ADAMTS13 is a metalloproteinase that cleaves von Willebrand factor (VWF) multimers. The metal ion dependence of ADAMTS13 activity was examined with multimeric VWF and a fluorescent peptide substrate based on Asp 1596 -Arg 1668 of the VWF A2 domain, FRETS-VWF73. ADAMTS13 activity in citrate-anticoagulated plasma was enhanced ϳ2-fold by zinc ions, ϳ3-fold by calcium ions, and ϳ6-fold by both ions, suggesting cooperative activation. Cleavage of VWF by recombinant ADAMTS13 was activated up to ϳ200-fold by zinc ions (K D app ϳ0.5 M), calcium ions (K D app ϳ4.8 M), and barium ions (K D app ϳ1.7 mM). Barium ions stimulated ADAMTS13 activity in citrated plasma but not in citrate-free plasma. Therefore, the stimulation by barium ions of ADAMTS13 in citrated plasma appears to reflect the release of chelated calcium and zinc ions from complexes with citrate. At optimal zinc and calcium concentrations, ADAMTS13 cleaved VWF with a K m app of 3.7 ؎ 1.4 g/ml (ϳ15 nM for VWF subunits), which is comparable with the plasma VWF concentration of 5-10 g/ml. ADAMTS13 could cleave ϳ14% of VWF pretreated with guanidine HCl, suggesting that this substrate is heterogeneous in susceptibility to proteolysis. ADAMTS13 cleaved FRETS-VWF73 with a K m app of 3.2 ؎ 1.1 M, consistent with an ϳ200-fold decrease in affinity compared with VWF. ADAMTS13 cleaved VWF and FRETS-VWF73 with roughly comparable catalytic efficiency of 55 M ؊1 min ؊1 and 18 M ؊1 min ؊1 , respectively. The striking preference of ADAMTS13for VWF suggests that substrate recognition depends on structural features or exosites on multimeric VWF that are missing from FRETS-VWF73.The von Willebrand factor (VWF) 2 cleaving proteinase ADAMTS13 is a member of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin repeats") family (1-3). Since the identification of ADAMTS13, evidence has increased concerning the association of severe ADAMTS13 deficiency with the disease thrombotic thrombocytopenic purpura (TTP) (4 -6). TTP is characterized by disseminated microvascular thrombi containing platelets and multimers of VWF, which is a plasma protein that mediates platelet adhesion by tethering platelets to the extracellular matrix (7,8). In the absence of ADAMTS13 activity, ultra-large multimers of VWF accumulate, causing persistent intravascular platelet aggregation and TTP. Congenital TTP, or Upshaw-Schulman syndrome, is caused by compound heterozygous or homozygous mutations in the ADAMTS13 gene (2, 9, 10). Acquired idiopathic TTP usually affects adults and is caused predominantly by autoimmune responses to ADAMTS13 (6). The mortality rate is ϳ90% if untreated; however, plasma exchange therapy has reduced this rate to ϳ20% (11-13).Many proteinases of the ADAMTS family are involved in extracellular matrix remodeling, angiogenesis, and development, where they typically cleave large multimeric proteins (14, 15 , and a conserved Met 249 that supports the active site zinc ion in a "Met turn"; these features identify ADAMTS13 as a member of the "metzincin" family (1,18,19). The me...