Abstract
C1 inhibitor is a heavily glycosylated plasma protein that regulates the activity of the first component of complement (C1) by inactivation of the serine protease subcomponents, C1r and C1s. C1 inhibitor cDNA clones have been isolated, and one of these (pC1INH1, 950 base pairs) has been partially sequenced. Sequence analysis demonstrates that the C1 inhibitor is a member of the serpin "superfamily" of protease inhibitors. In the region sequenced, C1 inhibitor has 22% identity with antithrombin III, 26% with alpha 1-antitrypsin and alpha 1-antichymotrypsin, and 18% with human angiotensinogen. C1 inhibitor has a larger amino-terminal extension than do the other plasma protease inhibitors. In addition, inspection of residues that are invariant among the other protease inhibitors shows that C1 inhibitor differs at 14 of 41 of these positions. Thus, it appears that C1 inhibitor diverged from the group relatively early in evolution, although probably after the divergence of angiotensinogen. Southern blot analysis of BamHI-digested DNA from normal individuals and from rodent-human somatic cell hybrid cell lines (that contain a limited but varied human chromosome complement) was used to localize the human C1 inhibitor gene to chromosome 11.