Description of Tissue Plasminogen Activator (tPA)

Tissue-type plasminogen activator (tPA) is one of two major physiologic activators of plasminogen in plasma. It is a serine protease of 68 kDa produced primarily in endothelial cells but is also present in monocytes and megakaryocytes. Normal plasma tPA antigen concentrations have been reported from 20 ng/ml to 5 μg/ml, depending on the assay used, but typically most of the tPA (> 90%) is in complex with it’s primary inhibitor, plasminogen activator inhibitor-1 (PAI-1). Structurally, tPA is a single-chain enzyme that consists of a catalytic domain followed by two kringle structures, an EGF domain and a finger domain. The activation of plasminogen by tPA is dependent on the presence of a fibrin cofactor. The binding of both tPA and plasminogen to fibrin is mediated in part through lysine binding sites within the kringle structures of both enzyme and substrate, but also through the finger domain of tPA. Activation of plasminogen by tPA occurs by cleavage after residue Arg560 to produce the two-chain active serine protease plasmin. The activity of tPA is regulated in part by a very short half life in circulation (t½ of ~4 minutes) and by circulating protease inhibitors PAI-1 and to a lesser extent α2macroglobulin^1-3.

References and Reviews

1. Bachmann F; The Plasminogen-Plasmin Enzyme System; in Hemostasis and Thrombosis, 3rd Edition, eds. RW Colman, J Hirsh, VJ Marder and EW Salzman, pp. 1592-1622, J.B. Lippincott Co., Philadelphia PA, USA, 1994.
2. Holvoet P, Cleemput H, Collen D; Assay of Human Tissue-Type Plasminogen Activator (t-PA) with an Enzyme-Linked Immunosorbent Assay (ELISA) Based on Three Murine Monoclonal Antibodies to t-PA. Thrombosis and Haemostasis 54, pp. 684-687, 1985.
3. Giles AR, Nesheim, et al; The fibrinolytic Potential of the Normal Primate following the Generation of Thrombin In Vivo; Thrombosis and Haemostasis 63, pp. 476-481, 1990.

Description of PAI-1 (Type-1 Plasminogen Activator Inhibitor)

Type I plasminogen activator inhibitor, PAI-1, is a 50 kDa single-chain glycoprotein which belongs to the serine protease inhibitor (SERPIN) family of proteins. The concentration of PAI-1 in normal human plasma is very low, with an average concentration of approximately 20 ng/ml. The plasma concentrations of PAI-1 can be affected by a number of factors including diurnal variations, age, sex, pregnancy, obesity and exercise status. PAI-1 is synthesized by various cell types including endothelial cells, hepatocytes, vascular smooth muscle cells, fibroblasts, mesothelial cells, granulosa cells and malignant cell lines. It is also found in the α-granules of platelets as well as plasma. PAI-1 exists in at least three different conformations, including an active form with a half-life of approximately 1 – 3 hours, a latent form and a proteolytically or oxidatively inactivated form. The plasma glycoprotein vitronectin has been shown to bind active PAI-1 and stabilize it in its active conformation, extending its functional half-life to greater than 24 hours. Little specific binding of the latent or inactive forms of PAI-1 to vitronectin occurs. PAI-1 is considered to be the primary regulator of plasminogen activation in vivo. It is the major physiologic inhibitor of both the single chain and two-chain forms of tPA, being able to inhibit the single-chain form at least 10000 times greater than other PAIs.^1-3

References and Reviews

1. Kruithof, E.K.O. Plasminogen activator inhibitor type 1: biochemical, biological and clinical aspects. Fibrinolysis, 2:59-70, 1988.
2. Edelberg, J.M, Sane, D.C., and Pizzo, S.V. Vascular regulation of plasminogen activator inhibitor-1 activity. Semin. Thromb. Hemost., 20(4):319-323, 1994.
3. Loskutoff, D.J., Sawdey, M., and Mimuro, J. Type 1 plasminogen activator inhibitor, Prog. Thromb. Haemost., 9:87-115, 1988.