RESUMO
The fifth component of the complement cascade, C5a, was iodinated using the Bolton-Hunter reagent. Results from the present study, using the high affinity ligand, [125I]Bolton-Hunter-labeled C5a ([125I]BH-C5a), revealed a single binding site on membranes prepared from human neutrophils, U-937 cells and human monocytes. Saturation studies demonstrated Bmax values in these cells of 11.5, 47.3 and 16.6 fmol/10(6) cells, respectively. The C5a receptor demonstrated a very high affinity for [125I]BH-C5a of approximately 4 pM in each cell type. Competition studies using analogs of C5a generated a similar order of potency in each of the cell types of C5a > or = C5a(1-74), Ser66Ala > C5a(1-73) > C5a(1-69). These studies indicate that [125I]BH-C5a labels a similar receptor in neutrophil, U-937 cell and monocyte membranes. Furthermore, C5a(1-73) produced shallow inhibition curves in competition experiments in each cell type. Computer analysis of the binding data revealed two components of binding. When 10 nM unlabeled C5a was used to initiate dissociation of [125I]BH-C5a binding in neutrophil membranes, two binding components were observed. In addition, dissociation of [125I]BH-C5a binding by 10 nM unlabeled C5a in the presence of 1 mM GppNHp decreased the percentage of binding to the slowly dissociating, high affinity binding component from 84 to 58%. These results suggest that multiple states of the C5a receptor exist.
