Background: Bradykinin formation typically requires interaction of Factor XII, prekallikrein (PK), and high-molecular-weight kininogen (HK) with negatively charged exogenous initiators or cell-surface proteins. Approximately 85% of plasma PK circulates as a complex with HK. Nonenzymatic cell-derived initiators, such as heat shock protein 90, can activate the HK-PK complex to generate kallikrein, bradykinin, and cleaved HK, even in the absence of Factor XII.
Objective: We sought to determine whether PK, without activation to kallikrein, can digest HK to release bradykinin.
Methods: Kallikrein was measured by using a chromogenic assay, and bradykinin levels were determined by ELISA. Cleavage of PK and HK were assessed by SDS-PAGE and Western blot analysis.
Results: Cleavage of HK by PK is demonstrated without any conversion of PK to kallikrein. HK cleavage by PK is distinguished from that of kallikrein by the following: (1) stoichiometric activation of HK by PK with release of bradykinin proportional to the PK input; (2) inhibition of PK cleavage of HK by corn trypsin inhibitor, which has no effect on kallikrein; and (3) inhibition of PK cleavage of HK by a peptide derived from HK, which inhibits binding of PK to HK. The same peptide has no effect on kallikrein activation of HK. C1 inhibitor (C1INH), the major control protein of the plasma bradykinin-forming cascade, inhibits PK cleavage of HK.
Conclusion: PK is an enzyme that can cleave HK to release bradykinin, and this reaction is inhibited by C1INH. This might account, in part, for circulating bradykinin levels and initiation of kinin formation in C1INH deficiency.