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Serpin peptidase inhibitor, clade G (C1 inhibitor), member 1, (angioedema, hereditary)
Available structures: 2oay
Symbol(s) SERPING1; C1IN; C1INH; C1NH; HAE1; HAE2
External IDs OMIM: 606860 MGI: 894696 Homologene: 44
RNA expression pattern

More reference expression data

Human Mouse
Entrez 710 12258
Ensembl ENSG00000149131 ENSMUSG00000023224
Uniprot P05155 P97290
Refseq NM_000062 (mRNA)
NP_000053 (protein)
NM_009776 (mRNA)
NP_033906 (protein)
Location Chr 11: 57.12 - 57.14 Mb Chr 2: 84.57 - 84.58 Mb
Pubmed search [1] [2]

C1-inhibitor (C1-inh, C1 esterase inhibitor) is a serine protease inhibitor (serpin) protein, the main function of which is the inhibition of the complement system to prevent spontaneous activation. C1-inh is an acute phase protein, it circulates in blood at levels of around 0.25 g/l, which levels rise ~2-fold during inflammation. C1-inh irreversibly binds to and inactivates C1r and C1s proteinases in the C1 complex of classical pathway of complement. MASP-1 and MASP-2 proteinases in MBL complexes of the lectin pathway are also inactivated. This way, C1-inh prevents the proteolytic cleavage of later complement components C4 and C2 by C1 and MBL. Although named after its complement inhibitory activity, C1-inh also inhibits proteinases of the fibrinolytic, clotting, and kinin pathways. Most notably, C1-inh is the most important physiological inhibitor of plasma kallikrein, fXIa and fXIIa.



C1-inh is the largest member among the serpin class of proteins. Remarkably, C1-inh has a 2-domain structure, unlike most family members. The serpin domain is similar to other serpins, and this part of C1-inh provides the inhibitory activity of C1-inh. The other domain, named N-terminal domain or N-terminal tail is not essential for C1-inh to inhibit proteinases. This domain has no similarity to other proteins. C1-inh is highly glycosylated glycosylated, bearing both N- and O-glycans. Especially the N-terminal domain is glycosylated heavily.


The human C1-inhibitor gene is located on the eleventh chromosome (11q11-q13.1).

Role in disease

Deficiency of this protein is associated with hereditary angioedema (or "hereditary angioneurotic edema"), which in layman's terms means swelling of the blood vessels. Deficiency of C1-inh permits plasma kallikrein activation, which leads to the production of the vasoactive peptide bradykinin. Also, C4 and C2 cleavage goes unchecked, resulting in auto-activation of the complement system. Most commonly, it presents as marked swelling of the face, mouth and/or airway that occurs spontaneously or to minimal triggers (such as mild trauma), but such swelling can occur in any part of the body. In 85% of the cases, the levels of C1-inh are low, while in 15% the protein circulates in normal amounts but it is dysfunctional. In addition to the episodes of facial swelling and/or abdominal pain, it also predisposes to autoimmune diseases, most markedly lupus erythematosus, due to its consumptive effect on complement factors 3 and 4.

Despite uncontrolled auto-activation, it is important to note that levels of key complement components are low during an acute attack, because they are being consumed - indeed, low levels of C4 are a key diagnostic test for hereditary angioedema. This situation is analogous to the low levels of clotting factors found in Disseminated intravascular coagulation (DIC).

Therapeutic use

In hereditary angioedema

Patients with frequent attacks of angioedema are most frequently treated using attenuated androgens, like Danazol. The obvious choice, replacement therapy using blood-derived C1-inh is feasible, but often too expensive for prophylactic treatment. Recently, new methods of treatment have emerged: a plasma kallikrein inhibitor and a bradykinin receptor antagonist. A cheaper C1-inh alternative, recombinant C1-inh was also developed.

For other conditions

The activation of the complement cascade can cause damage to cells, therefore the inhibition of the complement cascade can work as a medicine in certain conditions[1]. When someone has a heart attack, for instance, the lack of oxygen in heart cells causes necrosis in heart cells: dying heart cells spill their contents in the extracellular environment, which triggers the complement cascade. Activation of the complement cascade attracts phagocytes that leak peroxide and other reagents, which may increase the damage for the surviving heart cells. Inhibition of the complement cascade can decrease this damage.


C1-inhibitor is contained in the human blood; it can therefore be isolated from donated blood. Risks of viral transmission and relative expense of isolation prevented widespread use. It is also possible to produce it by recombinant technology, but Escherichia coli (the most commonly used organism for this purpose) lacks the eukaryotic ability to glycosylate proteins; as C1-inh is particularly heavily glycosylated, this recombinant form would be ineffective. C1-inh was also produced in glycosylated form using transgenic rabbits [3]. This form of recombinant C1-inh also has been given orphan drug status for delayed graft function following organ transplantation and for capillary leakage syndrome [4].


