TY - JOUR

T1 - Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part I. Amino acids in free state

AU - Zhuravleva, Yuliya S.

AU - Sherin, Peter S.

N1 - Publisher Copyright: © 2021 Elsevier Inc.

PY - 2021/8/20

Y1 - 2021/8/20

N2 - In the human eye lens the endogenous chromophores of UV-A light (315–400 nm) are able to sensitize radical reactions leading to protein modifications during normal aging and the cataract progression. Kynurenic acid (KNA−) is the most photochemically active dye of the human eye lens reported to date with pKa(KNAH2•) 5.5 for its radical form. Cataract is thought to develop under oxidative stress which could be accompanied by acidosis, an acidification of the intracellular environment. Protonation of kynurenyl radicals at mildly acidic conditions may change the outcome of radical reactions leading to additional damage to proteins. In this work we investigated the influence of pH on the degradation of initial reagents and the formation of products in photoinduced radical reactions between KNA− and amino acids tryptophan (Trp) and tyrosine (Tyr) in free states. Our results have shown that pH variation has minor influence on kinetics of reagent decay and accumulation of products in reactions between tyrosyl and kynurenic acid radicals. However in the case of Trp a two-fold decrease of the reagent degradation without visible changes in the composition of formed products was observed with pH decrease from 7 to 3. Time-resolved measurements have shown similar acidification-induced two-fold acceleration of decay of kynurenyl and tryptophanyl radicals via Back Electron Transfer (BET) with the restoration of initial reagents. Experiments with tryptophan derivatives with different pKa values for their radical forms point out the protonation of tryptophanyl radical as the driving force for BET acceleration at low pH. Our results demonstrate that the protonation of kynurenyl radical does not change its reactivity towards amino acids radicals but the total yield of radical photodamage decreases with the protonation of tryptophanyl radicals. It could be expected that radical induced damage to proteins will depend on the pKa of tryptophanyl radicals within a protein globule.

AB - In the human eye lens the endogenous chromophores of UV-A light (315–400 nm) are able to sensitize radical reactions leading to protein modifications during normal aging and the cataract progression. Kynurenic acid (KNA−) is the most photochemically active dye of the human eye lens reported to date with pKa(KNAH2•) 5.5 for its radical form. Cataract is thought to develop under oxidative stress which could be accompanied by acidosis, an acidification of the intracellular environment. Protonation of kynurenyl radicals at mildly acidic conditions may change the outcome of radical reactions leading to additional damage to proteins. In this work we investigated the influence of pH on the degradation of initial reagents and the formation of products in photoinduced radical reactions between KNA− and amino acids tryptophan (Trp) and tyrosine (Tyr) in free states. Our results have shown that pH variation has minor influence on kinetics of reagent decay and accumulation of products in reactions between tyrosyl and kynurenic acid radicals. However in the case of Trp a two-fold decrease of the reagent degradation without visible changes in the composition of formed products was observed with pH decrease from 7 to 3. Time-resolved measurements have shown similar acidification-induced two-fold acceleration of decay of kynurenyl and tryptophanyl radicals via Back Electron Transfer (BET) with the restoration of initial reagents. Experiments with tryptophan derivatives with different pKa values for their radical forms point out the protonation of tryptophanyl radical as the driving force for BET acceleration at low pH. Our results demonstrate that the protonation of kynurenyl radical does not change its reactivity towards amino acids radicals but the total yield of radical photodamage decreases with the protonation of tryptophanyl radicals. It could be expected that radical induced damage to proteins will depend on the pKa of tryptophanyl radicals within a protein globule.

KW - Back electron transfer

KW - Kynurenic acid

KW - Protonation

KW - Radicals

KW - Tryptophan

KW - Tyrosine

KW - UV-A light

KW - Free Radicals

KW - Oxidation-Reduction

KW - Kynurenic Acid

KW - Humans

KW - Amino Acids

KW - Kinetics

KW - Hydrogen-Ion Concentration

UR - http://www.scopus.com/inward/record.url?scp=85108583704&partnerID=8YFLogxK

U2 - 10.1016/j.freeradbiomed.2021.06.015

DO - 10.1016/j.freeradbiomed.2021.06.015

M3 - Article

C2 - 34146664

AN - SCOPUS:85108583704

VL - 172

SP - 331

EP - 339

JO - Free Radical Biology and Medicine

JF - Free Radical Biology and Medicine

SN - 0891-5849

ER -