Proteolytic inactivation of alpha-1-antitrypsin by human neutrophils: involvement of multiple and interlinked cell responses to phagocytosable targets.

. Neutrophil polymorphonuclear leukocytes (PMN) can inactivate the PMN-elastase inhibitor alpha-1-antitrypsin (A1AT) proteolytically, by using metalloproteinases normally stored as zymogens in myeloperoxidase (MPO)-negative granules. Supernatants from opsonized zymosan (OPZ)-triggered human PMN cleaved and inactivated human AlAT through a process inhibitable by metal-chelators, suggesting that the interaction of PMN with OPZ leads to the extracellular availability of active metalloenzymes. During OPZ-triggering, PMN used approximately 80% of the generated hydrogen peroxide (H2O2) to produce HOCl by means of the MPO pathway, while the remainder was catabolized by PMN themselves. No H2O2 was available as free compound in the extracellular environment and hydroxyl (·OH) or ·OH-like radicals were not generated. The selective deletion of single components of the HOCl-generating MPO pathway resulted in the generation of PMN supernatants free of active metalloenzymes but rich of the corresponding zymogens. Similar results were obtained by replacing normal PMN with cells from a patient with hereditary MPO deficiency. No evidence was obtained for the intervention or contribution of ·OH-like radicals, serine-proteinases and oxidized glutathione in the transformation of the zymogens into enzymes able to inactivate AlAT. On concluding. PMN undergoing phagocytosis release MPO in amount sufficient to handle the extracellular pool of the generated H2O2 entirely, leading to the generation of equimolar amounts of HOCl. In turn, HOCl or a similar compound derived from it interacts with concomitantly released metallozymogens, switching on their AlAT inactivating potential without the apparent contribution of other PMN-derived molecules. As HOCl and its derivatives are well-known to oxidize and inactivate AlAT, the herein shown ability of PMN to transform short-living HOCl into long-acting and AlAT-inactivating enzymatic systems suggest a central role for the HOCl-generating MPO pathway in the PMN-mediated impairment of lung anti-elastase defenses.