Variation in sialic acid expression on VWF and susceptibility to ADAMTS13 proteolysis. (A) To investigate whether sialic acid expression on VWF influences susceptibility to ADAMTS13 proteolysis, wild-type plasma–derived VWF (pd-VWF) α2-3,6,8,9 neuraminidase–treated VWF (Neu-VWF), and PNGase F–treated VWF (PNG-VWF) preparations were incubated with 3nM human rADAMTS13 in the presence of 1.5M urea. Rate of VWF cleavage was assessed by determining the rate of VWF:CB decrease over time. Results are expressed as residual VWF:CB (mean ± SEM). (B) VWF proteolysis was also studied by nonreducing 1% agarose gel electrophoresis. (C) To determine whether α2-3–linked sialic acid on VWF influenced ADAMTS13 proteolysis, pd-VWF and α2-3 neuraminidase–treated VWF (α2-3 Neu-VWF) were incubated with recombinant ADAMTS13 in the presence of 1.5M urea as before. Rate of proteolysis for α2-3Neu-VWF was compared with pd-VWF by determining residual collagen binding activity.