Isolation and characterization of a blue fluorophore from human eye lens crystallins: in vitro formation from Maillard reaction with ascorbate and ribose.

A blue fluorophore, named LM-1 was isolated from human eye lens crystallins. The fluorescence property of LM-1 (excitation/emission, 366/440 nm) is similar to the fluorescence originating during non-enzymatic glycation (Maillard reaction) of proteins with the reducing sugars. LM-1 accumulates linearly with age in highly cross-linked water insoluble crystallins and is present at higher levels in cataractous lenses. The fluorophore could be synthesized by incubation of bovine serum albumin (BSA) with ribose, but not with glucose or fructose. Incubation of bovine lens crystallins with ascorbic acid (ASA) and its oxidative products, dehydroascorbic acid (DHA) and 2,3-diketogulonic acid (DKG) in presence of oxygen resulted in LM-1 formation. When oxygen was removed from the system, only DHA and DKG could synthesize LM-1, but not ASA, suggesting that ASA oxidation is obligatory for LM-1 synthesis. Modification of lysine residues on BSA prior to incubation with ribose resulted in corresponding decrease in LM-1 formation. Since ASA concentration is unusually high in lens and has been found to be a powerful glycating agent of crystallins and since LM-1 does not form with hexoses, it is likely that ASA is the major precursor of LM-1.