High-molecular weight kininogen. A secreted platelet protein.

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Abstract
Human platelets were studied immunochemically to determine if they contain high-molecular weight kininogen. On crossed immunoelectrophoresis with total kininogen antisera (antisera that recognizes both high- and low-molecular weight kininogen) extracts of platelets contained total kininogen antigen. Platelet total kininogen antigen showed complete antigenic identity with plasma total kininogen and displayed the same electrophoretic migration as plasma total kininogen. Using antisera monospecific to high molecular weight kininogen, a competitive enzyme-linked immunosorbent assay (CELISA) was developed to directly measure platelet high-molecular weight kininogen. By CELISA, 27-101 ng of high molecular weight kininogen antigen per 10(8) platelets was quantitated in detergent-soluble lysates of washed human platelets from nine normal donors with a mean level of 60 ng +/- 24/10(8) platelets. Plasma high-molecular weight kininogen, either in the platelet suspending medium or on the surface of the platelets, could only account for 5% of antigen measured in the solubilized platelets. On the CELISA, platelet high-molecular weight kininogen was immunochemically identical to plasma and purified high-molecular weight kininogen. Platelet high-molecular weight kininogen was secreted from platelets after exposure to ionophore A23187 (3-15 microM), collagen (5-150 micrograms/ml), and thrombin (1.6 U/ml). Secreted platelet high-molecular weight kininogen did not become a part of the platelet Triton-insoluble cytoskeleton. On cross immunoelectrophoresis secreted platelet total kininogen antigen had a similar electrophoretic migration to plasma total kininogen. Thus, human platelets contain high-molecular weight kininogen that can be secreted from platelets and that may participate in plasma coagulation reactions.