Calcium-induced aggregation of the urea-deaggregated human lens insoluble protein

Calcium-induced aggregation of the urea-deaggregated insoluble protein (US-fraction) derived from human lens was investigated by use of a simple light scattering method. The initial rate of aggregation was shown to increase with progress of senescence of the lens, and to be much higher in the nucleus as compared to the cortex. The aggregation was found to be more active in the US-fraction from cataractous lens than in that of normal lens at same age. A calcium sensitive polypeptide (CSP) was isolated from the US-fraction from cataractous human lenses by gel filtration followed by DEAE-cellulose column chromatography in the presence of 7 M urea. The molecular size of the CSP in the dissociated state was assessed to be 4.2 kDa by urea-SDS-PAGE, and the size in the associated state to be 450 kDa by thin layer gel filtration. With addition of calcium ion to the latter, it aggregates rapidly to form much larger aggregates and eventually precipitates out. The aggregation rate of the CSP is shown to increase exponentially with an increase in the concentration of both CSP and calcium ion, indicating the involvement of complicated mechanisms in the aggregation process. Effects of divalent ions other than calcium ion on the aggregation process were also examined. It is revealed that the effect of manganese ion is little less than that of calcium ion, whereas magnesium and strontium ions are much less effective as compared to the former two ions.