Single-chain tPA is the native form of tPA secreted from endothelial cells, whereas the two-chain form is the result of the proteolytic activity of plasmin. Both forms are catalytically active and have similar enzymatic properties in the presence of fibrin.
tPA activates plasminogen to plasmin, and also binds to fibrin. It is synthesized mainly in vascular endothelial cell and is secreted into the plasma continuously, and also through the acute release of tPA. tPA is a relatively poor activator of plasminogen in the absence of fibrin due to the low affinity of tPA for its substrate. However, tPA has a high affinity for fibrin, and binding increases its activating capacity up to 1000 fold. This dramatic increase is attributed to specific binding sites on the fibrin surface that concentrate and correctly orientate tPA with its substrate, as well as promote efficient clot lysis.
tPA is inhibited in vitro by plasminogen activator inhibitor, so something must be done to avoid this. Acidification of whole blood is therefore performed immediately after withdrawal. This can be done by mixing 1 ml of the citrated blood with 1 ml acetate buffer.
There are numerous physiological factors that influence tPA and PAI antigen and activity level. For a complete list, please request the tPA monograph from DiaPharma. One interesting feature of the fibrinolytic system is the circadian variation in tPA and PAI-1level. Free tPA levels are lowest in the morning, increase during the day, and reach their peak activity level in the late afternoon. tPA and PAI-1 antigen are highest in the early morning and decrease during the day. This may help explain the high incidence of MI and stroke in the morning hours. Other factors that influence tPA and PAI-1 include alcohol, drugs, oral contraceptives, exercise, food, heparin administration, pregnancy, smoking, etc.
PAI-1, PAI-2, PAI-3, protease nexin, a2-macroglobulin, trypsin inhibitor, and C1 Inhibitor all inhibit tPA. Plasminogen activator inhibitor 1 (PAI-1) is the most efficient inhibitor of tPA in plasma. It is a serine protease inhibitor (serpin) that acts as a pseudo-substrate for its target protease, with which it forms an inactive complex. PAI-1 is synthesized by several cell types including endothelial cells and hepatocytes and is present in platelets, placenta, and serum. The normal concentration range of PAI-1 in plasma is 5-40 mg/l and the normal activity is 0-20 AU/ml.
PAI-2 is a serpin with a higher affinity for u-PA (urinary-type plasminogen activator, or urokinase) than for t-PA. It is often only detectable during pregnancy, specifically in the third trimester.
PAI-3 is also called Protein C inhibitor, and inhibits u-PA and thrombin, and is present in plasma and urine.
Elevated t-PA and PAI-1 antigen and reduced t-PA activity may be associated with cardiovascular disease. t-PA is given to stroke and heart attack victims shortly after the event to help break up the clot.
S-2765™, S-2366™, and S-2288™, S-2403™ are suitable for single-chain tPA, and S-2251™ was used in the discontinued Chromogenix Coaset® tPA kit, but there are no substrates specifically for two-chain tPA. A paper by Verheijen et al. (Thromb Res, 1985; 39: 281 – 288), however, describes a method comparing the direct amidolytic activity of tPA on S-2366™ and the plasminogen activating activity. Also, the substrate S-2288™ is suitable for measuring double-chain tPA because it has a slightly higher sensitivity than S-2366™. S-2288™ should be used with purified systems, though, since this substrate is sensitive for several proteases.
|Protein S||70||10 (free)||0.14|
|Heparin Cofactor II||66||80||1.2|
|Protein C Inhibitor||57||4||0.07|