Neutrophils autoinactivate secretory products by myeloperoxidase-catalyzed oxidation

The neutrophil response to inflammatory stimuli involves the formation of reactive oxygen species and secretion of granule enzymes. In studying secretion of vitamin B₁₂ binding protein by human neutrophils, we noted a major decrease in total recoverable activity from the extracellular fluid plus the stimulated cells (54% of resting cells). Recovery of B₁₂ binding protein from neutrophils exposed to phorbol myristate acetate or opsonized zymosan was significantly enhanced on addition of heme enzyme inhibitors (azide, cyanide) or catalase or when halide-free medium was used. The changes in B₁₂ binding protein recovery were attributable entirely to increases in extracellular fluid levels, and cell pellet content was unaffected. These data indicate extracellular destruction of functional B₁₂ binding protein by the halide-dependent heme enzyme myeloperoxidase and H₂O₂. Kinetic studies demonstrated rapid secretion of B₁₂ binding protein in the first 2 to 5 minutes, followed by its inactivation over the next 20 to 30 minutes. A cell-free extract of vitamin B₁₂ binding protein was readily inactivated on exposure to purified myeloperoxidase, H₂O₂, and a halide. These findings document a functional interaction among products of the neutrophil specific granules (B₁₂ binding protein), azurophil granules (myeloperoxidase), and metabolic burst (H₂O₂). They provide an interesting model for the modulation of the inflammatory response by oxidation of secretory products of neutrophils.