D-Aspartic acid (D-Asp) was found in a high content in a-crystallin isolated from the watersoluble fraction of aged human lens. a-Crystallin was separated to o:A-and aB-crystallins by reverse-phase chromatography. Racemization of aspartic acid proceeded to a larger extent in 01A-crystallin than in aB-crystallin. Purified aA-crystallin was treated with trypsin to yield peptides that were then separated by HPLC using a C18 column. Tryptic peptides of 0:A-crystallin were separated and analyzed for amino acid composition and racemization of aspartic acid. Each aspartyl and asparginyl residue in the protein was racemized to a different extent. Aspss (D/L ratio of Asp, 1.41) and Aspm (3.04) were highly racemized. The D/L ratio of other aspartyl residues ranged from 0.1 to 0.2.
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Fujii et al. (1991) studied this question.