The reactive center of Cl-inhibitor, a plasma protease inhibitor that belongs to the serpin superfamily, is located on a peptide loop which is highly susceptible to proteolytic cleavage.With plasma kallikrein, Cls andfl-Factor XIIa, this cleavage occurs at the reactive site residue PI (Arg4); with neutrophil elastase, it takes place near P1, probably at residue P3 (Val"2).After these cleavages, Cl-inhibitor is inactivated and its conformation is modified. Moreover, in vivo, cleaved Cl-inhibitor is removed from the blood stream more rapidly than the intact serpin, which suggests that proteolysis unmasks sites responsible for cellular recognition and the uptake of the cleaved inhibitor. In the study reported here, we show, using an MAb, that an identical neoepitope is created on Cl-inhibitor after the cleavage of its exposed loop by plasma kallikrein, Cls, ,8-Factor XIIa, and by neutrophil elastase.
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