An analysis of the H₂O₂-mediated crosslinking of lens crystallins catalyzed by the heme-undecapeptide from cytochrome c

In contrast to other tissues, the lens exists in a milieu containing relatively high (micromolar) concentrations of H₂O₂. It has been demonstrated that activation of H₂O₂ to more-potent oxidant species via the heme-undecapeptide from cytochrome c produces alterations in lens crystallin polypeptides similar to the changes found in cataract. These include crystallin polypeptide crosslinking and the development of a blue fluorescence not attributable to tryptophan. Of the three classes of mammalian crystallins, γ-crystallin is crosslinked by heme peptide-H₂O₂, whereas α and β are not. Hemepeptide plus H₂O₂ generates dityrosine from free tyrosine, and, concomitant with crosslinking, the γ-crystallin exposed to this system develops a new fluorophor with the characteristics of dityrosine. The findings with bovine and human crystallins are identical in this regard. In addition to the oxidation of tyrosine, exposure to heme peptide-H₂O₂ results in the oxidation of tryptophan. The intrinsic fluorescence of α, β, and γ-crystallins is due primarily to tryptophan, and the intrinsic fluorescence of each is decreased by heme peptide-H₂O₂. Thus, tryptophan oxidation occurs in all crystallins, but crosslinking occurs only in γ-crystallin and is associated with oxidation of tyrosine.