Photochemical studies on xanthurenic acid

The tryptophan metabolite xanthurenic acid (Xan) has been isolated from age d human cataractous lenses. The photophysical properties of Xan were examin ed to determine if it is a potential chromophore for age-related cataractog enesis. We found that Xan produces singlet oxygen (phi (A) = 0.17 in CD3OD) with the same efficiency as the lenticular chromophore N-formyl kynurenine and quenches singlet oxygen at a rate similar (2.1 x 10(7); CD,OD) to othe r tryptophan metabolites found in the eye. As the mechanisms of induction o f cataracts may also involve redox reactions, the interactions of hydrated electrons (e(aq)-), the azide radical (N-3(.)) and hydroxyl radical (OH.) w ith Xan were studied using the technique of pulse radiolysis. The reaction rate constants of e(aq)(-), N-3(.) and OH. with Xan were found to be of the same order of magnitude as other tryptophan metabolites. The rate constant for reaction of Xan with e(aq)(-) solvated electrons was found to be diffu sion controlled (k = 1.43 x 10(10) M-1 s(-1)); the reaction with N-3(.) was very fast (k = 4.0 x 10(9) M-1 s(-1)); and with OH. was also near diffusio n controlled (k = 1.0 x 10(10) M-1 s(-1)). Superoxide O-2(.-) production by irradiated Xan in methanol was detected by electron paramagnetic resonance and substantiated by determining that the enhanced rate of oxygen consumpt ion of Xan irradiated in the presence of furfuryl alcohol was lowered by su peroxide dismutase.