The effects of hydrogen peroxide on lens proteins: a possible model for nuclear cataract

Soluble proteins from the human lens nucleus were incubated for 12 weeks with 0, 10, 100 and 1000 microM-H2O2. Treatments were monitored for alterations in cysteine (sulphydryl), methionine, tryptophan and other amino acids; protein size, solubility and conformation; polypeptide sizes; and non-tryptophan fluorescence. Progressive changes were observed in several of these parameters. Cysteine (up to 100%) and methionine (up to 45%) were rapidly oxidized but no significant alterations were found in any other amino acids. No new chromophores were generated but the non-tryptophan fluorescence was enhanced three-fold. The gradually increasing solvent accessibility of tryptophan residues indicated that the proteins were undergoing conformational alterations. This was accompanied by the insolubilization of protein (up to 75%). The insoluble proteins consisted largely of covalently cross-linked polymers of the lens polypeptides. Disulphide bonds and dityrosine were shown not to be involved in the cross-linking. The modifications observed, as well as their order and extent, are very similar to those found in the cataractous lens. Our observations suggest that low concentrations of H2O2 may be responsible for the oxidative modification of lens proteins during the development of senile nuclear cataracts.