Human
meizothrombin, a transient intermediate in the activation of
prothrombin to
thrombin, was isolated in the presence of dansylarginine-N-(3-ethyl-1,5-pentanediyl)amide (DAPA), an active site inhibitor of
thrombin.
Meizothrombin autolysis to
meizothrombin autolysis to
meizothrombin des fragment 1 (cleavage at Arg273- Thr274) at different DAPA concentrations was monitoed by gel electrophoresis or by the changed fluorescence intensity of the DAPA-
protein complex.
Meizothrombin autolysis could be described by apparent pseudo-first order kinetics and was not eliminated even in the presence of a 100-fold molar excess of DAPA. By fitting the
autolysis rates observed at different DAPA concentrations to a simple inhibition model, the rate of uninhibited
autolysis at zero DAPA concentration, 22 h-1, and the dissociation constant of the DAPA/
meizothrombin complex, 5.9 x 10(-8) M, were obtained. The uninhibited
autolysis rate so obtained was consistent with an estimate (19 h-1) obtained from gel filtration chromatography experiments. A surprising finding was that the data could not be described adequately without inclusion of a DAPA-insensitive, or background, rate of
autolysis. We conclude that the active site of human
meizothrombin can never be completely blocked and
autolysis can never be completely prevented even in the presence of saturating concentrations of the active site inhibitor DAPA.