Evidence for extracellular processing of pro-von Willebrand factor after infusion in animals with and without severe von Willebrand disease

Although proteolytic processing of pro-von Willebrand factor (pro-vWF) resu lting in free propeptide and mature vWF is known to be initiated intracellu larly, 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 bl ood samples drawn before and after infusion. vWF:Ag increased promptly. No pro-vWF could be detected when the first postinfusion sample was drawn afte r 30 minutes (pigs) or 60 minutes (mice), but pro-vWF was detectable for sh ort periods when postinfusion samples were drawn after 15 minutes (dog) or 5 minutes (baboons). In contrast, free propeptide was increased at the firs t time-point measured, suggesting that it was generated from the pro-vWF in the rpvWF preparation. vWF multimers were analyzed in the rpvWF preparatio n and in plasma samples drawn before and after infusion of rpvWF using ultr a-high resolution 3% agarose gels to allow separation of homo- and hetero-f orms of the vWF polymers. Within 30 minutes after infusion in the pigs, 1 h our in the dog and the mice, and within 2 hours in the baboons, the multime r pattern had changed to that typically seen in mature vWF. These data indi cate that propeptide cleavage from unprocessed vWF can occur extracellularl y in the circulation. The enzyme or enzymes responsible for this cleavage i n plasma remain to be identified. (C) 1999 by The American Society of Hemat ology.