von Willebrand factor (VWF) multimeric composition is regulated in plasma by ADAMTS13. VWF deglycosylation enhances proteolysis by ADAMTS13. In this study, the role of terminal sialic acid residues on VWF glycans in mediating proteolysis by ADAMTS13 was investigated. Quantification and distribution of VWF sialylation was examined by sequential digestion and high-performance liquid chromatography analysis. Total sialic acid expression on VWF was 167nmol/mg, of which the majority (80.1%) was present on N-linked glycan chains. Enzymatic desialylation of VWF by α2-3,6,8,9 neuraminidase (Neu-VWF) markedly impaired ADAMTS13-mediated VWF proteolysis. Neu-VWF collagen binding activity was reduced to 50% (± 14%) by ADAMTS13, compared with 11% (± 7%) for untreated VWF. Despite this, Neu-VWF exhibited increased susceptibility to other proteases, including trypsin, chymotrypsin, and cathepsin B. VWF expressing different blood groups exhibit altered ADAMTS13 proteolysis rates (O ≥ B > A ≥ AB). However, ABO blood group regulation of ADAMTS13 proteolysis was ablated on VWF desialylation, as both Neu-O-VWF and Neu-ABVWF were cleaved by ADAMTS13 at identical rates. These novel data show that sialic acid protects VWF against proteolysis by serine and cysteine proteases but specifically enhances susceptibility to ADAMTS13 proteolysis. Quantitative variation in VWF sialylation therefore represents a key determinant of VWF multimeric composition and, as such, may be of pathophysiologic significance. © 2010 by The American Society of Hematology.