A heterogeneous group of proteolytic enzymes that convert plasminogen to plasmin. They are concentrated in the lysosomes of most cells and in the vascular endothelium, particularly in the vessels of the microcirculation.
Any of a group of substances able to convert plasminogen into plasmin. Plasminogen is a substance normally present in blood; plasmin is an enzyme that digests the blood-clotting protein fibrin. Tissue plasminogen activator (tPA) is a medication used to dissolve blood clots immediately after a heart attack or stroke. Because TPA acts directly on clots, it rarely causes systemic bleeding.