The Interaction of the Factor Vlll/von Willebrand Factor Complex with Hematin

SummaryIntravenous infusion of hematin, used in the treatment of acute porphyria, induces a decline in the plasma factor VIH/von Willebrand factor complex (VIII/vWF) and thrombocytopenia. We investigated this problem by studying the interaction between hematin, purified VIII/vWF, and platelets in vitro. Hematin was labeled with either 59Fe or 3H and characterized by gel chromatography. Hematin self-aggregated, forming a complex with an average molecular weight of approximately 10,000 daltons. When incubated with VIII/vWF for 30 min at 37° C and applied to Sepharose CL-4B, the hematin eluted with the VIII/vWF in the void volume. Hematin inhibited the dissociation of factor VIII antigen (VIII :Ag) from the von Willebrand antigen (vWF:Ag) in 0.25 M CaCl2, and reversed the aggregation of VIII:Ag induced by 0.1 M 6-aminocaproic acid. Both hematin and the hematin-VIII/vWF complex bound to washed normal platelets and to platelets from a patient with Bemard-Soulier syndrome. Thromb-asthenic platelets were not aggregatable by hematin, and bound significantly less hematin-VIII/vWF than normal platelets suggesting that hematin-induced platelet activation was required for binding. Likewise, binding was inhibited by PGE1 which also prevented aggregation. We conclude that hematin forms complexes with VIII/vWF, alters the functional activity and dissociation of this compound, and participates in the binding of VIII/vWF to platelets.

Factor VIII Coagulant Antigen (VIII. CAg) in Fetal Blood Samples

Quantitative immunoradiometric assays (IRMAS) for the coagulant portion of the FVIII complex [VIII: CAg) have recently been described several laborateries (Reisner et al. Thrombosis Research, in press). The antigen detected by these assays is absent or decreased in the plasma of most individuals with severe hemophilia A, Levels of VIII:CAg and FVIII related antigen (VIIIR.Ag) were measured in ten presumably non-hemophiiic samples obtained either by fetoscopy or after abortion. Ratios of VIII:CAg to VIIIR:Ag ranged from .07 to .52 in four samples where both values could be determined. VI11 :CAg could not be detected in four other samples. Three of these also lacked detectable V111R:Ag and were probably diluted with amniotic fluid which has neither VIII:CAg nor VIIIR:Ag. One Sample had 80% VIIIR. Ag in the absence of detectable VIII:CAg. VIII:CAg was detected in six samples with a range of 4 to 50% of an adult normal pool. An aborted male fetus from a potential carrier of severe hemophilia A had an VIII:CAg to VIIIR:Ag ratio of .42 and was probably not affected, The low values of VIII. CAg seen in this preliminary study may be explained by the decrease in antigenicity seen in serum versus plastra samples in our assay. Hence, extreme care in sample preparation and storage must be exercised should this IRMA for VIII:CAg be used in fetal diagnosis of hemophilia A.