Reduction of von Willebrand factor by endothelial cells

The haemostatic activity of plasma von Willebrand factor (vWF) is a functio n of multimer size. Only the large vWF multimers are effective in promoting platelet adhesion to a site of vascular injury. We observed that the condi tioned medium of cultured human umbilical vein, human microvascular and bov ine aortic endothelial cells contained an activity which reduced the averag e multimer size of plasma or purified vWF. The average multimer size of vWF produced endogenously by human umbilical vein endothelial cells was simila rly reduced following secretion. The reducing activity was ablated by pre-t reatment with heat or the thiol blocking agents, iodoacetamide, N-ethylmale imide or E-64, but not by a range of specific serine-, cysteine-, aspartic- , or metalloproteinase inhibitors. Reduction in vWF multimer size was assoc iated with formation of new thiols in vWF and there was no evidence for add itional proteolytic processing of vWF. The reducing activity was associated with a protein with an anionic pI that binds heparin and contains reactive thiol(s). These results suggested that the interchain disulfide bonds that link the vWF homodimers near the N-termini were being reduced by a vWF red uctase secreted by endothelial cells. In support of this hypothesis, incuba tion of vWF with the protein reductants, protein disulfide isomerase and th ioredoxin, resulted in formation of new thiols in vWF and reduction in the average multimer size of vWF. These findings may have consequences for cont rol of vWF haemostatic activity.