The hyaluronic acid binding serine protease (PHBSP), an enzyme with the abi
lity to activate the coagulation factor FVII and the plasminogen activator
precursors and to inactivate factor VIII and factor V, could be isolated fr
om human plasma in the presence of 6 M urea as a single-chain zymogen, wher
eas under native conditions only its activated two-chain form was obtained.
The total yield of proenzyme (proPHBSP) was 5-6 mg/l, corresponding to a c
oncentration of at least 80-100 nM in plasma. Upon removal of urea, even in
the absence of charged surfaces a rapid development of amidolytic activity
was observed that correlated with the appearance of the two-chain enzyme.
The highest activation rate was observed at pH 6. ProPHBSP processing was c
oncentration-dependent following a second order kinetic and was accelerated
by catalytic amounts of active PHBSP, indicating an intermolecular autocat
alytic activation. Charged macromolecules like poly-L-lysine, heparin, and
dextran sulfate strongly accelerated the autoactivation, suggesting that in
vivo proPHBSP activation might be a surface-bound process. The intrinsic a
ctivity of the proenzyme was determined to be 0.25-0.3%, most likely due to
traces of PHBSP. The presence of physiological concentrations of known pla
sma inhibitors of PHBSP, like alpha (2) antiplasmin and C1 esterase inhibit
or, but not antithrombin III/heparin, slowed down zymogen processing. Our i
n vitro data suggest that the autoactivation of proPHBSP during plasma frac
tionation is induced by the removal of inhibitors of PHBSP and is accelerat
ed by charged surfaces of the chromatographic resins.