MODEL STUDIES ON THE PHOTOCHEMICAL PRODUCTION OF LENTICULAR FLUOROPHORES

With aging, human lens proteins accumulate fluorophores having blue an d green emissions. Model studies were undertaken to determine the role of 3-hydroxykynurenine (3-HK) and its glucoside (3-HKG) in the photoc hemical production of those fluorophores. Experiments were carried out using 10(-3) M 3-HK solutions in the presence or absence of glycine ( 1 M), which was used to mimic the environment of the lens. The solutio ns were photolyzed (transmission above 295 nm) for various periods of time while the loss of starting material and the formation of fluoresc ent photoproducts (blue emission at 470 nm, and green emission at 520 nm) were monitored using fluorescence and UV-visible spectroscopy and thin-layer and high-pressure liquid chromatography analysis. Several p arameters were varied such as oxygen tension and the addition of the f ree radical scavenger, penicillamine. The photolytic loss of 3-HK in t he absence of glycine occurred approximately 5-10 times faster than in its presence. Conversely, blue and green fluorophores formed in irrad iated solutions containing glycine but not with the photolysis of 3-HK alone. The blue fluorophore was formed first and appeared then to be photochemically converted to the green one, with the rate of formation of the latter increasing with an increase in UV dosage or oxidizing c onditions. The addition of penicillamine drastically reduced the photo chemical formation of both fluorophores. Both the blue and green fluor ophores appear to result from the photochemically induced covalent att achment of 3-HK to glycine. In the human lens, these reactions can exp lain the age-related loss of 3-HKG with the concomitant formation of f luorophores covalently attached to lens proteins, probably via the ami no group of lysine.