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

Abstract

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 k Cys = 36 ± 4 M −1 s −1 and to glutathione k GSH = 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 k Cys and k GSH were determined: A GSH = (1.8 ± 0.7) × 10 5 M −1 s −1, E GSH = 29.2 ± 5.6 kJ mol −1, A Cys = (2.7 ± 0.9) × 10 8 M −1 s −1, E Cys = 40.4 ± 5.7 kJ mol −1. The large difference in frequency factors for k Cys and k GSH is attributed to steric hindrance, peculiar to the bulky GSH molecule.