of phosphoseryl residues reduces the calcium-binding capacity of phosphoproteins. Elimination and dehydration reactions can produce variability in molecular weight. The rates of decomposition may be rapid enough to cause the heterogeneity or "maturational" degradation seen in dentin phosphoproteins during mineralization.