Mechanism of aggregation of UV-irradiated beta(L)-crystallin

Thermal denaturation and aggregation of UV-irradiated beta(L)-crystallin from eye lenses of steers have been studied. The data on size-exclusion chromatography and SDS-PAGE indicated that UV irradiation of beta(L)-crystallin at 10 degrees C resulted in fragmentation of the protein molecule and formation of cross-linked aggregates. Fluorescence data showed that tryptophan fluorescence in the irradiated protein decreased exponentially with the UV dose. Decrease in tryptophan fluorescence is a result of photochemical destruction, but not of conformational changes of protein, because there is no red shift in the fluorescence maximum. The differential scanning calorimetry (DSC) profiles of the samples of UV-irradiated and wild type beta(L)-crystallin were registered. The area under curves, which is proportional to the amount of the native protein, decreased exponentially with increasing the irradiation dose. The shape of the DSC profiles for the samples of UV-irradiated beta(L)-crystallin was identical to that for wild type beta(L)-crystallin. The DSC data allowed estimating the portion of UV-denatured beta(L)-crystallin, which is not registered by DSC, and the portion of the combined fraction consisting of native and UV-damaged molecules retaining the native structure. A conclusion has been made that UV-induced denaturation of beta(L)-crystallin follows the one-hit model. The study of the kinetics of thermal aggregation of UV-irradiated beta(L)-crystallin at 37 degrees C using dynamic light scattering showed that the initial stage of aggregation was that of formation of the start aggregates with the hydrodynamic radius of 20 nm. Further sticking of the start aggregates proceeded in the regime of reaction-limited cluster-cluster aggregation. Splitting of the aggregate population into two components occurred above a definite point in time. (C) 2010 Elsevier Ltd. All rights reserved.