Proteolytic Cleavage of an Activated Subcomponent of the First Component of Rabbit Complement, C1¯s

Abstract

When rabbit C$1¯$ purified by affinity chromatography on IgG-Sepharose 6B was chromatographed on DEAE-cellulose in the presence of ethylenediaminetetraacetate, C$1¯$s was isolated as two forms, C$1¯$s(I) and C$1¯$s(II), having different molecular weights. On the other hand, incubation of the C$1¯$ with soybean trypsin inhibitor before the chromatography resulted in the isolation of C$1¯$s(I) alone, indicating that, during the purification, C$1¯$s(II) was derived from C$1¯$s(I) by proteolytic cleavage of C$1¯$s(I) by a contaminating protease, probably plasmin [EC 3.4.21.7]. In fact, C$1¯$s(I) was completely converted to C$1¯$s(II) or a C$1¯$s(II)-like fragment by highly purified plasmin. Analysis of the polypeptide chain structures revealed that C$1¯$s(I), which consisted of H and L chains with molecular weights of 70, 000 and 36, 000, respectively, was converted to C$1¯$s(II) by cleavage of the H chain, since C$1¯$s(II) consisted of two chains each with a molecular weight of 37, 000. This conversion proceeded without any alteration in C$1¯$ esterase activity, but was accompanied by loss of the ability to form C$1¯$r-C$1¯$s complex.