The human complement C1 complex has a picomolar dissociation constant at room temperature.

Periodic sampling of serum or reconstituted C1 initially diluted 1/2000 and 1/4000 (that is, to 0.1 and 0.05 nM) into a recombinant C1s-containing solution showed a gradual decline of hemolytic activity until equilibrium was approached, consistent with a simple dissociation, reassociation equilibrium, presumably C1 C1q + C1r2C1s2. The presence of excess (5 nM) recombinant C1s minimized further dissociation of the C1r2C1s2, allowing the first step to be studied independently of the dissociation of C1r2C1s2 C1r2 + 2 C1s. Reassociation experiments were also performed, starting with the dissociated C1 diluted to the same concentrations and following the regain of hemolytic activity to approximately the same values, showing that the same equilibrium had been achieved from both directions. Analysis of the kinetic data yielded forward and reverse rate constants and the equilibrium constant, for which values of approximately 72 and 3 pM were estimated at 0 and 23 degrees C, respectively. The effects of temperature, ionic strength, Ca2+ ion concentration, and activation of the zymogen on the equilibrium constants were explored; extreme sensitivity to temperature, ionic strength, and activation were found. At 23 and 30 degrees C, slow activation of C1 was also evident. Highly purified, reconstituted C1 yielded approximately the same values for the kinetic and equilibrium parameters as serum C1, suggesting that the structure of the reconstituted complex was similar to or identical with that of the serum C1 complex.