The Major in Vivo Modifications of the Human Water-insoluble Lens Crystallins Are Disulfide Bonds, Deamidation, Methionine Oxidation and Backbone Cleavage

This investigation of the water-insoluble crystallins from human lenses has used multiple chromatographic separations to obtain proteins of sufficient purity for mass spectrometric analysis. Each fraction was analysed to determine the molecular masses of the constituent proteins as well as peptides in tryptic digests of these proteins. The major components of the water-insoluble crystallins were identified as alphaA- and alphaB-crystallins. In addition, gammaS-, betaB1-, gammaD-, betaA3/A1- and betaB2-crystallins were found, in order of decreasing abundance. Although there was evidence of some backbone cleavage, the predominant forms of alphaA-, alphaB, betaB2-, gammaS- and gammaD-crystallins were the intact polypeptide chains. The major modifications distinguishing the water-soluble crystallins were increased disulfide bonding, oxidation of Met, deamidation of Gln and Asn and backbone cleavage. Of the many reactions hypothesized to lead to crystallin insolubility and cataract, these results most strongly support metal-catalysed oxidation, deamidation and truncation as initiators of conformational changes that favor aggregation.