vWF-cleaving protease activity in plasma from patients with chronic relapsing TTP, from asymptomatic family members and in NHP. Each plasma was diluted 1:20 with TBS/1 mmol/L Pefabloc before incubation with purified vWF. The SDS-1.4% agarose gel (A) shows multimeric patterns of the vWF substrate after incubation with either diluted plasma samples or with Tris-buffered saline (TBS). Analysis of the disulfide-reduced incubation mixtures was performed by SDS-polyacrylamide gel electrophoresis (B). The arrows indicate positions of the intact vWF subunit (250 kD) and of its proteolytic fragments (170 kD and 140 kD).