Age-related changes in protein conformation in bovine lens crystallins

In order to investigate the conformational changes associated with the aging process, circular dichroism (CD), absorption and fluorescence measurements of bovine lens crystallins isolated from the nucleus of old (cow) and young (calf) animals are reported. Results show considerable differences in spectroscopic parameters between the young and old alpha-crystallin; however, no such changes were observed for beta- and gamma-crystallins. Age-related changes include an increased absorption in near-u.v. and decreased intensity in the far-u.v. region; near-u.v. circular dichroism shows a considerable difference, whereas the dichroism in far-u.v. remains the same. The decrease in tryptophan fluorescence of old alpha-crystallin is of the same magnitude as is the increase in non-tryptophan fluorescence. The fluorescence of the sulfhydryl (SH) specific probe, 2-(4'-maleimidylanilino) naphthalene-6-sulfonate, indicates that accessible (to the probe) SH groups of cow alpha-crystallin are fewer than those of calf, and they are also in a more polar environment. This study demonstrates that, with aging, alpha-crystallin undergoes a change in the tertiary structure involving tryptophan, tyrosine and cysteine residues. This conformational change has been explained by the suggestion that a large portion of the protein unfolds during the aging process, resulting in a change in interaction properties between the aromatic amino acid residues and between the residues and the peptide backbone. The unfolding is also associated with the accessibility, reactivity and spatial arrangement of these residues, including the cysteine by which aggregation or cross-linking of the protein is likely to occur.