Soluble C3 proconvertase and convertase of the classical pathway of human complement. Conditions of stabilization in vitro.

Soluble classical-pathway C3 convertase and proconvertase were prepared from purified C4b-C2ox complex in the presence of Ni2+; the two complexes, stable for at least 15 h at 4 degrees C, were isolated by sucrose-density-gradient ultracentrifugation. The C3 convertase alone was able to cleave C3, and its decay was accelerated in the presence of C4-binding protein. The individual roles of Ni2+ and I2 treatment of C2 in the stabilization of the complexes seemed to be different and additive. 63Ni2+ binding coupled to h.p.l.c. analysis showed that 63Ni2+ bound only to the C2ox proteolytic fragment a (1 mol/mol) with a Kd of 26 microM. Competition studies between Ni2+ and Mg2+ indicated that only half of the Ni2+ bound to the C3 convertase was removed by Mg2+, whereas, in the same conditions, Ni2+ bound to C2ox proteolytic fragment a was not displaced, suggesting the presence of two sets of sites on the convertase. EDTA prevented the formation of both C3 convertase and proconvertase; EDTA had no effect on the preformed C3 convertase, whereas it dissociated the preformed proconvertase.