Activator-bound C1 is less susceptible to inactivation by C1 inhibition than is fluid-phase C1.
Parameters that influence the effective interaction of C1 with the serum regulatory glycoprotein C1 Inhibitor were investigated. C1 that bound to activator particles EAC4 or EA was strikingly less susceptible to inactivation by C1 Inhibitor than was fluid-phase C1. By using the conventional hemolytic assay, the concentrations of C1 Inhibitor required for inhibition of C1 bound to EAC4 were 1000-fold higher than those required for fluid-phase C1. With EA as the activator (and indicator) particle, 17- to 75-fold higher concentrations of C1 Inhibitor were required to inhibit bound vs free C1. These findings suggest that, on binding to these particulate immune complexes, the domain of the C1 molecule capable of interacting with C1 Inhibitor is less available for binding than when C1 is in fluid phase. Alternatively, the conformation of C1 may be altered when bound to EA or EAC4, resulting in a lower association constant of C1 Inhibitor for C1. As assessed by inhibition of classical complement pathway hemolysis, the inhibition of the enzymatic activity of C1 by C1 Inhibitor (both in the fluid phase and particle-bound) was markedly dependent on the concentration of the reactants. Incubation of C1 and C1 Inhibitor at serum concentrations resulted in the inhibition of more than 10 times the amount of C1 hemolytic activity than that which occurred when the same ratio of components was incubated at the more dilute concentrations used in the conventional hemolytic assays. These findings have allowed for the development of a more sensitive and rapid assay for C1 Inhibitor function.
D003166:Complement Activating Enzymes / Q000378:metabolism*; D003167:Complement Activation; D003172:Complement C1 / Q000032:analysis / Q000378:metabolism*; D003174:Complement C1 Inactivator Proteins / Q000032:analysis / Q000494:pharmacology*; D004306:Dose-Response Relationship, Immunologic; D006801:Humans; D007700:Kinetics
D003172:Complement C1; D003174:Complement C1 Inactivator Proteins; D003166:Complement Activating Enzymes