Oxidation of chloride and thiocyanate by isolated leukocytes

Peroxidase-catalyzed oxidation of chloride (Cl-) and thiocyanate (SCN-) was studied using neutrophils from human blood and eosinophils and macrophages from rat peritoneal exudates. The aims were to determine whether Cl- or SCN- is preferentially oxidized and whether leukocytes oxidize SCN- to the antimicrobial oxidizing agent hypothiocyanite (OSCN-). Stimulated neutrophils produced H2O2 and secreted myeloperoxidase. Under conditions similar to those in plasma (0.14 M Cl-, 0.02-0.12 mM SCN-), myeloperoxidase catalyzed the oxidation of Cl- to hypochlorous acid (HOCl), which reacted with ammonia and amines to yield chloramines. HOCl and chloramines reacted with SCN- to yield products without oxidizing activity, so that high SCN- blocked accumulation of chloramines in the extracellular medium. Under conditions similar to those in saliva and the surface of the oral mucosa (20 mM Cl-, 0.1-3 mM SCN-), myeloperoxidase catalyzed the oxidation of SCN- to OSCN-, which accumulated in the medium to concentrations of up to 40-70 microM. Sulfonamide compounds increased the yield of stable oxidants to 0.2-0.3 mM by reacting with OSCN- to yield derivatives analogous to chloramines. Stimulated eosinophils produced H2O2 and secreted eosinophil peroxidase, which catalyzed the oxidation of SCN- to OSCN- regardless of Cl- concentration. Stimulated macrophages produced H2O2 but had low peroxidase activity. OSCN- was produced when SCN- was 0.1 mM or higher and myeloperoxidase, eosinophil peroxidase, or lactoperoxidase was added. The results indicate that SCN- rather than Cl- may be the physiologic substrate (electron donor) for eosinophil peroxidase and that OSCN- may contribute to leukocyte antimicrobial activity under conditions that favor oxidation of SCN- rather than Cl-.