Kinetic and mechanistic behavior of the ascorbate ion-glutathione mixture in the visible-light-mediated oxidation of tryptophan

Abstract

Within the frame of the frequently invoked correlation between antioxidant contents and potential antioxidative protection, the stability of the amino acid (AA) tryptophan (trp), taken as a prototype of photooxidizable-relevant biological target, was evaluated in the presence of the known antioxidant ascorbic acid (AsA). The peptide glutathione (GSH) was also included in order to eliminate thermal decomposition of AsA. Riboflavin (Rf, vitamin B2), endogenous in most biological environments, was employed as a photosensitizer. A systematic kinetic and mechanistic study was conducted under aerobic conditions in pH 7 aqueous medium. The evaluation of oxygen uptake rates (OUR) in the photoirradiated mixtures, taken as an overall indicator of the total oxidation degree of the mixture components, constitutes an interesting and useful tool in this case. In the presence of Rf, the isolated AA and AsA undergo photodegradation under conditions currently found in natural media. The photodegradations occur in a non-simple reaction, mediated by reactive oxygen species (ROS) such as singlet molecular oxygen (O2(1g)), superoxide radical anion, and hydrogen peroxide and hydroxyl-radical. All ROS are photogenerated from triplet excited Rf after interaction with the ground state of AsA and trp. The photoirradiation also triggers an already described parallel thermally-driven and dark reaction on AsA which is stopped by the addition of GSH. The global outcome can be interpreted as a sort of conjunctive antioxidant protection exerted on the system AsA + trp + GSH upon Rf-photosensitization, a result that cannot be predicted from the individual behavior of the mixture components.