The structure and enzymic activities of the C1r and C1s subcomponents of C1, the first component of human serum complement

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Abstract
The subcomponents C1r [r fragment of complement component 1] and C1s and their activated forms C.hivin.1r and C.hivin.1s each had MW in dissociating solvents of about 83,000. The amino acid compositions of each were similar, but there were significant differences in the monosaccharide analyses of C1r and C1s, whether activated or not. Subcomponents C1r and C1s have only 1 polypeptide chain, but C.hivin.1r and C.hivin.1s each contain 2 peptide chains of approximate MW 56,000 (''a'' chain) and 27,000 (''b'' chain). The amino acid analyses of the ''a'' chains from each activated subcomponent are similar, as are those of the ''b'' chains. The N-terminal amino acid sequence of 29 residues of the C.hivin.1s ''a'' chain was determined, but the C.hivin.1r ''a'' chain has a blocked N-terminal amino acid. The 20 N-terminal residues of both ''b'' chains are similar, but not identical, and both show obvious homology and other serine proteinases. The difference in polysaccharide content of the C.hivin.1r and C.hivin.1s is most marked in the ''b'' chains. When tested on synthetic amino acid esters, C.hivin.1s hydrolyzed lysine and tyrosine ester bonds, but C.hivin.1r did not hydrolyze any amino acid esters tested nor any protein substrate except C1s. The lysine esterase activity of C1s provides a rapid and sensitive assay of the subcomponent.