Plasmin Digestion of factor VIII: Characterization of the Breakdown Products with Respect to Antigenicity and von Willebrand Activity

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Abstract
Highly purified factor VIII was incubated for up to 24 hours in the presence of plasmin, and the biological activities and peptide structure of the digestion products determined at intervals. Procoagulant activity (VIIIC) was rapidly lost, but 17-32% of the initial ristocetin cofactor (VIIIR: WF) activity remained after 24 hours. Immunoelectrophoresis showed a progressive increase in rocket height and a concomitant increase in electrophoretic mobility of the factor-VIII-related antigen (VIIIR: AG). Crossed immunoelectrophoresis of the 24- hour digest showed three distinct precipitin arcs, of which the major one, with intermediate anodal mobility, gave reactions of non-identity with the other two. On sepharose gel chromatography the 24-hour digest gave three peaks: peak II contained about 80% of the residual VIIIR: WF and resolved on SDS-polyacrylamide gels into a series of peptides with apparent molecular weights between 125,000 and 185,000; these were reduced by mercaptoethanol to fragments of 15,000-80,000 daltons, a 65,000 dalton fragment being particularly strongly PAS positive. We conclude that large molecular size is not a prerequisite for VIIIR :WF activity, and that the presence of factor-VIII breakdown products may be a cause of misleading results in the determination of VIIIR:AG by immunoelectrophoresis.

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