UV filter decomposition. A study of reactions of 4-(2-aminophenyl)-4-oxocrotonic acid with amino acids and antioxidants present in the human lens

Deamination of UV filters, such as kynurenine (KN), in the human lens results in protein modification. Thermal reactions of the product of kynurenine deamination, 4-(2-aminophenyl)-4-oxocrotonic acid (CKA), with amino acids (histidine, lysine, methionine, tryptophan, tyrosine, cysteine) and antioxidants (ascorbate, NADH, glutathione reduced) were studied. The rate constants of the reactions under physiological conditions were measured. The rate constants of CKA addition to cysteine kCys = 36 ± 4 M−1 s−1 and to glutathione kGSH = 2.1 ± 0.2 M−1 s−1 are 4–5 orders of magnitude higher than the rate constants of CKA reactions with the other amino acids and antioxidants. The Arrhenius parameters for kCys and kGSH were determined: AGSH = (1.8 ± 0.7) × 105 M−1 s−1, EGSH = 29.2 ± 5.6 kJ mol−1, ACys = (2.7 ± 0.9) × 108 M−1 s−1, ECys = 40.4 ± 5.7 kJ mol−1. The large difference in frequency factors for kCys and kGSH is attributed to steric hindrance, peculiar to the bulky GSH molecule.