The role of glycation and autoxidation on crystallin aggregation

Incubation of α-crystallin with glucose and CuSO4 resulted in crystallin changes similar to those observed in cataracts. Examination of the reaction mixtures by polyacrylamide slab gel electrophoresis showed progressive crystallin aggregation through non-disulfide covalent bonds and parallel increases in ultraviolet absorbance and non-tryptophan fluorescence. Both glucose and copper were required; iron was less effective. The reaction can be accelerated by increasing glucose concentration or by utilizing ribose which has a higher percentage of free aldehyde groups than glucose. These observations are consistent with a mechanism involving crystallin glycation. The reaction is mediated by hydrogen peroxide and transition metals since it is inhibited by catalase and by chelating agents. These results, in turn, are consistent with copper-catalyzed autoxidation of glucose and of glycated crystallin. This reaction generates superoxide free radical which dismutates to yield hydrogen peroxide. The latter, in turn, generates hydroxyl radicals in presence of transition metal ions (Fenton reaction). Hydroxyl radical attack leads to cross-linking which is enhanced in glycated proteins. Under hyperglycemic conditions, such as in diabetes mellitus, high levels of glucose occur in insulin-independent tissues such as the lens. Elevated cupremia and oxidative stress are also known to occur in diabetic patients. There-fore, our findings are consistent with crystallin glycation and superimposed oxyradical generation during diabetic cataractogenesis.