Role in disease
Deficiency of this protein is associated with hereditary angioedema ("hereditary angioneurotic edema"), or swelling due to leakage of fluid from blood vessels into connective tissue. Deficiency of C1-inhibitor 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. In its most common form, 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-inhibitor 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. Mutations in the gene that codes for C1-inhibitor, SERPING1, may also play a role in the development of age-related macular degeneration. At least 97 disease-causing mutations in this gene have been discovered.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).
Synthesis
C1-inhibitor is contained in the human blood; it can, therefore, be isolated from donated blood. Risks of infectious disease transmission (viruses, prions, etc.) 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-inhibitor is particularly heavily glycosylated, this sialylated recombinant form would have a short circulatory life (the carbohydrates are not relevant to the inhibitor function). Therefore, C1-inhibitor has also been produced in glycosylated form using transgenic rabbits. This form of recombinant C1-inhibitor also has been given orphan drug status for delayed graft function following organ transplantation and for capillary leakage syndrome. References
Further reading
External links
Human SERPING1 genome location and SERPING1 gene details page  in the UCSC Genome Browser. "SERPING1". Drug Information Portal. U.S. National Library of Medicine. The MEROPS online database for peptidases and their inhibitors: I04.024
B02AB03 (WHO)
Overview of all the structural information available in the PDB for UniProt: P05155 (Plasma protease C1 inhibitor) at the PDBe-KB.