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Activation of human factor VII in the initiation of tissue factor-
dependent coagulation
LV Rao, SI Rapaport and SP Bajaj
We have used activation peptide release assays to compare factor VII and
activated factor VII (VIIa) activation of factor X, normal factor IX (IXN),
and a variant factor IX (IXBmLE), which, after activation, is unable to
back-activate factor VII. In purified systems, factor VII and VIIa each
rapidly activated factor X, but after a one minute lag for factor VII. VIIa
also readily activated both IXN and IXBmLE. Factor VII initially failed to
activate substantial amounts of either IXN or IXBmLE; on further incubation
factor VII activated IXN but not IXBmLE. Activation of IXN began when
approximately 10% of factor VII had been converted to VIIa, as measured by
125I-factor VII radioactivity profiles. Adding factor VII to VIIa slowed
its activation of IXBmLE. However, in the presence of factor X, factor VII
alone rapidly activated IXBmLE. Unlike purified systems, 1 nmol/L VIIa
added to factor VII-deficient plasma failed to activate factor IX.
Increasing factor VII to 10 nmol/L (plasma concentration) either as native
VII or VIIa yielded similar activation curves for factor IX and similar
activation curves for factor X. Adding 5% VIIa to factor X-deficient plasma
and to factor XII-deficient plasma substantially shortened the dilute
tissue factor clotting time of only the former. These data support the
hypothesis that factor VII/tissue factor complex initiates tissue
factor-dependent clotting through a minimal generation of Xa. This Xa then
rapidly back-activates a small amount of factor VII, following which the
rates of activation of both factors IX and X increase dramatically.
Volume 68,
Issue 3,
pp. 685-691,
09/01/1986
Copyright © 1986 by The American Society of Hematology
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