BINDING AND INHIBITION OF MYELOPEROXIDASE (MPO) - A MAJOR FUNCTION OFCERULOPLASMIN

Interactions between plasma proteins and MPO were studied. The protein fraction of normal plasma and serum was shown to exhibit an inhibitor y effect on the peroxidase activity of MPO. Most of the inhibitory eff ect could be retained on an MPO-coupled affinity chromatography column . In particular, a protein with apparent mel, wt of 130 kD showed affi nity for MPO. The protein was identified as ceruloplasmin by N-termina l amino acid sequencing and immunochemistry. During separation procedu res the peroxidase inhibitory effect was limited to ceruloplasmin-cont aining fractions of plasma. Purified ceruloplasmin inhibited the perox idase activity of MPO in a concentration-dependent manner, and exhibit ed selective binding to MPO-coated microtitre plates. This binding cou ld be inhibited by MPO dissolved in buffer. Correspondingly the bindin g of MPO to ceruloplasmin-coated plates could be blocked by ceruloplas min in solution, showing a physical interaction to occur between the t wo proteins under physiological conditions. We also found affinity to exist between MPG and C3 (and its C3d-containing fragments). However, C3 and C3 fragments did not inhibit the peroxidase reaction in vitro. We propose that ceruloplasmin takes part in the clearance and inactiva tion of MPG, in vivo. We also speculate that impaired inactivation of MPO may have a pathophysiological role in inflammatory diseases charac terized by autoantibodies to MPG, such as rapidly progressive glomerul onephritis with P-ANCA (perinuclear anti-neutrophil cytoplasmic antibo dies).