1. A substance that renders another substance, or catalyst, active, or that accelerates a process or reaction. 2. The fragment, produced by chemical cleavage of a proactivator, that induces the enzymic activity of another substance. 3. An apparatus for making substances radioactive; e.g., neutron generator, cyclotron. 4. A removable type of myofunctional orthodontic appliance that acts as a passive transmitter of force, produced by the function of the activated muscles, to the teeth and alveolar process that are in contact with it. 5. a protein that binds to a DNA sequence before RNA polymerase transcription.
- catabolite gene a. (CGA) SYN: catabolite (gene) a. protein.
- plasminogen a. a proteinase converting plasminogen to plasmin by cleavage of a single (usually Arg-Val) bond in the former. SYN: urokinase.
- polyclonal a. (pol-e-klo′nal) a substance that will activate T cells, B cells, or both regardless of their specificities.
- tissue plasminogen a. (TPA, tPA) 1. a naturally occurring thrombolytic serine protease that catalyzes the conversion of plasminogen to plasmin; 2. a genetically engineered protein used as a thrombolytic agent in myocardial infarction, stroke, and peripheral vascular thrombosis.TPA is a single-chain glycoprotein with a molecular weight of about 70 kD. Produced by endothelial cells at sites of vascular injury, it modulates thrombogenesis by converting fibrin-bound plasminogen to plasmin, cleaving the arginine-valine bond in the 560–561 position of plasminogen. As a result, fibrin strands in a clot are chemically degraded and platelet adhesion and aggregation are inhibited. TPA has little effect on plasminogen in the absence of fibrin, and its release does not significantly reduce systemic concentrations of fibrinogen. Alteplase, a synthetic TPA produced by recombinant DNA technology, improves outcome when administered intravenously in acute myocardial infarction and in selected cases of stroke and peripheral ischemia due to thrombosis. It has a circulating half-life of only 4–6 minutes, but persists in clots up to 7 hours. see thrombolytic therapy.

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ac·ti·va·tor 'ak-tə-.vāt-ər n
1) a substance (as a chloride ion) that increases the activity of an enzyme compare COENZYME
2) a substance given off by developing tissue that stimulates differentiation of adjacent tissue also a structure giving off such a stimulant

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ac·ti·va·tor (akґtĭ-va″tər) 1. a substance that combines with an enzyme to increase its catalytic activity. 2. a substance that stimulates the development of a particular structure in the embryo. Cf. inductor and organizer. 3. functional a. 4. a chemical or other form of energy that causes another substance to become reactive or that induces a chemical reaction.

Medical dictionary. 2011.