The intrinsic lysis concept

Thrombin-activatable fibrinolysis inhibitor (TAFI) deficiency is compatible with murine life

To investigate the consequence of deficiency in thrombin-activatable fibrinolysis inhibitor (TAFI), we generated homozygous TAFI-deficient mice by targeted gene disruption. Intercrossing of heterozygous TAFI mice produced offspring in the expected Mendelian ratio, indicating that transmission of the mutant TAFI allele did not lead to embryonic lethality. TAFI-deficient mice developed normally, reached adulthood, and were fertile. No gross physical abnormalities were observed up to 24 months of age. Hematological analysis of TAFI-deficient mice did not show any major differences including plasma fibrinogen level, prothrombin time, and activated partial thromboplastin time. TAFI-deficient mice did not suffer from excess bleeding as determined by blood loss following tail transection, although their plasma failed to prolong clot lysis time in vitro. In vivo, TAFI deficiency did not influence occlusion time in either an arterial or a venous injury model. TAFI deficiency did not improve survival rate compared with the wild-type in thrombin-induced thromboembolism, factor X coagulant protein-induced thrombosis, and endotoxin-induced disseminated intravascular coagulation. Furthermore, TAFI deficiency did not alter kaolin-induced writhing response, implying that TAFI does not play a major role in bradykinin catabolism. The current study demonstrates that TAFI deficiency does not change normal responses to acute challenges.

An inhibitor of activated thrombin-activatable fibrinolysis inhibitor potentiates tissue-type plasminogen activator-induced thrombolysis in a rabbit jugular vein thrombolysis model

When activated in vitro, thrombin-activatable fibrinolysis inhibitor (TAFI) slows clot lysis by cleaving the C-terminal lysine and arginine residues from partially degraded fibrin. An inhibitor of carboxypeptidase isolated from potato (CPI) reverses prolongation of clot lysis by inhibiting activated TAFI. We investigated in vivo effect of TAFI inhibition on tissue-type plasminogen activator (t-PA)-induced clot lysis using CPI in a rabbit jugular vein thrombolysis model. It was found necessary to further purify the CPI preparations from commercial sources by HPLC chromatography to remove endotoxin and anti-plasmin activity that would affect the endogenous fibrinolytic system. The effect of intravenous administration of the purified CPI with t-PA was determined by measuring thrombus weight at the end of 90 minutes in six groups of animals. In the control group receiving saline, the median thrombus weight was 116 mg. In the group that received CPI only (0.5 mg/kg bolus injection followed by 0.3 mg/kg/h infusion), the median thrombus weight was 121 mg. In the group that received t-PA at a dose of 10 microg/kg bolus followed by 67 microg/kg/h infusion, the median thrombus weight decreased to 86 mg. When CPI was coadministered with the same regimen of t-PA, the median value further decreased to 58 mg. When animals were given three times higher the dose of t-PA (30 microg/kg bolus followed by 200 microg/kg/h infusion) in the absence or presence of CPI, median thrombus weights were 56 mg and 0 mg, respectively. Our results demonstrate that systemic coadministration of the purified CPI improves clot lysis induced by t-PA.

Amyloid beta peptides stimulate tissue-type plasminogen activator but not recombinant prourokinase

Tissue-type plasminogen activator (rt-PA) and prourokinase (rscu-PA) have been tested with respect to the influence of amyloid beta peptides on plasminogen activation which was monitored by cleavage of the chromogenic plasmin substrate S-2251. It was shown that rt-PA is stimulated by amyloid beta peptides at concentrations of 10 micrograms/ml in contrast to prourokinase, which does not alter its catalytic properties in presence of amyloid beta peptides. The stimulation of rt-PA can be inhibited by tranexamic acid indicating a molecular mode of stimulation similar to the fibrin mediated stimulation of rt-PA.

Dual effect of synthetic plasmin substrates on plasminogen activation

The effect of plasmin substrates D-valyl-L-leucyl-lysine-p-nitroanilide (S-2251) and H-D-norleucyl-hexahydrotyrosyl-lysine-p-nitro-anilide (Spectrozyme-PL) on the rate of activation of native human plasminogen in physiological salt solution is studied. Plasminogen activation by two-chain urokinase-type plasminogen activator (urokinase), two-chain tissue-type plasminogen activator (tc-tPA) or trypsin, but not by single chain tPA (sc-tPA) is increased 5- to 10-fold by both substrates, as determined by electrophoretic and spectrophotometric kinetic analysis. The amidolytic activity of sc-tPA, on the other hand, is inhibited by the plasmin substrates in a non-competitive manner (K1 of 6.4 . 10(-4) M for S-2251 and 2.9 . 10(-4) M for Spectrozyme-PL), whereas urokinase and tc-tPA activities are not affected. It is concluded that plasmin substrates containing a lysine residue have a general capacity to enhance plasminogen activation presumably by inducing a conformational change in the native zymogen in a manner similar to 6-aminohexanoate, while the same substrates are inhibitory both on the amidolytic activity of sc-tPA and the activation of native and des1-77-plasminogen by sc-tPA.