tPA

tPA

Tissue plasminogen activator (tPA, tissue plasminogen activator) is a serine protease found on endothelial cells (cells located in blood vessels) involved in the breakdown of blood clots (plasmin). The tPA enzyme catalyzes the conversion of plasminogen to plasmin. tPA can be used for embolic or thrombotic stroke research. TPA can be manufactured using recombinant biotechnology. The tPA created in this way can be called recombinant tissue plasminogen activator (rtPA).

Figure 1. Protein structure of tPA.

Introductions

Blood clotting is an enzymatic event that is triggered by material from injured tissue and eventually forms a fibrin monomer that forms a clot. After a few days, the fibrin clot is degraded by the fibrinolytic enzyme system. The central enzyme component in this system is the glycoprotein plasminogen present in plasma and most extravascular fluids. Plasminogen is a zymogen of a serine protease, and after being partially cleaved by a plasminogen activator, it is converted into its active form of plasmin. Two plasminogen activators were found in the human body, namely tissue-type plasminogen activator (t-PA) and urine-type activator (u-PA). t-PA is the main activator of plasminogen in blood, while u-PA has a major function in tissue-related proteolysis, and is second only to t-PA in removing intravascular fibrin. Fibrinolysis is mainly initiated and propagated by the fibrin surface. The fibrin surface provides binding sites to optimize the many components of the fibrinolysis system (most notably plasminogen and t-PA). This stimulating effect ensures that the concentration of plasminogen and t-PA on the fibrin deposits is high, and enables the localization of plasmin activity. Plasminogen activator inhibitor-1 (PAI-1) and plasmin inhibitors can provide inhibitory regulation. PAI-1 is the most effective t-PA inhibitor in plasma, and most t-PA in circulation is bound to this inhibitor.

Structure

Human t-PA is a 68 kDa serine protease consisting of 530 or 527 amino acids and contains 7% to 13% carbohydrates. The molecule is composed of five different domain structures with autonomous functions. The finger domain and two kringle domains are involved in the binding of t-PA to fibrin, while the epidermal growth factor domain is involved in rapid liver clearance. molecule. The serine protease domain contains an active site region composed of serine, histidine, and aspartic acid, which are relatively far apart from each other in the primary structure, but very close in folded proteins. This region cleaves the Arg561-Val562 bond in plasminogen, thereby activating it as plasmin.

Synthesis and secretion

t-PA is mainly synthesized in vascular endothelial cells and is continuously secreted into plasma by the acute release of t-PA. The latter situation occurs when stimulating certain endothelial cell receptors. Different areas of the vascular system secrete different amounts of t-PA. The amount of t-PA secretion in the upper limb is four times that of the lower limb. There are two forms of t-PA, single-chain t-PA (sct-PA) and two-chain t-PA (tct-PA). Single-stranded molecules are the natural form of t-PA secreted by endothelial cells, while double-stranded forms are the result of the proteolytic activity of plasmin. Both forms are catalytically active and have similar enzymatic properties in the presence of fibrin.

Regulation

The interaction between t-PA and PAI-1 and its synergistic appearance in plasma are very complicated. The total amount of t-PA and PAI-1 antigen in plasma usually shows a strong positive correlation, indicating that the synthesis and clearance of t-PA and PAI-1 are biologically linked. However, the increase in PAI-1 levels leads to an increase in t-PA production, which does not automatically lead to an increase in active t-PA content. In contrast, t-PA activity appears to be inversely related to PAI-1 and t-PA antigens. Since the increase in the total amount of t-PA leads to a decrease in t-PA activity, the latter correlation seems confusing.

t-PA analysis method

Various methods for measuring t-PA levels in human plasma have been described. These can be divided into specific t-PA detection and non-specific global detection. Today, two common methods are the determination of the dissolution time of whole globulin clots and fibrin plate. Specific t-PA analysis includes immunological methods for measuring t-PA antigen (ie free t-PA and t-PA / PAI-1) and functional methods. The latter is either a chromogenic assay using chromogenic plasmin substrate and stimulant, or a biological immunoassay using a combination of monoclonal antibodies and chromogenic plasmin substrate.

Reference

1. Wardlaw JM, et al. Recombinant tissue plasminogen activator for acute ischaemic stroke: an updated systematic review and meta-analysis. Lancet. 2012,379 (9834): 2364–72.