  1. ^ Caliezi C, Wuillemin WA, Zeerleder S, Redondo M, Eisele B, Hack CE. C1-esterase inhibitor: an anti-inflammatory agent and its potential use in the treatment of diseases other than hereditary angioedema. Pharmacol Rev 2000;52:91-112. PMID 10699156

Further reading

  • Lappin D, Whaley K (1989). "Regulation of C1-inhibitor synthesis by interferons and other agents.". Behring Inst. Mitt. (84): 180-92. PMID 2478116.
  • Stein PE, Carrell RW (1995). "What do dysfunctional serpins tell us about molecular mobility and disease?". Nat. Struct. Biol. 2 (2): 96-113. PMID 7749926.
  • Davis AE, Bissler JJ, Cicardi M (1994). "Mutations in the C1 inhibitor gene that result in hereditary angioneurotic edema.". Behring Inst. Mitt. (93): 313-20. PMID 8172583.
  • Davis AE (2005). "The pathophysiology of hereditary angioedema.". Clin. Immunol. 114 (1): 3-9. doi:10.1016/j.clim.2004.05.007. PMID 15596403.
  • Siddique Z, McPhaden AR, McCluskey D, Whaley K (1992). "A single base deletion from the C1-inhibitor gene causes type I hereditary angio-oedema.". Hum. Hered. 42 (4): 231-4. PMID 1339401.
  • Davis AE, Aulak K, Parad RB, et al. (1993). "C1 inhibitor hinge region mutations produce dysfunction by different mechanisms.". Nat. Genet. 1 (5): 354-8. doi:10.1038/ng0892-354. PMID 1363816.
  • Frangi D, Aulak KS, Cicardi M, et al. (1992). "A dysfunctional C1 inhibitor protein with a new reactive center mutation (Arg-444-->Leu).". FEBS Lett. 301 (1): 34-6. PMID 1451784.
  • Lappin DF, Guc D, Hill A, et al. (1992). "Effect of interferon-gamma on complement gene expression in different cell types.". Biochem. J. 281 ( Pt 2): 437-42. PMID 1531292.
  • Siddique Z, McPhaden AR, Lappin DF, Whaley K (1992). "An RNA splice site mutation in the C1-inhibitor gene causes type I hereditary angio-oedema.". Hum. Genet. 88 (2): 231-2. PMID 1684567.
  • Frangi D, Cicardi M, Sica A, et al. (1991). "Nonsense mutations affect C1 inhibitor messenger RNA levels in patients with type I hereditary angioneurotic edema.". J. Clin. Invest. 88 (3): 755-9. PMID 1885769.
  • Carter PE, Duponchel C, Tosi M, Fothergill JE (1991). "Complete nucleotide sequence of the gene for human C1 inhibitor with an unusually high density of Alu elements.". Eur. J. Biochem. 197 (2): 301-8. PMID 2026152.
  • Parad RB, Kramer J, Strunk RC, et al. (1990). "Dysfunctional C1 inhibitor Ta: deletion of Lys-251 results in acquisition of an N-glycosylation site.". Proc. Natl. Acad. Sci. U.S.A. 87 (17): 6786-90. PMID 2118657.
  • Stoppa-Lyonnet D, Carter PE, Meo T, Tosi M (1990). "Clusters of intragenic Alu repeats predispose the human C1 inhibitor locus to deleterious rearrangements.". Proc. Natl. Acad. Sci. U.S.A. 87 (4): 1551-5. PMID 2154751.
  • Levy NJ, Ramesh N, Cicardi M, et al. (1990). "Type II hereditary angioneurotic edema that may result from a single nucleotide change in the codon for alanine-436 in the C1 inhibitor gene.". Proc. Natl. Acad. Sci. U.S.A. 87 (1): 265-8. PMID 2296585.
  • Theriault A, Whaley K, McPhaden AR, et al. (1990). "Regional assignment of the human C1-inhibitor gene to 11q11-q13.1.". Hum. Genet. 84 (5): 477-9. PMID 2323781.
  • Aulak KS, Cicardi M, Harrison RA (1990). "Identification of a new P1 residue mutation (444Arg----Ser) in a dysfunctional C1 inhibitor protein contained in a type II hereditary angioedema plasma.". FEBS Lett. 266 (1-2): 13-6. PMID 2365061.
  • Skriver K, Radziejewska E, Silbermann JA, et al. (1989). "CpG mutations in the reactive site of human C1 inhibitor.". J. Biol. Chem. 264 (6): 3066-71. PMID 2563376.
  • Ariga T, Igarashi T, Ramesh N, et al. (1989). "Type I C1 inhibitor deficiency with a small messenger RNA resulting from deletion of one exon.". J. Clin. Invest. 83 (6): 1888-93. PMID 2723063.
  • Tosi M, Duponchel C, Bourgarel P, et al. (1986). "Molecular cloning of human C1 inhibitor: sequence homologies with alpha 1-antitrypsin and other members of the serpins superfamily.". Gene 42 (3): 265-72. PMID 3089875.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "C1-inhibitor". A list of authors is available in Wikipedia.
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