Transition metal-catalyzed. oxidation of ascorbate in human cataract extracts: Possible role of advanced glycation end products

PURPOSE. With age, human lens crystallins become more pigmented, oxidized, modified by ascorbate oxidation and advanced glycation end products (hGEs), and bind copper. The hypothesis was tested that the major AGE and ascorbyl ation product in the human lens, N-epsilon-carboxymethyl-L-lysine (CML), ha s an EDTA-Iike structure, which may predispose it to bind redox active copp er. METHODS. Young, old, and cataractous human lens protein fractions were glyc ated with ascorbic acid and tested for their ability to bind Cu(II) by atom ic absorption spectroscopy and oxidize (C-14(1))-ascorbate by radiometric t hin-layer chromatography method. hGEs were assayed by highperformance liqui d chromatography (HPLC). CML-rich proteins were immunoprecipitated from you ng, old, and cataractous crystallins using affinity-purified CML antibody a nd tested for their ability to oxidize ascorbate and generate hydroxyl radi cals in the presence of H2O2 using 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) spin-trap and EPR spectroscopy. RESULTS. Ascorbate oxidizing activity at 24 hours of native crystallins was significantly increased in both the water soluble (WS; P < 0.001) and inso luble (WIS; P < 0.05) fractions from cataractous and normal lenses. The che lator DTPA completely prevented oxidation up to 24 hours of incubation but less effectively thereafter. Mean endogenous Cu content in pooled young, ol d, and cataract fractions increased from 0.016 to 0.026 nmol/mg protein, re spectively, in WS (P < 0.05) and WIS (P ( 0.001) fractions, and Cu(II) bind ing was 20% to 30% increased in cataractous versus old and young lenses in WS (P < 0.01) and WIS (P < 0.001) fractions. Mean levels of the AGEs, CML, and pentosidine were markedly elevated in WS and WIS fractions from catarac tous versus old or young crystallins (20% to severalfold, P < 0.05 to Pc 0. 001), in a separate experiment, protein-bound Fe was not elevated. Crystall ins ascorbylated in vitro showed an increase in CML as well as Cu(II) bindi ng. CML-rich proteins (immunoprecipitated from cataractous lenses) oxidized ascorbate similar to 4 times faster than similar proteins from young and o ld normal lenses (P < 0.01) and generated hydroxyl radicals in the presence of H2O2 and DMPO. CONCLUSIONS. The association between CML formation, copper binding, and gen eration of free radicals by cataractous lens crystallins can be duplicated by ascorbylation in vitro. These effects are only in part attributable to C ML itself, and other modifications (AGEs, conformational changes) may parti cipate in the process. A vicious cycle between AGE formation, lipoxidation, and metal binding may exist in the aging lens, suggesting that chelation t herapy could be beneficial in delaying cataractogenesis.