Synergistic inhibition of the intrinsic factor X activation by protein S
and C4b-binding protein
SJ Koppelman, C van't Veer, JJ Sixma and BN Bouma
Department of Haematology, University Hospital, Utrecht, The Netherlands.
The complement protein C4b-binding protein plays an important role in the
regulation of the protein C anticoagulant pathway. C4b-binding protein can
bind to protein S, thereby inhibiting the cofactor activity of protein S
for activated protein C. In this report, we describe a new role for
C4b-binding protein in coagulation. We observed inhibition of the intrinsic
factor X activating reaction by the complex of C4b- binding protein and
protein S. At the plasma concentration of protein S, the factor X
activation was inhibited for 50% and addition of C4b- binding protein led
to a potentiation of the inhibition to almost 90%. Because C4b-binding
protein alone had no effect on the activation of factor X, we hypothesized
that binding of C4b-binding protein to protein S was a prerequisite for
optimal inhibition of factor X activation. C4b-binding protein lacking the
beta-chain, which is unable to bind to protein S, did not potentiate the
inhibitory effect of protein S. In an earlier study, we observed that
C4b-binding protein increased the binding affinity of protein S for factor
VIII. Therefore, a possible interaction of C4b-binding protein with factor
VIII was investigated. C4b-binding protein bound to factor VIII and to
thrombin activated factor VIII in a saturable and specific way. Also,
factor VIII in complex with von Willebrand factor was able to bind
C4b-binding protein. The beta-chain of C4b-binding protein was not required
for the interaction with factor VIII because C4b-binding protein lacking
the beta-chain also bound to factor VIII. Monoclonal antibodies directed
against the alpha-chain of C4b-binding protein inhibited the binding to
factor VIII, whereas monoclonal antibodies directed against the beta- chain
had no effect on the binding to factor VIII. This finding indicates that
the binding site for factor VIII on C4b-binding protein is localized on the
alpha-chains of C4b-binding protein. The potentiation by C4b-binding
protein of the inhibition of the factor X activation by protein S was
blocked by a monoclonal antibody directed against the alpha-chain of
C4b-binding protein. This finding indicates that the potentiation of the
inhibitory effect of protein S was mediated via an interaction of
C4b-binding protein with factor VIII. C4b-binding protein did not bind to
factor V and was not able to potentiate the inhibitory effect of protein S
on prothrombinase activity.(ABSTRACT TRUNCATED AT 400 WORDS)
Volume 86,
Issue 7,
pp. 2653-2660,
10/01/1995
Copyright © 1995 by The American Society of Hematology
