Human C1 inhibitor: improved isolation and preliminary structural characterization.

An improved procedure for the isolation of the C1 inhibitor (C1-INH) component of human complement is reported. Following preliminary steps to remove plasminogen, fibrinogen, and aggregated material, three conventional chromatographic steps are used to isolate C1-INH in high (70%) overall yield. An extinction coefficient (E 1%, 1 cm 280nm) of 3.60 has been determined. The isolated protein exhibits a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with a mobility corresponding to an apparent molecular weight (Mr) of 105 000. After removal of carbohydrate, the protein shows an increased mobility, corresponding to an apparent Mr of 78 000. A total carbohydrate content of 33% has been calculated, and from this and the size of the deglycosylated polypeptide, a true molecular weight of 116 000 was estimated. Further analysis of the carbohydrate has indicated a galactose:mannose ratio of 2:1 and approximately equimolar amounts of N-acetylglucosamine and N-acetylgalactosamine. This composition is unusual for a plasma protein and suggests that much of the carbohydrate is contained in linkages other than the typical N-glycosidic structures. Values found for the amino acid composition are compared with those reported previously. The amino-terminal sequence (40 residues) of C1-INH is also reported. Asparagine lies at the amino terminus. Neither high-performance liquid chromatography of the released phenylthiohydantoin derivative nor back-hydrolysis of the thiazolinone permitted identification of the residue contained at position 3. The sequence around this position is compatible, however, with an N-glycosidic linkage to residue 3.(ABSTRACT TRUNCATED AT 250 WORDS)