Isolation of two forms of activated C1s, a subcomponent of the first component of rabbit complement

Two forms of activated C1s, a subcomponent of the first component of complement, were present in preparations of C1 specifically purified from rabbit serum by affinity chromatography on IgG-Sepharose 6B and were separated by DEAE-cellulose chromatography in the presence of EDTA. These two activated C1s, designated C1s(I) and C1s(II), were indistinguishable with regard to hemolytic activity as well as C1s esterase activity, though they had different molecular weights. C1s(I) had a molecular weight of 106,000, consisting of H and L chains connected by disulfide bonds; the molecular weights of the chains were 70,000 and 36,000, respectively. On the other hand, C1s(II), with a molecular weight of 72,000, consisted of two chains each with a molecular weight of about 37,000, which were also connected by disulfide bonds. These results suggest that, in the case of rabbit C1s, the primary product of activation with C1r, C1s(I), may be susceptible to further cleavage of its H chain without any loss of C1s activity, resulting in the formation of C1s(II), though the active principle responsible for this conversion remains to be elucidated.