Human eosinophilic peroxidase: role in bactericidal activity

Abstract

Peroxidase, present in both neutrophils and eosinophils, is thought to be involved in the bactericidal activity of leukocytes, through either the decarboxylation of amino acids in the presence of chloride or the iodination of proteins in the presence of iodide. We examined these activities in sonic extracts of purified neutrophils and eosinophils and related them to the ability of the extracts to kill bacteria. In contrast to the neutrophil, the eosinophil cell-free system is unable to decarboxylate L-alanine-1- 14 C. Decarboxylation by the neutrophil peroxidase is dependent on the hydrogen peroxide concentration, with inhibition at high levels. Peroxidase-mediated decarboxylation showed a pH optimum at 4.5 and was dependent on the type of buffer employed, in order of increasing activity, N-(2-acetamido)-iminodiacetic acid (ADA), Tris-maleate, sodium phosphate, and sodium acetate. Iodination, measured as the ability to convert 125I into a trichloroacetic acid-precipitable form, was greater in resting eosinophils than in resting neutrophils. Both leukocyte types show increased iodinating ability upon phagocytosis of zymosan. When iodide was used as the halide, eosinophil and neutrophil cell-free preparations showed bactericidal activity as measured by the reduction in viability of Staphylococcus aureus and Escherichia coli. In contrast, with chloride as the halide only the neutrophil system could effectively kill both organisms. This finding is in agreement with the observation that eosinophilic peroxidase is unable to catalyze the decarboxylation reaction, and it suggests the importance of this mechanism in bactericidal activity.