Elucidating mechanisms of chlorine toxicity: Reaction kinetics, thermodynamics, and physiological implications

Department of Environmental Health Sciences, School of Public Health, University of Alabama at Birmingham, Birmingham, Alabama 35294-0022, USA.
AJP Lung Cellular and Molecular Physiology (Impact Factor: 4.08). 09/2010; 299(3):L289-300. DOI: 10.1152/ajplung.00077.2010
Source: PubMed


Industrial and transport accidents, accidental releases during recreational swimming pool water treatment, household accidents due to mixing bleach with acidic cleaners, and, in recent years, usage of chlorine during war and in acts of terror, all contribute to the general and elevated state of alert with regard to chlorine gas. We here describe chemical and physical properties of Cl(2) that are relevant to its chemical reactivity with biological molecules, including water-soluble small-molecular-weight antioxidants, amino acid residues in proteins, and amino-phospholipids such as phosphatidylethanolamine and phosphatidylserine that are present in the lining fluid layers covering the airways and alveolar spaces. We further conduct a Cl(2) penetration analysis to assess how far Cl(2) can penetrate the surface of the lung before it reacts with water or biological substrate molecules. Our results strongly suggest that Cl(2) will predominantly react directly with biological molecules in the lung epithelial lining fluid, such as low-molecular-weight antioxidants, and that the hydrolysis of Cl(2) to HOCl (and HCl) can be important only when these biological molecules have been depleted by direct chemical reaction with Cl(2). The results from this theoretical analysis are then used for the assessment of the potential benefits of adjuvant antioxidant therapy in the mitigation of lung injury due to inhalation of Cl(2) and are compared with recent experimental results.

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    • "The pH change that this reaction produces can likely be mitigated by the buffering capacity of the lung lining fluid. Oxidative reactions between hypochlorite and nucleophilic lung lining constituents may also occur, with thiols being particularly sensitive to these oxidations (Squadrito et al., 2010). Disruption of local redox homeostasis may promote the inflammatory response through induction of redox sensitive stress signaling, or as a secondary response to protein oxidation with consequent alteration in function. "
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    ABSTRACT: Acute Cl2 exposure following industrial accidents or military/terrorist activity causes pulmonary injury and severe acute respiratory distress. Prior studies suggest that antioxidant depletion is important in producing dysfunction, however a pathophysiologic mechanism has not been elucidated. We propose that acute Cl2 inhalation leads to oxidative modification of lung lining fluid, producing surfactant inactivation, inflammation and mechanical respiratory dysfunction at the organ level. C57BL/6 J mice underwent whole-body exposure to an effective 60 ppm-hour Cl2 dose, and were sacrificed 3, 24 and 48 hours later. Whereas pulmonary architecture and endothelial barrier function were preserved, transient neutrophilia, peaking at 24 hours, was noted. Increased expression of ARG1, CCL2, RETLNA, IL-1b, and PTGS2 genes was observed in bronchoalveolar lavage (BAL) cells with peak change in all genes at 24 hours. Cl2 exposure had no effect on NOS2 mRNA or iNOS protein expression, nor on BAL NO3- or NO2-. Expression of the alternative macrophage activation markers, Relm-α and mannose receptor was increased in alveolar macrophages and pulmonary epithelium. Capillary surfactometry demonstrated impaired surfactant function, and altered BAL phospholipid and surfactant protein content following exposure. Organ level respiratory function was assessed by forced oscillation technique at 5 end expiratory pressures. Cl2 exposure had no significant effect on either airway or tissue resistance. Pulmonary elastance was elevated with time following exposure and demonstrated PEEP refractory derecruitment at 48 hours, despite waning inflammation. These data support a role for surfactant inactivation as a physiologic mechanism underlying respiratory dysfunction following Cl2 inhalation.
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    • "In contrast, preferential effects at higher chlorine concentrations separate from a threshold effect (i.e., a > 1) are possible, but specific underlying mechanisms are difficult to identify. Chlorine is thought to undergo multiple types of reactions with constituents of the epithelial lining fluid of the respiratory tract with consequent depletion of antioxidant defenses, production of secondary reactive products, and damage to cellular and extracellular constituents, such as proteins and lipids (Squadrito et al., 2010). The ultimate toxicity of chlorine is the result of the combined effects of the myriad potential reactions. "
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    ABSTRACT: Chlorine gas is considered a chemical threat agent that can cause acute lung injury. Studies in the early 20th century on war gases led Haber to postulate that the dose of an inhaled chemical expressed as the product of gas concentration and exposure time leads to a constant toxicological effect (Haber's Law). In the present work, mice were exposed to a constant dose of chlorine (100 ppm-h) delivered using different combinations of concentration and time (800 ppm/7.5 min, 400 ppm/15 min, 200 ppm/30 min, and 100 ppm/60 min). Significant effects of exposure protocol on survival evaluated 6 h after exposure were observed, ranging from 0% for the 7.5-min exposure to 100% for the 30- and 60-min exposures. Multiple parameters indicative of lung injury were examined to determine if any aspects of lung injury were differentially affected by the exposure protocols. Most parameters (pulmonary edema, neutrophil influx, and levels of protein, immunoglobulin M, and the chemokine KC [Cxcl1] in lavage fluid) indicated that lung injury was most pronounced for the 15-min exposure and least for the 60-min exposure. In contrast, changes in pulmonary function at baseline and in response to inhaled methacholine were similar following the three exposure regimens. The results indicate that the extent of lung injury following chlorine inhalation depends not only on total dose but also on the specifics of exposure concentration and time, and they suggest that evaluation of countermeasures against chlorine-induced lung injury should be performed using multiple types of exposure scenarios.
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    • "Its reactivity with most substrates exceeds that of hydrogen peroxide, hydroperoxides, and peroxynitrite by several orders of magnitude [25]. The rates constants for the reactions of hypochlorous acid, however, vary over many orders of magnitude [26] [27] [28]. Thus, hypochlorous acid should be considered as a highly reactive but selective oxidant. "
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    ABSTRACT: Myeloperoxidase (donor:hydrogen-peroxide oxidoreductase) is the most abundant protein in neutrophils and is also found in monocytes. It contains two heme-prosthetic groups and is a unique peroxidase that catalyzes the conversion of hydrogen peroxide and chloride to hypochlorous acid. Hydrogen peroxide is formed from the spontaneous dismutation of superoxide, which is produced by an NADPH oxidase in the cell membrane. Hypochlorous acid is the major strong oxidant produced by neutrophils. It has powerful antimicrobial activity, and it is extremely reactive with biological molecules. It inactivates enzymes and α1-antitrypsin, cross-links proteins, and reacts with unsaturated fatty acids to form chlorohydrins, which may destabilize cell membranes. Given this broad spectrum of reactivity, hypochlorous acid is an obvious candidate for causing much of the damage mediated by neutrophils in inflammatory diseases. The ferric or native enzyme (MP3+) reacts with hydrogen peroxide (H2O2) to form the active redox intermediate compound I, which oxidizes chloride (CI-) to hypochlorous acid (HOCl).
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