Identification and quantification of alpha-amino adipic acid in bovine dentine phosphoprotein

A unique phosphoprotein, which contains an uncommon amino acid, alpha-amino adipic acid (alpha-AAA), was isolated from unerupted bovine teeth by extensive EDTA demineralization. The alpha-AAA in the protein hydrolysates was identified based on the elution time on an amino acid analyzer using the sodium and lithium citrate buffer in a dual column system, and its identity was confirmed by comparisons of its DNS derivative with that of authentic alpha-AAA. This result suggests that the lysine residues in the phosphoprotein may be deaminated by an amine oxidase into allysine and further oxidized to alpha-AAA.