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.
- Low-molecular-weight kininogen, which is produced locally by numerous tissues, and secreted together with tissue kallikrein.
They are both spliced from the same precursor.
A third type, T-kininogen, is found in rats but not humans.
Closely related proteins include cystatin.