Relative importance of N-linked sialic acid and ABO blood group antigens in determining VWF proteolysis by ADAMTS13. (A) To investigate the effect of sialic acid removal on the cleavage of blood group–specific VWF, purified O or AB VWF was incubated with or without neuraminidase and then subjected to ADAMTS13 proteolysis in the presence of 1.5M urea. Rate of VWF cleavage was assessed by determining the decrease in VWF:CB over time. Results are expressed as residual collagen-binding activity (mean ± SEM). (B) To determine whether loss of the A antigenic carbohydrate structure from blood group A VWF (A-VWF) affects susceptibility to ADAMTS13 proteolysis, A-VWF was digested with A-Zyme (N-acetyl-galactosaminidase), and ADAMTS13 cleavage assay was carried out as before. A-VWF– and A-Zyme–treated VWF (AZ-VWF) were cleaved at the same rate by ADAMTS13, and both were cleaved significantly more slowly than O-VWF (P < .05).