Heterozygosity for a mutant dysfunctional C1 inhibitor protein, a member of the serine proteinase inhibitor (serpin) superfamily, results in type II hereditary angioneurotic oedema. We identified a “hinge” region mutation in C1 inhibitor with a Val to Glu replacement at P14 Val–432. Recombinant C1 inhibitors P10 Ala→Thr and P14 Val→Glu did not form stable complexes with fluid phase C1s or kallikrein. The P14 Val→Glu mutant, however, was cleaved to a 96K form by C1s, while the P10 Ala→Thr mutant was not. The recombinant P10 mutant also did not complex with C1s, kallikrein or β–factor XIIa–Sepharose. The two mutations, therefore, result in dysfunction by different mechanisms: in one (P14 Val→Glu), the inhibitor is converted to a substrate, while in the other (P10 Ala→Thr), interaction with target protease is blocked.
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