An antimicrobial system in polymorphonuclear neutrophils (PMN) consisting of myeloperoxidase and hydrogen peroxide has been proposed. This system appears to be activated during phagocytosis as a result of the stimulated metabolic activities. A lysed-granules (LG) fraction was prepared from guinea pig exudative PMN. LG alone possessed bactericidal activity which was related to the pH of the reaction; the lower the pH, the more marked the activity. When low concentrations of both H202 and LG were combined under conditions where neither factor alone exhibited significant killing power, there was a striking increase in bactericidal activity. This enhanced activity was much greater than an additive effect. The LG-peroxide antibacterial system was most active over a pH range of 4.0 to 6.0. The activity of the LG-peroxide system was essentially abolished by peroxidase inhibitors, NaN3, KCN, and aminotriazole. The antibacterial activity of this system was nonspecific in nature, being equally effective against gram-negative and gram-positive organisms.

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