Cleavage of ULVWF strings in plasma under flow. (A) WT ADAMTS-13, MDTCS, and MDTC were mixed with ADAMTS-13-depleted plasma to final concentrations of 670, 670, and 800 nM, respectively, and were perfused over HUVECs under a shear stress of 2.5 dyne/cm². The percent cleavage of ULVWF strings for MDTCS mutant was similar to that of WT at 100%, whereas it was 54.8 ± 6.8% for the MDTC mutant, significantly lower than those of WT and MDTCS mutant, but higher than its activity in the absence of plasma. The figures are mean ± SEM; n = 22; *P < .01 compared with WT. (B) WT ADAMTS-13 was perfused over histamine-stimulated HUVECs in the presence or absence of the enzyme-depleted plasma. The percent cleavage for the 2 enzyme preparations was similar at 670, 335, and 168 nM, but significantly higher for ADAMTS-13 in the depleted plasma at 84 nM. The figures are mean ± SEM; n = 4; *P < .05.