Oxygen Uptake in the Vitamin B₂‐sensitized Photo‐oxidation of Tyrosine and Tryptophan in the Presence of Uracil: Kinetics and Mechanism

Abstract

Considering the significance of visible light-promoted reactions in complex biological media, the photo-oxidation of the amino acids (AAs) tyrosine (tyr) and tryptophan (trp) was studied in the presence of the naturally occurring oxidative scavenger uracil (ur). The involved photoprocesses, studied at pH 7 and 9, are driven through the reactive oxygen species (ROS) singlet molecular oxygen (O2(1Deltag)), superoxide radical anion (O2*-) and hydrogen peroxide (H2O2). The effect on the effectiveness of the overall photo-oxidation process due to the presence of an added electron-donating substrate such as ur is not straightforwardly predictable. The addition of the pyrimidine compound, a much lesser photo-oxidizable substrate than the AAs themselves, produced different results: (1) antioxidative for tyr at pH 9, decreasing the overall rate of oxygen uptake; (2) synergistic for tyr at pH 7, increasing the oxidation rate more than the corresponding addition value of the respective individual rates and (3) no effect for trp at both pH values. The final result depends on the respective abilities of the substrates as quenchers of both the long-lived riboflavin triplet excited state and the generated ROS and the pH of the medium. An interpretation for the different cases is attempted through a kinetic and mechanistic analysis.

Antiglycating Effect of Phenolics from the Chilean Currant Ribes cucullatum under Thermal Treatment

“…The increase in the amount of oxygen after the addition of catalase indicates the generation of H 2 O 2 in our system, considering that catalase decompose H 2 O 2 to O 2 and H 2 O [ 44 ]. This approach has been used before to demonstrate the generation of reactive oxygen species [ 45 , 46 ]. Therefore, a contribution of free radical reactions to the protein crosslinking and oxidation cannot be discarded.…”

Chloroplast redox state changes mark cell‐to‐cell signaling in the hypersensitive response

“…Uracil, a ROS inhibitor (Montaña et al ., 2009 ; Abdollahi & Ghahremani, 2011 ), was infiltrated into PVY‐N605(123)‐GFP‐inoculated leaves of pt‐roGFP L2 plants with 100 μM uracil (Sigma; U0750‐25G in 224 μM NaOH diluted in ddH 2 O; left side of the leaf) at 4 dpi. As a control, the right side of the leaf was inoculated with 224 μM NaOH diluted in ddH 2 O.…”