Substrate-dependent variation of VWF cleaving activity of mouse plasma samples and recombinant mouse ADAMTS13 proteins. (A) Activity levels of FVB/NJ and C57BL/6J plasma samples against VWF multimers (multimers), human VWF73 fusion peptide (hVWF73), and mouse VWF73 fusion peptide (mVWF73). Mouse plasma samples were assayed for activity of cleaving the indicated substrates, and all values were expressed using normal human plasma as the reference. Each column in the graph represents the mean and standard deviation of results from 23 FVB/NJ or 21 C57BL/6J different mouse plasma samples for VWF multimers and 5 different plasma samples each for the VWF peptide substrates. FVB/NJ plasma ADAMTS13 was much more effective than C57BL/6J plasma in cleaving VWF multimers (P < .001). This difference diminished from 11- to 1.6-fold when either human or mouse VWF73 was used as the substrate. (B) Activity levels (means and standard deviations) of recombinant full-length ADAMTS13 protein and its IAP-a variant against various types of substrates. FRETS indicates FRETS-VWF73. Recombinant proteins were assayed for enzymatic activity against the indicated substrates. All values were expressed against normal human plasma as the reference. Each column represents the mean and standard deviation of results obtained with 3 lots of the protein. The IAP-a protein was less effective than full-length mouse ADAMTS13 in cleaving VWF multimers (P < .001). The difference diminished from 9- to 1.4- to 2.0-fold when VWF73 peptides were used as the substrates.