Although proteolytic processing of
pro-von Willebrand factor (
pro-vWF) resulting in free propeptide and mature vWF is known to be initiated intracellularly, vWF released from endothelial cells may contain a high proportion of incompletely processed
pro-vWF. Because
pro-vWF is only rarely detectable in normal human plasma, we investigated whether extracellular processing of
pro-vWF is possible. A recombinant preparation (rpvWF) containing both
pro-vWF and mature vWF subunits was infused into 2 pigs and 1 dog with severe
von Willebrand disease, 2 mice with a targeted disruption of the vWF gene, and 2 healthy baboons. Total vWF
antigen (vWF:Ag), free propeptide, and
pro-vWF were measured using
enzyme-linked
immunosorbent assay techniques in blood samples drawn before and after infusion. vWF:Ag increased promptly. No
pro-vWF could be detected when the first postinfusion sample was drawn after 30 minutes (pigs) or 60 minutes (mice), but
pro-vWF was detectable for short periods when postinfusion samples were drawn after 15 minutes (dog) or 5 minutes (baboons). In contrast, free propeptide was increased at the first timepoint measured, suggesting that it was generated from the
pro-vWF in the rpvWF preparation. vWF multimers were analyzed in the rpvWF preparation and in plasma samples drawn before and after infusion of rpvWF using ultra-high resolution 3%
agarose gels to allow separation of homo- and hetero-forms of the vWF
polymers. Within 30 minutes after infusion in the pigs, 1 hour in the dog and the mice, and within 2 hours in the baboons, the multimer pattern had changed to that typically seen in mature vWF. These data indicate that propeptide cleavage from unprocessed vWF can occur extracellularly in the circulation. The
enzyme or
enzymes responsible for this cleavage in plasma remain to be identified.