Ozone Uptake in the Human Nasal Cavity: The contribution of Uric Acid
Open Access
- Author:
- Fassih, Ali
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 12, 2007
- Committee Members:
- James S Ultman, Committee Chair/Co-Chair
Abdellaziz Ben Jebria, Committee Member
Ali Borhan, Committee Member
Antonios Armaou, Committee Member
Rebecca Bascom, Committee Member - Keywords:
- uric acid
nasal
antioxidant
uptake
ozone
respiratory - Abstract:
- Ozone, a highly reactive, oxidative gas, is the most harmful component of urban smog. The large surface area of the respiratory airway makes it particularly vulnerable to O3 oxidation. As a protective barrier, a thin coating of mucus blankets the airway surface to prevent direct contact between inhaled toxins and underlying epithelial cells. Furthermore, this lining layer contains an array of biomolecules including antioxidants, which reactively deplete O3 levels. Previous work has proposed that the antioxidant uric acid (UA) is the major target of O3 oxidation in the nasal lining fluid (NLF). It has been postulated that O3 is so reactive that it is depleted before fully penetrating the NLF and contacting epithelial cells. This suggests that harmful effects of O3 must be mediated by toxic products of O3 oxidation, such as lipid peroxides and aldehydes. This work investigated the role of NLF uric acid in modulating O3 absorption in the human nasal cavity. Ozone absorption was measured as the fractional uptake (L) of O3 from a humidified air stream containing 0.36 parts per million (ppm) O3, and flowing through the nose unidirectionally at 3 liters per minute (lpm). Contents of the NLF were sampled by nasal lavage with saline. Previous studies report that a relationship between daily measurements of L and UA concentration in the NLF (CUA,NLF) cannot be observed due to significant day-to-day variations in parameters affecting L. To overcome this problem, several perturbations were imposed to induce changes in CUA,NLF, and therefore L, allowing same-day comparison of these parameters. In a preliminary study, we investigated the effect of continuous O3 exposure on Lin fifteen subjects. It was hypothesized that O3 exposure would temporarily deplete CUA,NLF, resulting in lowered L. Results showed that L was significantly reduced (p<0.001) following exposure to 0.36 ppm O3 for 30 minutes at 3 lpm. In a subsequent study of twenty-five subjects, we aimed to relate values of L and CUA,NLF before and after O3 exposure. Values of L and CUA,NLF were significantly reduced following O3 exposure and were strongly correlated with each other (p<0.001). Regression of these data indicated that L=0when CUA,NLF=0, suggesting a major contribution of UA in modulating O3 uptake. Reaction-diffusion modeling of the data yielded an apparent second order reaction rate constant between O3 and UA (k2) of 1.56x109 M-1s-1. In a third study, we investigated effects of the oxidant gas nitrogen dioxide (NO2) on L in twelve subjects. Exposure to 1.0 ppm NO2 at 3 lpm for 30 minutes resulted in a small reduction of L, and no change in CUA. Results of in vitro experiments verified minimal reactivity between gaseous NO2 and UA in aqueous solution. Although NO2 is an oxidative gas capable of depleting various NLF compounds, it appears that none of these compounds played a major role in modulating O3 absorption. Therefore, the facts that 1) CUA,NLF was not influenced by NO2 exposure and 2) NO2 exposure induced only a small reduction in L are in agreement with the assumption that UA is the major NLF target of O3. Analysis of data from a previous study (Santiago, 2001) that employed serial nasal lavages to dilute UA levels in NLF showed a strong correlation (p<0.001) between values of CUA and the corresponding L values at each sampling time. The apparent k2 obtained from this data was 6.04 x108 M-1s-1, which is the same order of magnitude as the value obtained in O3 exposure studies. However, the results of this analysis indicated that there can be O3 transport in the absence of UA, likely due to oxidation of exposed cell membranes following serial nasal lavage challenges. A concentration profile for O3 in the NLF was simulated with the reaction-diffusion model employing a value for k2=109 M-1s-1 and CUA,NLF=200 uM. Ozone penetration distance into the NLF was estimated to be 0.6 um, which was less than reported NLF thickness values of 5-10 um, indicating that O3 itself cannot not penetrate the NLF to reach underlying cells. Therefore, as a part of the third study, we attempted to detect secondary products of O3 oxidation, such as lipid peroxides and aldehydes, that might explain how O3 exposure causes adverse health effects. Results demonstrated significant production of TBARS, a marker of lipid peroxidation, immediately following and 60 minutes following O3 exposure. Interestingly, increased levels of CUA,NLF appeared to reduce absolute TBARS formation, suggesting a secondary protective role for UA. Increased cellular production of gaseous nitric oxide (NO) has been implicated during inflammation. Therefore, in the first study, nasal air was monitored before and after O3 exposure as an indicator of an inflammatory response to O3. Our data showed a small, but significant elevation of NO one hour following initiation of O3 exposure, suggesting an emerging inflammatory response. In summary, we have provided in vivo evidence that UA is the major target of O3 oxidation in the human nose. The observed value for k2, and corresponding O3 penetration depth, provides evidence that O3 does not reach underlying cells, suggesting that secondary ozonation compounds are responsible for harmful effects of O3 exposure. In support of this, TBARS levels were significantly elevated following O3 exposure. Additionally, we show that there may be an inhibitory role of UA in preventing the formation of secondary ozonation compounds. Finally, our results indicate a cellular response to O3 exposure as measured by NO.