Is there a precursive, relatively procoagulant-inactive form of normal antihemophilic factor (factor VIII)?

When factor VIII/von Willebrand factor (FVIII/vWF) is chromatographed on 4% agarose in 0.25 M CaCI2. the procoagulant activity partially separates from the void volume protein peak. If FVIII/vWF is reacted with thrombin prior to chromatography, both the magnitude and resolution of the proe.oagulant activity peak are greatly increased. These observations suggest that activated species of FVIII/vWF are contained in the late-eluting procoagulant activity peak, and therefore, a precursive form of FVIII/vWF may exist. Such a precursor might be expected to: (1 ) lack procoagulant activity; (2) elute in the void volume from 4% agarose-CaCl2; and (3) be activated by thrombin, after which the procoagulant activity should elute in the inner volume from 4% agarose-CaCI2 columns. The specific procoagulant activities for FVIII/vWF purified in the presence and absence of the protease inhibitors, heparin-bovine pancreatic trypsin inhibitor and diisopropylfluorophosphate, were 27 and 92 U/mg. respectively. For FVIII/vWF isolated in the presence of this mixture of inhibitors. we observed that a peak with very low specific procoagulant activity. but containing at least 80% of the total activity. consistently eluted in the void volume from 4% agarose-O.25 M CaCI2 gel filtration. The FVIII/vWF prepared in the absence of protease inhibitors could be activated 13.8 ± 3.0-fold by thrombin. compared to 6.6 ± 1 .7-fold for FVIII/vWF prepared without protease inhibitors. When FVIII/vWF prepared in the presence of inhibitors is activated by thrombin prior to 4% agarose-0.25 M CaCI2 chromatography, virtually no FVIII procoagulant activity elutes in the void volume; instead, a considerably enhanced symmetrical peak of FVIII procoagulant activity elutes in the inner volume. These data imply that FVIII/vWF circulates in vivo as a precursor with little or no procoagulant activity until activated by thrombin or a thrombin-like enzyme.