Summary
Inhibition of thrombin by endogenous inhibitors plays a central role in the spatiotemporal
control of clot formation. A failure to adequately inactivate thrombin such as in
antithrombin deficiency generates a strong prothrombotic phenotype. To study if and
to what extent delayed thrombin inactivation rates beyond antithrombin deficiency
contribute to the prothrombotic phenotype we measured thrombin inhibition profiles
in plasma samples obtained from 16 healthy individuals and 39 thrombophilic patients,
including 17 patients diagnosed positive for anti-prothrombin/phospholipid antibodies.
To test thrombin inhibition, thrombin was added to plasma, and endogenous thrombin
inhibition stopped by addition of the reversible thrombin inhibitor argatroban. Subsequently,
the amount of argatroban-complexed thrombin was measured using an oligonucleotide-based
enzyme capture assay. In normal human plasma thrombin at concentrations up to 4 ng/ml
(109 pM) became inactivated with an average half-life time of 56.4 ± 4.7 seconds (s).
In antithrombin-deficient plasma the thrombin half-life was prolonged to 168.2 ± 14.9
s. Among the thrombophilic patients, only one with mild antithrombin deficiency showed
impaired thrombin inactivation rates, whereas all other patients including the antiphospholipid
positive patients showed thrombin inhibiting capacities within the normal range. We
conclude that thrombin added to normal human plasma at subthreshold levels of ∼100
pM or below becomes inactivated with a half-life time below 1 minute. Antiphospholipid
antibodies do not prolong thrombin half-life times, making it unlikely that delayed
thrombin inactivation contributes to the thrombotic phenotype of the antiphospholipid
syndrome. In contrast, plasma levels of antithrombin falling below 80% of normal markedly
prolong the thrombin half-life.
Keywords
Thrombin - half-life time - inactivation - antithrombin - antiphospholipid syndrome
