Identification of Hydrogen Peroxide Oxidation Sites of αA- and αB-Crystallins

Abstract

The α-crystallins are the most abundant structural proteins of the lens and, because of their chaperone activity, contribute to the solubility of the other crys-tallins. With aging, the lens crystallins undergo a variety of modifications which correlate with a loss of solubility and the development of cataract. A recent study demonstrating that α-crystallins exposed in vitro to FeCl3 and H2O2 exhibit decreased chaperone activity, implicates metal catalyzed oxidations of α-crystallins in this loss of solubility. The present study has determined that α-crystallins incubated with FeCl3 and H2O2 are modified by the nearly complete oxidation of all methionine residues to methionine sulfoxide, with no other detectable reaction products. The modifications were identified from the molecular weights of peptides formed by enzymatic digestion of the α-crystallins and located by tandem mass spectrometric analysis of the fragmentation pattern of the modified peptides. A dominant pattern in the mass spectra of the fragments from peptides with oxidized methionine is loss of 64 Da, which corresponds to loss of CH3SOH from the methionine sulfoxide. These fragments are useful in identifying peptides that include oxidized methionine residues.