Influence of ozone on traffic-related particulate matter on the generation of hydroxyl radicals through a heterogeneous synergistic effect
Introduction
High levels of ambient traffic-related particulate matter (PM) have been reported to be associated with increased human morbidity and mortality [1]. Ambient PM contains numerous carcinogenic and toxic substances, heavy metals and stable quinoid radicals. Also, traffic-related particles are potent oxidants, oxidizing important biological molecules, producing reactive oxygen species (ROS) and directly attacking cellular compartments [2], [3]. Ozone (O3) is a highly reactive oxidant gas and a major component of air pollution, especially photochemical smog [4].
Many epidemiologic studies have indicated substantial association between ambient ozone concentrations and adverse respiratory lung diseases [5], [6]. When inhaled, ozone can induce adverse health effects in human lungs and cause airway inflammation. Ozone induces oxidative stress in two stages, the first by O3 and its bioactive reaction products, and the second by the activated respiratory tract inflammatory processes [7], [8]. A primary target of ozone in the lungs is the alveolar epithelium. Ozone is known to induce epithelial cytotoxicity, DNA damage and injury through acute chronic oxidative stress, which ultimately produces necrosis, sloughing and increased epithelial permeability [9], [10]. Ozone has been shown that is able to induce lung tumors in experimental animals, through free radical mechanisms and especially by the formation of HO radicals [11]. Experiments with DNA in aqueous solution showed that ozone itself interacts with DNA in a dose-dependent increase of the 8-oxo-7, 8-dihydro-2′-deoxyguanosine (8-oxo-dG) oxidative damage, which can partially inhibited by hydroxyl radical scavengers. These results suggest that DNA mutagenic damages were caused by ozone via the production of HO radicals (enhanced with piperidine treatment) [12].
Ozone and PM have traditionally been considered as separate environmental pollutants, but several recent studies and epidemiological evidence indicate that, there is a positive association between airborne PM and O3 and hospital admissions for respiratory diseases. The relative risks are mainly for increases of 100 μg/m3 in daily PM10 and 50 ppb in daily ozone concentrations [13], [14], [15]. Diesel exhaust particles (DEP) generate substantial inflammatory effects in airways of healthy subjects, but when exposed to DEP and ozone together a significant increase in neutrophils and myeloperoxidase was seen in their sputum [16]. Experiments in vivo indicate a synergistic or additive effect of ozone and PM. When rats were exposed to mixtures of O3 and various fine ambient PM there was an increase in the toxicity effects [17]. A number of recent studies, showed that ambient concentrations of ozone can increase the biological potency of diesel exhaust particles in rat lungs [18]. Also, inhalation of urban particles at a level that causes few effects in the lung of rats, when inhaled in combination with ozone appeared to potentiate cell injury and interstitial inflammation [19]. Ozone exposure enhances the toxicity of DEP in human airway epithelial cells by augmenting IL-8 gene expression (a potent chemoattractant of neutrophils in the lungs) [20].
The role of ambient traffic-related particles in the atmosphere has elicited a great deal of recent interest due to the possible influence of heterogeneous reactions with ozone and other gases. The heterogeneous uptake of ozone by mineral dust aerosols, the influence of relative humidity and the process of catalytic transformation at active surface sites have been studied [21]. A recent study showed that ozone-initiated oxidation reactions of volatile organic compounds significantly affected concentrations of ultra-fine particles in confined spaces [22].
Also, relevant studies observed the transformation of ozone into HO from experiments measuring its high oxidation efficiency in water treatment. Ozone's oxidation potential increases substantially with the presence of activated carbon (with high basicity pyrrol groups) or carbon black, enhancing its transformation into HO. Activated carbon acts as an initiator or promoter for the ozone decomposition. Textural characteristics, such as large pores and surface area, of activated carbon are governing factors for ozone decomposition into HO [23], [24].
Additionally, it has been shown by various experimental results that ROS produced by airborne particulate matter, including the highly reactive and damaging HO, can cause severe oxidative stress within cells through the formation of oxidized cellular macromolecules, including nucleic acids, proteins and lipids [25], [26]. The formation of the oxidatively damaged DNA nucleosides such as 8-hydroxy-2′-deoxyguanosine (8-OHdG) has been associated with aging and carcinogenesis [27], [28].
The intent of the present study was to determine the ability of ozone and traffic-related airborne PM (in aqueous buffered mixtures, at various pH values) to generate synergistically increasing amounts of hydroxyl radicals (HO), compared to individual action of O3 or PM. Particles used in this study include: PM10, PM2.5, diesel exhaust particles and gasoline exhaust particles (GEP). This work was set-up to test also the mutagenic oxidative damage caused by synergistic or additive effect of O3 and PM in aqueous mixtures (at physiological pH 7.4) by the hydroxylation of 2′-deoxyguanosine (dG) in the C(8) position and the formation of 8-hydroxy-2′-deoxyguanosine (8-OHdG), as a measure of direct generation of ROS.
Section snippets
Chemicals
5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Aldrich Chemical Co., Milwaukee, WI. 2′-Deoxyguanosine was from Aldrich. Other fine chemicals of reasonable purity were purchased from Merck and Fluka. Water used in our experiments was filtered with ion-exchange resins and double distilled (also, water was checked for iron ions).
Particulate matter samples
Three samples from PM10 and PM2.5 of ambient suspended particles were collected from a sampling site (considered as highly polluted urban area) in the center
Generation of HO in O3 or PM aqueous mixtures
The science of electron paramagnetic resonance spectroscopy is very similar in concept to the more familiar nuclear magnetic resonance (NMR) technique. EPR is used to record the spectra of free radicals, i.e. an atom, molecule or ion containing one unpaired electron. Free radicals (like the hydroxyl radical, HO) are extremely unstable, thus spin traps (nitroxides, e.g. DMPO) are used to stabilize them temporarily in order to obtain their EPR spectra by forming the DMPO–OH adduct.
Experimental
Acknowledgements
We gratefully acknowledge the financial support by the Research Grants Committee of the University of Athens, and the Director of the Laboratory of Organic Chemistry for the use of chemicals and various scientific instruments.
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