J. E. Roberts, J. F. Wishart, L. Martinez, C. F. Chignell
Photochem.Photobiol. 72, 467-471 (2000)
Abstract:
The tryptophan metabolite xanthurenic acid has been isolated from aged human cataractous lenses. The photophysical properties of xanthurenic acid were examined to determine if it is a potential chromophore for age-related cataractogenesis. We found that xanthurenic acid produces singlet oxygen (F*= 0.17; CD3OD) with the same efficiency as the lenticular chromophore N-formyl kynurenine and quenches singlet oxygen at a rate similar to other tryptophan metabolites (2.1 x 107 M-1 s-1; CD3OD) found in the eye. As the mechanisms of induction of cataracts may also involve redox reactions, the interactions of hydrated electrons (e-aq), the azide radical (N3°), and hydroxyl radical (OH°) with xanthurenic acid were studied using the technique of pulse radiolysis. The reaction rate constants of e-aq, N3° and OH° with xanthurenic acid were found to be of the same order of magnitude as other tryptophan metabolites. The rate constant for reaction of xanthurenic acid with hydrated electrons was found to be diffusion controlled (k = 1.43 x 1010 M-1 s-1); the reaction with N3° was very fast (k = 4.0 x 109 M-1 s-1); and with OH° was also near diffusion controlled (k = 1.0 x 1010 M-1s-1). Superoxide O2°- production by irradiated xanthurenic acid in methanol was detected by ESR. Further evidence of interaction of xanthurenic acid with superoxide was substantiated by determining that the enhanced rate of oxygen consumption of xanthurenic acid irradiated in the presence of furfuryl alcohol was quenched by superoxide dismutase.