The ability of H to modulate C5 utilization by the cell-bound alternative pathway C5 convertase differs on sheep and rabbit erythrocytes

Abstract

The ability of H to inhibit formation of the alternative pathway C3 convertase differs on the non-activating cells sheep erythrocytes (E^s) and on the activating cells rabbit erythrocytes (E^r) or desialated sheep erythrocytes (E^des). C3 convertase sites are converted to C5 convertase sites by deposition of additional C3b molecules and C5 binds to cell-bound C3b in a reaction that is inhibited by H. C5 convertase sites P(C3b)_nBb were formed on E^s, E^r, E^des and revealed by incubation of the cells with purified C5 and with rat serum treated with KSCN and hydrazine. When incremental amounts of H were added to the reaction, a dose-dependent inhibition of C5 utilization was observed. The amount of H that inhibited 50% of C5 utilization was the same for a given cell regardless of the number of C5 convertase sites that were expressed on the cell. However the 50% inhibitory dose of H was 92 ng/10⁷ E^sP(C3b)_nBb, 87 ng/10⁷ E^desP(C3b)_nBb while it was at least 380 ng/10⁷ E^rP(C3b)_nBb. These results suggest that H competes with C5 for uptake within the C5 convertase site and that the regulatory effect of H on C5 cleavage depends on the surface to which the convertase is bound.