Isolation and Characterization of the Proenzyme Form of the C1s Subunit of the First Complement Component

Abstract

The proenzyme form of C1s was isolated by sequential chromatography of the euglobulin fraction of human serum on DE-32 cellulose, TEAE cellulose, and Sephadex G-200. C1s was found to be an α-globulin with a molecular weight calculated from S and D, of 86,200 daltons. The amino acid composition was unremarkable for a serum protein. Isolated C1s preparations were unable to cleave natural (C4) or synthetic (ALTEE) C1s substrates unless first activated by incubation with C̄1̄r or trypsin. Activation was accomplished by limited proteolytic cleavage. Isolated C1q, C1r, and C1s formed a macromolecular complex in the presence of calcium which possessed C1 activity. C1 formed in this manner was in the proenzyme form as evidenced by ability to cleave ALTEE. Reconstituted C1, however, was readily activated on addition of aggregated IgG.