The mechanisms of simultaneous stereoinversion, racemization, and isomerization at specific aspartyl residues of aged lens proteins.

Proteins have been considered to consist exclusively of L-amino acids in living tissues. However, we found biologically uncommon D-aspartyl (Asp) residues at specific sites in alphaA- and alphaB-crystallin from the aged human lens (mean age: 80 years). In alphaB-crystallin, the Asp-36 and Asp-62 residues are highly racemized (D/L ratios: 0.92 for Asp-36; 0.54 for Asp-62). More interestingly, the configuration of the Asp-58 and Asp-151 residues in alphaA-crystallin is inverted to the D-isomer (D/L ratio: 3.1 for Asp-58, 5.7 for Asp-151). A D/L ratio > 1.0 is not considered to be due to racemization, but rather is thought to result from stereoconfiguration inversion. Our report was the first observation that inversion occurred in the configuration of amino acids in vivo during the natural aging process. We also found that these enantiomers were simultaneously isomerized to form beta-Asp residues. We propose that the mechanism of D- and beta-Asp formation in the protein depends on the primary structure and the presence of a chiral reaction field, which induces formation of D-Asp.