Environ. Sci. Technol. 2005, 39, 1673-1678
Protein Nitration by Polluted Air THOMAS FRANZE, MICHAEL G. WELLER, REINHARD NIESSNER, AND ULRICH PO ¨ SCHL* Technical University of Munich, Institute of Hydrochemistry, Marchioninistrasse 17, D-81377 Munich, Germany
The effects of air pollution on allergic diseases are not yet well-understood. Here, we show that proteins, in particular birch pollen proteins including the allergen Bet v 1, are efficiently nitrated by polluted air. This posttranslational modification of proteins is likely to trigger immune reactions and provides a molecular rationale for the promotion of allergies by traffic-related air pollution. Enzyme immunoassays have been used to determine equivalent degrees of nitration (EDN) for protein samples exposed to urban outdoor air and synthetic gas mixtures. The observed rates of nitration were governed by the abundance of nitrogen oxides and ozone, and concentration levels typical for summer smog conditions led to substantial nitration within a few hours to days (EDN up to 20%). Moreover, nitrated proteins were detected in urban road dust, window dust, and fine air particulate matter (EDN up to 0.1%).
Introduction Numerous studies indicate that allergic diseases have been on the increase during the past decades (1, 2), and various possible explanations have been invoked, including environmental pollution effects (3). Classical air pollution (type I) with high sulfur dioxide concentrations seems not to be associated with allergies, but traffic-related air pollution (type II) characterized by high concentrations of nitrogen oxides and ozone (summer smog conditions) appears to enhance allergic diseases (4-6). For example, traffic-related air pollution was found to be associated with an increased occurrence of asthma and allergies among children (7); allergic asthma patients exhibited increased bronchial responsiveness to allergen challenges after exposure to traffic exhaust (8), and rhinoconjunctivitis symptoms of individuals allergic to birch and grass pollen were strongly correlated with ambient nitrogen oxides and ozone (9). In animal experiments, airway sensitization was enhanced by NO2 (10) and by O3 (11), and exposure to NO2 abrogated immunological tolerance against inhaled ovalbumin (12). The chemical mechanisms and molecular processes that lead to adverse health effects of NO2 and O3 are, however, still poorly understood. We suggest that protein nitration may play a central role in the promotion of allergies by air pollutants. In the course of inflammatory biological processes, proteins undergo a nitration reaction that leads to the formation of 3-nitrotyrosine residues (i.e., to the introduction of a NO2 group vicinal to the OH group on the phenyl ring of the aromatic amino acid tyrosine (2-amino-3-(4-hydroxyphenyl)-propanoic acid) (13, 14)). It is not clear for which purpose this nitration reaction occurs in biological systems. A possible explanation is that NO2 groups may serve as * Corresponding author phone: +49-89-2180-78238; fax: +4989-2180-78255; e-mail:
[email protected]. 10.1021/es0488737 CCC: $30.25 Published on Web 01/29/2005
2005 American Chemical Society
markers for foreign proteins and guide the immune system. For example, 2,4-dinitrophenyl groups are coupled to proteins and peptides to boost the immune response (15), and peptides containing 3-nitrotyrosine were recently found to evade central immune tolerance and cause robust immune response (16). Landsteiner and Jacobs have shown in 1935 that guinea pigs are easily sensitized by injections or skin contact of nitrated aromatics such as 1,2,4-chlorodinitrobenzene, which can form covalent conjugates with proteins (17). Berrens et al. showed that posttranslational modification of pollen allergens significantly increased their IgE binding capacity; even nonallergenic proteins were rendered active by a conjugation reaction (18). Petersen et al. found significantly enhanced IgE binding to posttranslational modifications of the grass pollen allergen Phl p 1 (19), and Huby reported that posttranslational modifications appear generally to enhance allergenicity (20). Because of antibody multispecifity and promiscuous binding, both nitrated and native proteins could cause allergic symptoms if a nitrated protein led to immunological sensibilization (21, 22). Thus, the contact with nitrated proteins or nitrating substances from the environment might trigger immune reactions and promote the genesis of allergies. From atmospheric research, it is known that phenols and other aromatic compounds can be nitrated by NO3 radicals formed upon reaction of NO2 with O3 (23-28), indicating that tyrosine residues contained in proteins may as well be nitrated when exposed to atmospheric nitrogen oxides. Recently, we have shown that proteins can indeed be nitrated by NO2 and O3 at atmospherically relevant concentrations (29). Nitration of tyrosine residues also has been observed for proteins in aqueous solution reacting with peroxyacetylnitrate (PAN) (30), which is another reactive nitrogen species occurring in the atmosphere. Therefore, proteins contained in biogenic aerosol particles (pollen, spores, bacteria, plant debris, etc.) may be nitrated in polluted environments before inhalation and deposition in the human respiratory tract. Moreover, proteins in living organisms may be nitrated upon exposure to polluted air, leading to some of the adverse health effects observed upon inhalation of nitrogen oxides (3, 5, 10). Proteins account for up to ∼5% of urban air particulate matter, influence the physicochemical properties of atmospheric particles, and play a major role as airborne allergens from wind-driven and traffic-related suspension of soil and road dust (31-33). They are not only contained in coarse biological particles such as pollen grains (diameter >10 µm) but also in the fine fraction of air particulate matter (diameter
