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PJ Declerck, HR Lijnen, M Verstreken, H Moreau and D Collen
Center for Thrombosis and Vascular Research, University of Leuven, Belgium.
A murine monoclonal antibody (MA-12E6A8) was raised against human
urokinase-type plasminogen activator (u-PA), which, in an enzyme-linked
immunosorbent assay (ELISA), reacted 15,000-fold better with recombinant
two-chain u-PA (rtcu-PA) than with recombinant single-chain u-PA (rscu-PA).
The antibody had no effect on the activity of rtcu-PA or on its inhibition
by a chloromethylketone, but reduced the inhibition of rtcu-PA by
recombinant plasminogen activator inhibitor-1 (rPAI-1) at least 10-fold.
The dissociation constant of the rtcu-PA/MA- 12E6A8 complex was 7 nmol/L.
An ELISA was developed using MA-12E6A8 as capture antibody and a
horseradish peroxidase conjugated u-PA specific antibody for tagging. It
recognized free and active site blocked rtcu- PA but not rtcu-PA in complex
with rPAI-1 or with alpha 2-antiplasmin. This ELISA was used to monitor the
generation of rtcu-PA during fibrin clot lysis with rscu-PA in human
plasma. Addition of 5 micrograms/mL rscu-PA to 3 mL plasma containing a 0.2
mL 125I-fibrin labeled plasma clot caused 50% clot lysis in 62 +/- 13
minutes (mean +/- SD, n = 6), at which time 99 +/- 28 ng/mL rtcu-PA was
detected but no fibrinogen breakdown had occurred. Fifty percent fibrinogen
breakdown did occur only when rtcu-PA had reached a level of 1,000 +/- 270
ng/mL (at 150 +/- 21 minutes). rscu-PA, 2 micrograms/mL, induced 50% clot
lysis in 160 +/- 41 minutes (n = 6); no fibrinogen degradation occurred
within 4 hours and rtcu-PA levels did not exceed 80 ng/mL. In the absence
of a fibrin clot, 5 micrograms/mL rscu-PA added to human plasma did not
result in significant generation of rtcu-PA (less than 50 ng/mL after 4
hours) and no fibrinogen degradation was observed. These results indicate
that clot lysis with rscu-PA in a plasma milieu does not require extensive
systemic conversion of rscu-PA to rtcu-PA, and that fibrinogen degradation
occurs secondarily to systemic conversion of rscu-PA to rtcu-PA.
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