Proteolytic Cleavage of an Activated Subcomponent of the First Component of Rabbit Complement, C_??_s

Abstract

When rabbit C_??_ purified by affinity chromatography on IgG-Sepharose 6 B was chromatographed on DEAE-cellulose in the presence of ethylenediaminetetraacetate, C_??_s was isolated as two forms, C_??_s (I) and C_??_s (II), having different molecular weights. On the other hand, incubation of the C_??_ with soybean trypsin inhibitor before the chromatography resulted in the isolation of C_??_s (I) alone, indicating that, during the purification, C_??_s (II) was derived from C_??_s (I) by proteolytic cleavage of C_??_s (I) by a contaminating protease, probably plasmin [EC 3. 4. 21. 7]. In fact, C_??_s (I) was completely converted to C_??_s (II) or a C_??_s (II)-like fragment by highly purified plasmin. Analysis of the polypeptide chain structures revealed that C_??_s (I), which consisted of H and L chains with molecular weights of 70, 000 and 36, 000, respectively, was converted to C_??_s (II) by cleavage of the H chain, since C_??_s (II) consisted of two chains each with a molecular weight of 37, 000. This conversion proceeded without any alteration in C_??_ esterase activity, but was accompanied by loss of the ability to form C_??_r-C_??_s complex.