High correlation between pentosidine protein crosslinks and pigmentation implicates ascorbate oxidation in human lens senescence and cataractogenesis.

Abstract

Pentosidine is a recently discovered protein crosslink, involving lysine and arginine residues linked together in an imidazo [4,5,6] pyridinium ring formed by a 5-carbon sugar during nonenzymatic browning (Maillard reaction). The presence of high ascorbate levels in the human lens and its ability to undergo nonenzymatic browning led us to investigate pentosidine formation in the aging human lens. Incubation of lens crystallins with ascorbate and its oxidation products dehydroascorbate and 2,3-diketogulonate leads progressively to the formation of pentosidine crosslinks in the presence of oxygen. Under nitrogen, however, pentosidine forms only from 2,3-diketogulonate or xylosone, a degradation product of 2,3-diketogulonate. A high correlation between pentosidine crosslinks and the degree of lens pigmentation is noted in cataractous lenses. Pentosidine is found to be primarily associated with a-crystallin fractions of 300-5000 kDa. These results suggest that redox imbalance in cellular senescent systems such as the ocular lens may lead to irreversible ascorbate oxidation and protein crosslinking by xylosone. This mechanism may play an important role in the pathogenesis of "brunescent" cataracts.

AGEs in human lens capsule promote the TGFβ2‐mediated EMT of lens epithelial cells: implications for age‐associated fibrosis

“…After digestion, the mixture was passed through a 3‐kDa cutoff filter, vacuum–concentrated, and stored at −86°C until LC‐MS/MS analysis. Before analysis, we measured the amino acid content in the digested material using leucine as the standard, as previously described (Nagaraj et al ., 1991). …”

Deamidation Affects Structural and Functional Properties of Human αA-Crystallin and Its Oligomerization with αB-Crystallin

“…Present literature suggests a multifactorial mechanism for the development of cataract-specific cross-linked species, which might be driven by post-translational modifications. These included disulfide bonding (12), glycation (13), oxidation of Trp and His residues (14,15), deamidation (16), and transglutaminase-mediated cross-linking (17). However, despite identification of these modifications in crystallins, their exact roles in the mechanism of crystallin cross-linking remain poorly understood.…”

3-Hydroxykynurenine-mediated Modification of Human Lens Proteins

“…The most striking changes in these proteins include yellowing and browning of proteins, intra-and intermolecular cross-linking, and cross-linking with fiber cell membrane proteins (1)(2)(3)(4). Several mechanisms have been proposed for such changes, including oxidation (5,6) and glycation (7)(8)(9). Recent studies suggest that these two processes are interrelated (10,11) and may thus synergistically contribute to the observed lens protein modifications in aging and cataract formation.…”