Calcium and high molecular weight protein aggregates in bovine and human lens.

The calcium content of bovine and human lens proteins has been determined. High molecular weight (HMW) bovine a-crystallin contains about three times more calcium than other soluble bovine lens proteins, while the calcium content of HMW human lens protein is approximately 10 times greater than its low molecular weight (LMW) counterpart. The calcium could not be removed by exhaustive dialysis at 4° C. and a pH of 7.6. At more alkaline pH's and higher temperatures most of the calcium could be eliminated from both bovine and human HMW protein. Higher pH's were required to obtain a decrease of the calcium levels of HMW human protein comparable to that observed with bovine HMW a-crystallin. Investigation of a group of potential calcium-binding compounds indicated that threonine and penicillamine were most effective in reducing the calcium level of HMW human lens protein at neutral pH, removing approximately 58 per cent and 52 per cent, respectively. When the calcium level of bovine HMW a-crystallin was decreased to that found in LMW protein, most of the protein was converted to LMW species. The partial removal of calcium from HMW human lens protein produced a small but significant shift to LMW protein. Deaggregation of human HMW protein under conditions which removed most of the calcium followed by reaggregation in the presence or absence of 5 to 8 mM of calcium indicates that this cation is needed for reaggregation of most of the protein to HMW aggregates. The above observations strongly suggest that calcium is required for the formation of HMW lens protein and that removal of this cation will cause a reversion to LMW species.