Myeloperoxidase, iodide, and H2O2 have a bactericidal effect on Escherichia coli. Myeloperoxidase can be replaced in this system by lactoperoxidase or by a guinea pig leukocyte particulate preparation, H2O2 by an H2O2-generating system such as glucose and glucose oxidase, and iodide by thyroxine or triiodothyronine. The bactericidal effect was high at pH 5.0 and fell as the pH was increased.
Preincubation of myeloperoxidase, iodide, and H2O2 for 30 min before the addition of the bacteria largely prevented the bactericidal effect. Thus, the organisms must be present in the reaction mixture during iodide oxidation for maximum killing, which suggests the involvement of labile intermediates of iodide oxidation rather than the more stable end products of oxidation such as iodine.
Iodination of the bacteria by the myeloperoxidase-iodide-H2O2 system was demonstrated chemically and radioautographically. Iodination and the bactericidal effect were similarly affected by changes in experimental conditions in all the parameters tested (effect of preincubation, pH, and inhibitors).
Phagocytosis of bacteria by guinea pig leukocytes was associated with the conversion of iodide to a trichloroacetic acid-precipitable form. Iodide was localized radioautographically in the cytoplasm of human leukocytes which contained ingested bacteria. Iodide fixation was not observed in the absence of phagocytosis or in the presence of Tapazole.