Mode of action of poly(vinylpyridine-N-oxide) in preventing silicosis: Effective scavenging of carbonate anion radical

Small particles of crystalline silicon dioxide (crystallites) are exceptionally toxic. Inhalation of quartz crystallites causes silicosis, a devastating lung disease afflicting miners, particularly coal and stone workers. Poly(vinylpyridine-N-oxide)s (PVPNOs) have been applied in the prevention and treatment of silicosis, but their mode of action has been obscure. Recently, the sites of inducible ^.NO synthase activation and of nitrotyrosine formation were associated anatomically with the pathological quartz particle-caused lesions in the lungs. It has been suggested that the ^.NO formed combines rapidly with O₂^. to yield ONOO^-, a potential mediator of lung injury following silica exposure. Here, we show that PVPNOs do not react with peroxynitrite but scavenge exceptionally rapidly CO3^.- radicals, which are produced in the decomposition of ONOO^- in bicarbonate solutions. The rate constant for the reaction of CO₃^.- with PVPNO was found to be independent of the type and size of PVPNO, i.e., k = (1.9 ± 0.2) 10 ⁵ M^-1 s^-1 per monomer. In contrast, the rate constant for the reaction of CO₃^.- with the small molecule 4-methylpyridine N-oxide did not exceed 1 × 10⁴ M^-1 s^-1. The underlying reason for the difference is that, in the dissolved polymeric PVPNOs, the electrostatic repulsion between the N-oxide zwitterions destabilizes them, increasing dramatically their pK_a. The protonated N-oxides at physiological pH have abstractable hydrogen atoms and are expected to react rapidly with CO₃^.-, just as cyclic hydroxylamines do. It is also shown that PVPNO inhibits tyrosine nitration by peroxynitrite at pH 7.6 in the presence of excess of CO₂ in a concentration-dependent manner. Hence, binding of PVPNO to the quartz particles and eliminating CO₃^. could prevent the killing of macrophages, the associated release of macrophage-recruiting cytokines, and the amplification of the local concentration of ^.NO by the recruited macrophages. The latter causes necrosis of the macrophage-infiltrated lung tissue and, upon repair of the necrotic lesion, results in the growth of the dysfunctional fibrotic tissue, which is the hallmark of silicosis.