Sugar-induced change in near ultraviolet circular dichroism of α-crystallin

In order to study the effect of sugar molecules on the structure of α-crystallin, the protein from bovine lenses was isolated and treated with D-glucose and D-glucose-6-phosphate. Structural studies of the products were carried out using circular dichroism (CD) measurements. In the near-ultraviolet (UV) region, three positive and two negative CD bands are observed for α-crystallin. Upon incubation with the saccharides, the positive ellipticities of these near-UV CD bands increase greatly. The CD spectrum of α-crystallin in the far-UV region, which is typical of the β-conformation of a protein, does not change. The increase does not appear to be caused by glycosylation, or by sugar metabolites such as sorbitol or glycolytic products. When the concentration of sugar is higher than that has been used in this study, the protein tends to aggregate. The results strongly suggest that sugar, upon interaction with α-crystallin, induces a change in the tertiary structure of the protein while its secondary structure remains intact. The interaction of sugar molecules causing such change is weak, noncovalent, and temperature-dependent. The near-UV CD of α-crystallin appears to be a useful probe for the study of such structural changes, which may be significant in the pathogenesis of cataract.