Neutrophil-mediated endothelial angiotensin-converting enzyme dysfunction: role of oxygen-derived free radicals

We examined the mechanisms whereby phorbol 12-myristate 13-acetate (PMA)-activated rabbit peritoneal neutrophils [polymorphonuclear leukocytes (PMN)] altered endothelial-bound angiotensin-converting enzyme (ACE) activity in cultured bovine pulmonary arterial endothelial cells (EC). PMA or PMN alone had no effect on ACE activity. When PMN were coincubated with PMA (10 ng/ml) for 4 h in Earle's salt solution, endothelial ACE activity was decreased by 87%. No EC cytotoxicity was observed at this time as determined by 51Cr release from prelabeled EC. Activated PMN-mediated decreased ACE activity was inhibited by catalase (2,000 U/ml) but not by superoxide dismutase (300 U/ml). The decrease in ACE activity was also inhibited by the hydroxyl radical scavenger dimethylthiourea (5 mM) but not mannitol (5 mM), which does not cross cell membranes. Pretreatment of EC with the iron chelator deferoxamine mesylate (1-10 mM) for 4 h attenuated the PMN-mediated decrease in ACE activity, as did the thiol reducing agent, 2-mercaptoethanol (0.1 mM), and the myeloperoxidase inhibitor, cyanide (5 mM), but not azide (1-50 mM). Treatment with the proteinase inhibitor phenylmethylsulfonyl fluoride, with human alpha-antitrypsin, or with the nitric oxide synthase inhibitor N omega-nitro-L-arginine had no effect on PMN-mediated ACE dysfunction. These results suggest that PMN-mediated ACE dysfunction may be due to the production of hydrogen peroxide by PMN and its subsequent conversion into hydroxyl radicals.