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Kininogens are proteins that are defined by their role as precursors for kinins, but that also can have additional roles. Kinins are biologically active peptides, the parent form is bradykinin.
The two main kininogen types are:
High-molecular-weight kininogen, which is produced mostly by the liver but is synthesized in endothelial cells and is present in platelets and neutrophils. It acts as a cofactor for prekallikrein, factor XI, and factor XII in the coagulation and inflammation systems. It has no intrinsic enzymatic activity. These high molecular weight kininogens are cleaved into bradykinin and kallidin by tissue and plasma kallikreins.