In vitro multimerization of human von Willebrand factor from its subunits.

von Willebrand factor (vWF) is a multimeric glycoprotein composed of subunits having an estimated molecular mass of 220-240 kDa which are linked by disulphide bonds. Intact, unreduced vWF, has ristocetin co-factor activity, which is lacking in the reduced subunits. To determine whether reduced vWF subunits are capable of being re-multimerized vWF was purified, reduced and then subsequently dialysed in the absence of reducing agents. (1) Purified vWF, (2) reduced vWF, and (3) reduced and dialysed vWF were compared for size composition and for ristocetin co-factor activities. SDS-PAGE showed that the purified vWF failed to enter the gels, while the reduced vWF migrated to a position consistent with a mass of approximately 240 kDa. Reduced and subsequently dialysed vWF subunits multimerized into larger forms, most of which did not enter the gels. No residual 240 kDa material was detectable following remultimerization. vWF multimer analysis confirmed the loss of monomer in the reduced and dialysed material and the appearance of new multimers with a size range smaller than the native material. Reduced vWF showed no detectable ristocetin co-factor activity, while the re-multimerized material regained some activity. Thus, we deduce that vWF can spontaneously re-multimerize from its reduced subunits and regain a small but measurable quantity of ristocetin co-factor activity.