Kinetics of ozone reaction with low-molecular weight antioxidants at physiologically relevant conditions

Ozone (O3), a major component of photochemical smog is a highly reactive gas that causes a decrement in lung function and lung inflammation upon inhalation. Respiratory tract lining fluid (RTLF) contains natural antioxidants such as uric acid (UA), ascorbic acid (AH2), and glutathione (GSH) that protect the underlying epithelium from toxic gases such as O3. The purpose of this study was to better understand and quantify the reaction kinetics between O3 and these antioxidants under conditions that are similar to those encountered when polluted air is inhaled into the respiratory system. To achieve this objective, we developed a semi-batch reactor in which a 300 to 800 ml/min flow of air containing 1-5 parts per million by volume (ppm) of O3 contacted 3 ml of a well-stirred test solution. The concentration of O3 at the outlet of the reactor and the concentration of antioxidant in the aqueous solution were measured at reaction times of 5, 10 and 15 minutes, allowing the simultaneous determination of O3 absorbed and antioxidant consumed.; Data was collected on test solutions consisting of single-component, binary and ternary mixtures of 50-200 muM UA, AH2 and GSH. Kinetic parameters were determined using a mathematical model in which the squared-error between the observed amounts of O3 absorbed and antioxidant consumed and their simulated values was minimized. The results obtained in single-component solutions of the antioxidants indicated that: (1) mass transfer resistances in the gas and liquid phases were negligible; (2) the reaction of O 3 with UA or with AH2 has a one-to-one (1:1) stoichiometry; (3) the reaction of O3 with GSH has a 1:2.5 stoichiometry; and (4) the reactivity between O3 and these antioxidants is in the following order: UA ≈ AH2 > GSH. The rate constants estimated from these data for O3 reaction with UA, AH2, and GSH were 5.8 x 104 M-1 sec-1, 5.5 x 104 M-1 and 57.5 M -0.75 sec-1, respectively. Comparing the previous studies with the current investigation indicates: (1) with the exception of one study, all the previously-reported rate constants of UA and AH2 with O3 are two to three orders of magnitude larger than our values; (2) all the reported data agree with our conclusion that the reaction of O3 with UA or with AH2 is first-order with respect to O3, as well as the antioxidant; (3) our study is the only one that provides clear evidence of a non-first-order reaction between O 3 and GSH.; For binary and ternary mixtures of the antioxidants, we found that: (1) the consumption of UA and AH2 are decreased in the presence of one another. Numerical simulations verified that this protective effect is mostly due to the competitive reaction of UA and AH2 toward O3; (2) both UA and AH2 can attenuate the consumption of GSH in the binary mixtures whereas GSH does not have the same effect on UA and AH 2. It was necessary to employ an interaction parameter in the numerical simulation in order to account for this; and (3) the presence of UA and AH2 simultaneously in the mixture also diminished GSH consumption.; We also studied the kinetics of reaction between O3 and albumin. Concentrations of tryptophan residues in albumin ranged from 6 to 20 muM. By employing a procedure similar to the one used for the antioxidants we found a stoichiometry of 1.5:1.0 and an optimal rate constant of for 1.22 x 108 M-1.5 sec-1 reaction between O3 and a tryptophan residue in albumin. The effectiveness of GSH for protecting tryptophan groups from O3 oxidation was examined in binary solutions of GSH and albumin. The Concentrations of GSH in the solutions ranged from 0 to 200 muM, whereas equivalent tryptophan concentrations were varied from 0 to 20 muM. The results indicated that GSH and tryptophan groups can attenuate the consumption of one other. However, the changes in GSH consumption caused by tryptophan residues were not as drastic as the changes in tryptophan consumption caused by GSH. Therefore, GSH is more effective in protecting tr...