Binding of low-molecular-weight canine factor VIII coagulant from von Willebrand plasma to canine factor VIII-related antigen.

Plasmas having no detectable factor VIII-related antigen but moderate factor VIII coagulant were obtained from two unrelated dogs homozygous for von Willebrand's disease and with the severe clinical expression of the disease. when these plasmas were gel-filtered in a buffer at physiologic ionic strength, the factor VIII coagulant eluted in the bed volume as a single well-defined peak. Addition of protease inhibitors, including diisopropylfluorophosphate, did not change the elution pattern. Each plasma was then combined individually with plasmas from six different mutants of canine haemophilia, all of which had normal factor VIII-related antigen but no detectable factor VIII coagulant. The factor VIII coagulant elution profile of these combined plasma resembled that of normal canine plasma. Slightly over half of the recovered factor VIII coagulant coeluted in the void volume with the factor VIII-related antigen; the rest eluted as a second, distinct peak of lower molecular weight. These results demonstrated that part of the factor VIII coagulant of the von Willebrand plasmas had bound to the factor VIII-related antigen of the haemophilic plasmas. This finding supports the theory that factor VIII exists as a macromolecular complex of nonidentical components in normal citrated plasma.