Abstract
Summary Single chain tissue plasminogen activator (sctPA) is not a zymogen form of two chain tPA (tctPA) but its activity can be enhanced by this conversion (by plasmin) and also in the presence of various promoter species, e.g. fibrin, fibrinogen fragments, heparin. Promoter stimulation is usually found to be more effective than proteolytic activation of tPA and this has been confirmed here using time courses from sc- to tctPA reactions in plasminogen activation assays with a wide range of promoters under similar conditions. Promoters investigated were fibrinogen fragments (FF), soluble fibrin (DESAFIB), unfractionated heparin (UFH), low molecular weight heparin (LMWH) and solid fibrin for comparison with cell surfaces from a number of transformed cell lines grown in culture. Sc- to tctPA conversion gave up to 4-fold improvements in plasminogen activation rates, much less than the stimulation due to promoters, up to 77-fold. The best promoters were FF and cell surfaces. Promoters were active on both sc- and tctPA. More forcing plasmin treatment of sctPA produced a further degradation product of tctPA, termed low molecular weight tPA (LMW tPA). This product has lost the finger domain (amino acids 1–49) and has a significantly different activity profile from full length tPA. LMW tPA maintains activity with chromogenic substrates and in the presence of fibrin and DESAFIB, but activity is reduced close to unstimulated levels in the presence of FF, heparin and cell surfaces. Thus, two mechanistic groups of promoters emerge: (1) FF, heparin and cell surface receptors and (2) DESAFIB and fibrin, where tPA finger-promoter interactions are essential for group 1, partially involved in group 2.
Published Version
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