Interaction of human monomeric C3b with its receptor (complement receptor type 1, CR1) on neutrophils. Evidence for negative cooperativitiy

The binding of highly purified monomeric ¹²⁵I-C3b to its receptor (CR1) on resting human polymorphonuclear neutrophils (PMN) was analyzed under equilibrium conditions, at 4°C and low ionic strength. Scatchard analysis of specific binding data yielded curvilinear concave upward plots, which resulted from the presence of site-site interactions of the negative type among PMN C3b-receptors (negative cooperativity), as shown by dissociation kinetic experiments. Indeed, the dissociation rate of ¹²⁵I-C3b from PMN was markedly increased in the presence of an excess of unlabeled C3b in the dilution medium and was directly dependent on the degree of initial receptor occupancy with the radioligand. These interactions occurred when 2% of the receptors were occupied with ¹²⁵I-C3b and resulted in a 4-fold decrease in CR1 affinity when the receptor went from its 'empty' to its 'filled' conformation. In a disease associated with a continuous production of C3b (factor I deficiency), CR1 on in vivo circulating PMN was found to be in a 'low affinity' and 'high dissociating' state similar to that of normal CR1 at high occupancy. Finally, negative cooperativity among CR1 sites disappeared after PMN activation with chemotactic peptides.