Thrombin-activatable fibrinolysis inhibitor (TAFI) is synthesized by the li
ver and is thought to circulate in plasma as a plasminogen-bound zymogen. W
hen it is activated by the thrombin/thrombomodulin complex, activated TAFI
exhibits carboxypeptidase B-like activity. To study the structure-function
relationship of TAFI, we expressed recombinant human TAFI in insect cells.
During the cloning of TAFI cDNA from several human liver cDNA libraries, we
identified a second TAFI cDNA which differed from the published sequence a
t 2 positions. One of these sequences resulted in a substitution of alanine
for threonine at residue 147, the other was a silent mutation. These subst
itutions were found in several cDNA libraries from different sources. Using
Southern blot analysis, we confirmed the existence of this TAFI polymorphi
sm in the population. In order to compare the activation and activity of TA
FI isoforms, we expressed both isoforms in the baculovirus expression syste
m, and compared the enzyme kinetics of the purified proteins. The molecular
weight of recombinant TAFI is lower than plasma TAFI due to differences in
glycosylation. The two recombinant TAFI isoforms had similar activation ki
netics and the activated enzymes had similar carboxypeptidase B-like activi
ty towards small molecule substrates. Their ability to retard clot lysis wa
s found to be similar in a plate clot lysis assay.