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Plasma collagen cofactor correlates with von Willebrand factor antigen and ristocetin cofactor but not with bleeding time.

Collagen cofactor (CCo), an activity of von Willebrand factor (vWF) which increases the rate of adhesion of human fixed washed platelets (FWP) to collagen, was measured in plasma from normal individuals and individuals with von Willebrand's disease (vWD). CCo in vWD plasma was compared to vWF antigen (vWF:Ag), ristocetin cofactor (RCo), factor VIII (VIII) coagulant activity (VIII:C) and the quantitative bleeding time. There was close correlation between CCo and VIII:C (r = 0.909), vWF:Ag (r = 0.975), and RCo (r = 0.936). However, there was no correlation between CCo and the quantitative bleeding time. Plasma CCo in type IIA vWD was markedly lower than vWF:Ag and the ratio of CCo/vWF:Ag was 0.08, which was less than a mean value of 0.92 in type I vWD. CCo activity in normal plasma was completely inhibited by monoclonal antibody CLB-RAg 201, an antibody that inhibits the binding of vWF to collagen, suggesting that the binding of vWF to collagen is required for the expression of CCo. Furthermore, the partial inhibition of CCo by monoclonal antibody CLB-RAg 35 that inhibits the binding of vWF to platelet in the presence of ristocetin, suggests that CCo is partly mediated through platelet membrane glycoprotein Ib. Large multimers of vWF:Ag in normal plasma were preferentially absorbed by collagen. These studies demonstrate that CCo is another functional activity of vWF and the measurement of CCo may be useful for the detection of new variant forms of vWD.