Summary
Previous studies of factor (F)Va inactivation on human umbilical vein endothelial
cells have shown that α-thrombin cleaves the heavy chain near the COOH-terminus to
produce a Mr 97,000 fragment containing the NH2-terminal portion of the heavy chain and a Mr 8,000 peptide containing the rest of the molecule. The α-thrombin cleavage appeared
to occur between amino acid residues 586 and 654 of FV. This region contains a consensus
sequence for α-thrombin cleavage located at residues 640–644 (S-S-P-R-S). To test
the hypothesis that α-thrombin cleaves the FVa heavy chain at Arg643 and to evaluate the functional importance of this cleavage for FVa inactivation,
sitedirected mutagenesis was used to create recombinant FV molecules with mutations
R643→Q (FVR643Q) and R643→A (FVR643A). All recombinant molecules were purified to homogeneity and assayed for activity
following extended activation with α-thrombin. Under similar experimental conditions,
appearance of the Mr 97,000 heavy chain fragment in the plasma and wild-type FVa molecules correlated
with partial loss of cofactor activity, while following extended incubation of FVR643Q and FVR643A with α-thrombin no cleavage of the heavy chain at Arg643 was detected and no presence of the Mr 97,000 heavy-chain fragment was noticed. Further, no loss in cofactor activity was
observed using these mutant recombinant FVa molecules. Our data demonstrate that cleavage
of FVa at Arg643 by α-thrombin results in a partially inactive cofactor molecule and provides for
an activated protein C (APC)-independent anticoagulant effect of α-thrombin.
Keywords
Factor Va - α-thrombin - inactivation - heavy chain - coagulation