In vivo and in vitro processing of recombinant pro-von Willebrand factor.

Von Willebrand factor (vWF) is synthesized in endothelial cells as pre-pro-vWF and processed intracellularly to propeptide (vWFpp) and mature vWF. Recombinant pro-vWF when infused into animals can also be processed extracellularly in vivo. Within 1 h of infusion in a dog and mice the multimer pattern changed to that typically seen in mature vWF indicating that propeptide cleavage from unprocessed vWF occurs extracellularly in the circulation. Incubation of a recombinant pro-vWF preparation with canine and human vWF-deficient plasma induced a time-dependent decrease in pro-vWF antigen and an increase in vWFpp antigen without changing total vWF antigen or collagen-binding activity. Multimer analysis showed the gradual transformation of the pro-vWF multimers to mature vWF multimers and cleaved vWFpp was visualized on autoradiograms of SDS-polyacrylamide electrophoresis gels using (125)I-labeled pro-vWF. When recombinant pro-vWF was incubated with increasing amounts of purified thrombin, the extent of pro-vWF processing was dose dependent. The specific cleavage of vWFpp was confirmed by immunoblots using an anti-vWFpp antibody and by amino-terminal amino acid analysis. Hirudin preconditioning of vWF-deficient mice attenuated processing of infused recombinant pro-vWF suggesting that thrombin plays a part in the processing events in vivo.