Tissue Plasminogen Activator (tPA) is a serine protease enzyme found in endothelial cells, the cells lining the blood vessels. It is involved in the breakdown of blood clots by converting plasminogen into plasmin, an enzyme that dissolves fibrin clots. This process is known as fibrinolysis. tPA plays a crucial role in maintaining vascular health and preventing conditions such as myocardial infarction and stroke.