The inhibition of myeloperoxidase by ceruloplasmin can be reversed by anti-myeloperoxidase antibodies

Background. The purpose of this study was to characterize the recently repo rted inhibition of myeloperoxidase (MPO) by ceruloplasmin and to determine whether this may be disturbed in the presence of anti-MPO antibodies. Methods. Specificity of the binding between ceruloplasmin and MPO was confi rmed by Western blotting and enzyme-linked immunosorbent assay (ELISA), and the enzymatic activity of MPO was measured in the presence of ceruloplasmi n, affinity-purified anti-MPO antibodies, or both. The affinity of the bind ing between MPO and ceruloplasmin and MPO and the anti-MPO antibodies was m easured using a biosensor, with the results confirmed by chaotrope ELISA. Results. Affinity-purified anti-MPO antibodies from patients with microscop ic polyangiitis and florid renal vasculitis inhibited the binding between c eruloplasmin and MPO to a maximum of 72.9 +/- 12.8%, whereas those from pat ients with Wegener's granulomatosis and only minimal renal involvement inhi bited the binding to a maximum of only 36.8 +/- 10.9% (P < 0.001), with com parable reversal of the ceruloplasmin-mediated inhibition of MPO activity. Measurement of the affinity of the interactions demonstrated that binding b etween MPO and the anti-MPO antibodies is stronger than that be tween MPO a nd ceruloplasmin (1.61 x 10(7) to 1.33 x 10(8) vs. 7.46 x 10(6) M-1), indic ating that binding to the autoantibody would be favored in vivo. Conclusions. This study confirms a role for ceruloplasmin as a physiologica l inhibitor of MPO, and demonstrates how the inhibition may be disrupted in the presence of anti-MPO antibodies. Because a majority (16 of 21) of the antibodies did not themselves inhibit MPO activity, their interference with the inhibition mediated by ceruloplasmin may be brought about by steric hi ndrance consequent upon the binding of the antibody to a dominant epitope a t or near the active site.