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Platelets modulate the proteolysis of factor VIII:C protein by plasmin
ME Rick and DM Krizek
Factor VIII coagulant protein (VIII:C) functions as a critical cofactor
with factor IXa, calcium ions, and phospholipid during the activation of
factor X. In the course of this reaction, the activity of VIII:C is first
increased and then is destroyed by one or more serine proteases that are
part of the coagulation sequence. In this study, we have investigated the
influence of platelets on the inactivation of VIII:C by plasmin. Platelets
were separated from plasma proteins in the presence of granule release
inhibitors and were incubated with plasmin and isolated VIII:C or the
complex of purified VIII:C/von Willebrand factor (vWF); VIII:C activity and
antigen levels were assessed over time. In the presence of platelets, the
isolated VIII:C showed an initial increase in VIII:C activity that was not
present when platelets were absent, and the VIII:C/vWF showed an increase
in VIII:C activity over that seen when platelets were absent. In addition,
platelets stabilized VIII:C activity over a one-hour time course when
compared with buffer. The VIII:C antigen did not increase and decreased
slowly whether platelets were present or absent. Preincubating the
platelets with ristocetin, collagen, or plasmin did not alter the results,
and experiments using platelets from a patient with severe von Willebrand's
disease also showed a pattern similar to that seen with normal platelets.
Experiments using fixed platelets or phospholipid vesicles showed that they
did not support the activation reaction or delay the inactivation reaction.
These studies demonstrate that platelets modulate the activation and
inactivation of VIII:C by plasmin, apparently by a mechanism that is
independent of the platelet release reaction.
Volume 67,
Issue 6,
pp. 1649-1654,
06/01/1986
Copyright © 1986 by The American Society of Hematology
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