rhADAMTS13. (A) Cleavage of ULVWF-platelet strings by dilutions of rhADAMTS13. HUVECs were stimulated with histamine (20 mM) for 10 minutes and exposed to normal human platelets under flow. The ULVWF-platelet strings that formed were then cleaved over the course of 4 minutes by different dilutions of added rhADAMTS13 (n = 2-6 at each dilution). (Inset) Arrow shows rhADAMTS13 band in the gel lane at approximately 160 Mr, detected by anti-His tag antibody. (B) Neither Stx-1 nor Stx-2 inhibits the rhADAMTS13 cleavage of large or unusually large VWF forms under static nonphysiologic conditions. Inhibition of cleavage was observed under these conditions only if the known ADAMTS13 inhibitor, EDTA (1 mM), was also present. VWF enriched in ULVWF, derived from cultured HUVEC supernatant, was incubated with rhADAMTS13 for 24 hours in the presence of BaCl₂ and urea, and 10 nM of either Stx-1 or Stx-2; VWF multimers were displayed as described in Figure 2(n = 3).