The potent cytotoxic capacity of eosinophils for parasites and host tissue has in part been attributed to the catalytic action of eosinophil peroxidase (EPO), which preferentially oxidizes Br- to the powerful bleaching oxidant HOBr in buffers that mimic serum halide composition (100 mM Cl-, 20-100 μM Br-, < 1μM I-). However, serum also contains 20-120 μM SCN-, a pseudohalide whose peroxidative product, HOSCN, is a weak, primarily sulfhydryl-reactive oxidant. Because of its relative abundance and high oxidation potential, we hypothesized that SCN-, not Br- or I, is the major substrate for EPO in physiologic fluids. We find that in Earle's buffer (100 mM Cl-) supplemented with 100 μM Br- and varying concentrations of SCN-, HOBr production by activated eosinophils and purified EPO, assayed by conversion of fluorescein to dibromofluorescein, was 50% inhibited (ID50) by only 1 μM SCN-. SCN- also blocked (ID50 10 μM) EPO oxidation of I- to HOI, assayed as iodofluorescein, despite the presence of 100 μM (i.e. grossly supraphysiologic) I-. Thionitrobenzoic acid oxidation kinetics indicate that SCN- is the initial species oxidized by EPO in equimolar mixtures of SCN- and Br- and in human serum. EPO also catalyzed the covalent incorporation of [14C]SCN- into proteins in buffers regardless of Br- concentration and in human serum. Comparing the cytotoxicity of HOSCN and HOBr for host cells, we find that even subphysiologic concentrations of SCN- (3.3-10 μM) nearly completely abrogate the potent Br--dependent toxicity of EPO for 51Cr-labeled aortic endothelial cells and isolated working rat hearts, recently developed models of eosinophilic endocarditis. Thus, HOSCN, hitherto best known as a bacteriostatic agent in saliva and milk, is likely also the major oxidant produced by EPO in physiologic fluids, and the presence of SCN- averts damage to EPO-coated host tissues that might otherwise accrue as a result of HOBr generation. In view of these findings, the potential role of HOSCN in eosinophil killing of parasitic pathogens deserves close examination.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - 1991|