Myeloperoxidase reduces the opsonizing activity of immunoglobulin G and complement component C3b

The effect of myeloperoxidase, hydrogen peroxide (H₂O₂) and a halide (Cl) on the opsonizing molecules in immunoglobulin G (IgG) and complement factor C3b was assayed. At concentrations of the enzyme (1 μg/ml) that can be found in the extracellular fluid during inflammation, the myeloperoxidase-H₂O₂-Cl system inhibited the opsonizing effect of IgG and C3b measured as phagocytiuptake and superoxide generation. The effect was related to the enzymatic peroxidative activity of the protein. The presence of albumin (10 mg/ml) reduced the effect of myeloperoxidase with 10–20%. Taurine, which in the presence of myeloperoxidase-H₂O₂-Cl forms hydrophilic chloramines, and D-penicillamine, which scavenges HOCl, neutralize the inhibitory effect of myeloperoxidase. This suggests that either hypochlorous acid or lipophilic chloramines may exert its effect by oxidizing free sulphydryl groups exposed on the opsonizing ligands. Since the myeloperoxidase-H₂O₂-halide system also affects chemotactic factors, leukotrienes, proteinases and membrane receptors, the system may in several ways affect the development of the inflammatory response.