ADAMTS-13 Activity and Antigen in Commercial Von Willebrand Factor Concentrates.

The plasma metalloprotease ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin type I motif - 13) cleaves the Von Willebrand factor (VWF) at the peptide bond Tyr1605-Met1606 and thereby controls the hemostatic activity of the VWF. We tested three commercial VWF concentrates with respect to their ADAMTS-13 activity and antigen contents. ADAMTS-13 activity in the VWF concentrates was tested by measuring the capacity of the concentrates for auto-proteolysis both in the presence and in the absence of neutralising ADAMTS-13 auto-antibodies. To achieve this, the concentrates were reconstituted to a final VWF concentration of 100 I.E. U VWF:Ristocetin Cofactor activity (VWF:RCo). The VWF solutions were diluted with 5 mol/l urea and then incubated for 14–16h at 37°C in low ionic TRIS buffer containing BaCl2and different plasma samples or Imidazole buffer as plasma replacement.The residual VWF:RCo was subsequently measured using the BC von Willebrand Reagent from Dade Behring (Marburg, Germany). The residual VWF:RCo in the presence of plasma from a patient with acquired TTP (ADAMTS-13 activity: <6.25% due to high levels of neutralising ADAMTS-13 auto-antibodies) was then compared to the residual VWF:RCo in the mixtures containing normal plasma, Imidazole buffer or plasma from a patient with congenital TTP (ADAMTS-13 activity: <6.25% due to mutations in the ADAMTS-13 gene). ADAMTS-13 antigen was measured by a commercial ADAMTS-13 ELISA from American Diagnostica (Stamfort, USA).