It is well known that on artificial surfaces, binding and autoactivati
on of factor XII (FXII) is the initiating event of plasma prekallikrei
n (PK) activation. We performed investigations to examine whether this
mechanism was true for FXII activation on endothelial cells (HUVEV).
Activation of PK on HUVEC required an optimal substrate and Zn2+ conce
ntration, the latter of which varied with the buffer's carrier protein
, Maximal PK activation required the addition of 250 mu M or 10 mu M Z
n2+ to buffers containing bovine serum albumin (BSA) or gelatin, respe
ctively. However, the actual free Zn2+ concentration in these buffers
was the same at 8 mu M. In both BSA- and gelatin-containing buffers an
d using two different chromogenic substrates for FXII, no autoactivati
on of FXII on HUVEC was seen when incubated for up to 60 min. Rather.
initiation of FXII enzymatic activity required the presence of PK. FXI
I activation after PK activation contributed to the extent of measured
enzymatic activity, but its role was secondary because treatment with
corn trypsin inhibitor or a neutralizing antibody to FXIIa did not ab
olish the measured enzymatic activity. They also reduced the activity
to the level seen with PK activation alone. Alternatively. soybean try
psin inhibitor abolished the proteolytic activity associated with PK a
nd FXII activation on HUVEC. Further, only normal human and FXII-defic
ient plasmas, not PK-deficient plasma, had the ability to generate pro
teolystic activity when incubated over endothelial cells. In a purifie
d system, maximal PK activation was measured after a 10-15 min incubat
ion depending upon the concentration of reactants. When FXII was added
with the PK, maximal activation occurred within 7.5-10 min. In normal
human or FXII-deficient plasmas, but not in PK-deficient plasma. maxi
mal activation was seen in 4 min. These data indicate that on HUVEC, u
nlike artificial surfaces, PK activation when bound to HK is the initi
ating activation event in this system. FXII activation is secondary to
PK activation and contributes to the extent of measured enzymatic act
ivity. These data challenge the accepted dogmas of ''contact activatio
n'' and suggest that on biologic membranes a new notion as to how this
system is activated needs to be considered.