Tissue plasminogen activator

The tissue plasminogen activator (t-PA) is synthesized in the endothelium and released into the flowing blood. The highest t-PA concentration is found in the small veins and in the vessels of the kidney. In the large vessels only little t-PA can be detected. Only minimal amounts of t-PA are found in plasma, because these substances, which are released from the endothelium, disappear very quickly in free plasma and are usually degraded in the liver. Plasminogen activators are also released by macrophages, which are stimulated by cytokines from the lymphocytes. The different tissue plasminogen activators show certain variations depending on the tissue from which they originate.

The original form of t-PA is a single-chain molecule with a molecular weight of 68,000 D. These activators are called tissue-type plasmin activators. The release of t-PA is stimulated by vasoconstriction and by thrombin.
The t-PA is not very active in free form. It is only activated when bound to fibrin, increasing its activity by a factor of 100.

1. HA Neumann (2014) The coagulation system. ABW-Wissenschaftsverlag GmbH Berlin S. 104f.