Inhibition of hypochlorous acid by lidocaine and native components of alveolar epithelial lining fluid

Alveolar epithelial lining fluid (ELF) contains several antioxidant substances that may provide in vivo protection. We studied the ability of ELF and ELF components to inhibit the neutrophil oxidant hypochlorous acid (HOCl). Normal bronchoalveolar lavage fluid containing ELF was incubated with physiologically relevant concentrations of HOCl (0.04 mM). After incubation, residual HOCl activity was titered by the iodide method. The inhibitory activity of lavage fluid was unexpectedly strong. For example, lavage fluid diluted 20-fold in the assay system quenched 49% of starting HOCl. We initially postulated that ELF total protein and glutathione would account for most of the inhibition of HOCl. However, several experimental approaches demonstrated that the total protein and glutathione concentrations in diluted lavage fluid were too low to explain the observed inhibition. Instead, the majority of HOCl inhibition was due to the lidocaine used for upper airway anesthesia. Reagent lidocaine exhibited strong reactivity in the HOCl assay system. Furthermore, the lavage fluid lidocaine concentration (32.4 ± 6.9 μg/ml) was sufficient to explain most of the observed quenching activity. Additional experiments explored the hypothetical quenching activity of ELF components devoid of lidocaine. These findings demonstrate the technical problems posed by lidocaine in antioxidant studies involving lavage fluid or ELF.