Interaction between myeloperoxidase and antibodies against myeloperoxidase measured by chemiluminescence

We investigated interaction between antibodies directed against myeloperoxidase (anti-MPO) and myeloperoxidase (MPO) with chemiluminescence. Whole serum diluted 1/10 containing circulating anti-MPO antibodies as well as serum from normal blood donors inhibited myeloperoxidase (MPO) enzyme activity when incubated with 1.42 micrograms MPO. When further diluted the inhibitory effect was abolished. Incubation with purified human IgG fraction of anti-MPO, did not cause any inhibition when diluted 1/10 and incubated with 1.42 micrograms MPO. When adding MPO to normal sera a rapid increase of the enzyme activity was seen above 5 micrograms, and the inhibitory effect was completely abolished when 10 micrograms was added. Both sera from healthy individuals, as well as sera from patients with circulating anti-MPO inhibited MPO activity. The inability of pure IgG fractions from anti-MPO sera to inhibit MPO enzyme activity, clearly indicates the presence of an inhibitory factor, unspecific or specific, in serum.