A pulse radiolysis study of the reactions of 3-hydroxykynurenine and kynurenine with oxidizing and reducing radicals.

Pulse radiolysis has been used to study the reactions of 3-hydroxykynurenine and kynurenine with solvated electrons, superoxide radicals, hydroxyl radicals and azide radicals. Both 3-hydroxykynurenine and kynurenine react with solvated electrons with diffusion controlled rate constants (k = 2.5 x 10(10) M-1 s-1 and 2.3 x 10(10) M-1s-1, respectively). Neither compound was observed to react with superoxide radicals under our experimental conditions, an upper limit of 1.2 x 10(5) M-1s-1 for the rate constant of this reaction was estimated for both compounds. However, we do observe that a stable product of autooxidation of 3-hydroxy-kynurenine reacts with superoxide radicals and we calculate a lower limit for the rate of this reaction of 5.8 x 10(6) M-1s-1. Reactions of 3-hydroxykynurenine and kynurenine with hydroxyl radicals proceed with diffusion controlled rate constants (1.2 x 10(10) M-1 s-1 and 1.3 x 10(10) M-1 s-1, respectively). The measured values for the rate constants for reaction of 3-hydroxykynurenine and kynurenine with azide radicals are 2.1 x 10(10) M-1s-1 and 4.8 x 10(9) M-1 s-1, respectively. The differences in these rate constants are attributed to differences in the measured oxidation potentials for 3-hydroxykynurenine (+1.0 V vs. NHE) and kynurenine (+1.15 V vs. NHE).