Summary
An evaluation of the mode of action of antithrombin in the temporary inhibition of
purified 3.7 S bovine thrombin was made according to traditional enzyme inhibition
theory. Using enzyme clotting activity and concentration of active sites synonymously,
it was observed that the binding of antithrombin to thrombin followed a second order
reaction that could be distinguished into an essentially irreversible phase and a
slower, potentially reversible phase. The level of steady state enzyme inhibition
depended upon the ratio of the initial concentrations of enzyme to antithrombin. In
addition, there occurred a rapid enhancement of enzyme esterolytic activity during
the concomitant loss in clotting activity. This enhancement was also observed for
acetylated thrombin which had no proteolytic activity. Under high concentrations of
antithrombin, this enhancement subsequently deteriorated. A working hypothesis, including
equations, for a mutual depletion system is presented to account for the known parameters
of enzyme inhibition by antithrombin. We compared the neutralization of thrombin C
with autoprothrombin C and found that each enzyme responds to antithrombin according
to similar kinetic relationships.
