Stability of Prothrombin in the Presence of Thrombin

Abstract

In earlier numbers of these PROCEEDINGS, Mertz, Seegers and Smith, allege that thrombin can inactivate its precursor prothrombin. We have published evidence that a trypsin-like enzyme is not only a thromboplastic agent but also a progressive antithrombin of major importance in natural thrombin destruction. It is present, to a varying extent, in serum and in most prothrombin and thrombin preparations. There is doubt whether such a factor was ruled out in the Iowa experiments. It should be pointed out that Smith, et al., use extremely potent thrombic agents and assay them by a dilution technic which yields values up to several thousands. In our experience, strong thrombins (and prothrombins) appear unusually stable but turn out to be much less so when diluted to, say, about 5–25 Iowa units. In Table I of another Iowa communication, there is a 9% loss of activity of thrombin in the presence of prothrombin as compared with a much more startling 95% fall in the prothrombin titer during the same 4-hour period. Reagents and Methods. Our routine prothrombins, when maximally activated with CaCl2 and brain thromboplastin, usually clot a test fibrinogen in some 5-20+ seconds (temp. = 37.5°C, pH = 7.5). A typical prothrombin solution, assayed through the courtesy of Dr. H. P. Smith (Iowa) gave 47 units per cc (38 units per mg N). Persisting in our unwillingness to use any system of thrombin “units” for comparing one preparation with another, without convincing proof of control of instability factors, we restrict the assay to a single prothrombin preparation and the thrombins freshly prepared from it. Under these narrowed conditions, thrombin concentrations are measured, with a 5–10% accuracy, in a relative manner from the clotting-times obtained with a series of dilutions tested under strictly comparable conditions.