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The assembly of the classical-pathway C3 convertase from C4 and I2-treated C2 by the action of C1s is an Mg2(+)-dependent reaction. The Mg2+ concentration necessary for the assembly of C3 convertase in the fluid phase was found to be dependent on NaCl concentration. In the absence of NaCl more than 5 mM-MgCl2 was found to be required, whereas 0.5 mM-MgCl2 was adequate for the assembly of C3 convertase in the presence of 150 mM-NaCl. The C3 convertase assembled in a low-ionic-strength buffer was extremely labile compared with that assembled in buffer of physiological ionic strength, and the stability of C3 convertase was improved with the increase in NaCl concentration. It was found that the stabilizing effect of NaCl on C3 convertase was due to inhibition of the dissociating activity of C2b, which was formed during the assembly of C3 convertase. In addition to the dissociation-accelerating effect, C2b inhibited the assembly of C3 convertase in low-ionic-strength buffer, and this effect also was diminished with increase in NaCl concentration. An increase in NaCl concentration to more than 200 mM resulted in a decrease in the assembly of C3 convertase. This effect was not due to the lability of the assembled C3 convertase but due rather to the inhibition of C2 cleavage by C1s. Purified C3 convertase itself is stable in dilute medium or high-ionic-strength medium such as 500 mM-NaCl, suggesting that the interactions between C4b and C2a are hydrophobic. In these respects C2b seemed to be functionally similar to C4bp, but C2b failed to act as a cofactor for the Factor I-catalysed C4b cleavage.

Effect of sodium chloride concentration on fluid-phase assembly and stability of the C3 convertase of the classical pathway of the complement system