The dimeric Aα chain composition of dysfibrinogenemic molecules with mutations at Aα 16

Abstract In the last stage of fibrinogen synthesis, two Aα-Bβ-γ half-molecules are disulfide linked in their N-terminal regions to form a dimeric fibrinogen molecule. It is not known whether intracellular hepatocyte assembly of fibrinogen half-molecules occurs randomly or is a directed process. One analysis based on partitioning of coagulable components of fibrinogen from a heterozygous dysfibrinogenemic subject having a mutation at the thrombin cleavage site (Fibrinogen Louisville, Aα16 R→H), suggested that only homodimeric molecules containing two normal fibrinopeptides A (FPA, FPA) or two abnormal fibrinopeptides A (FPA ★ , FPA ★ ) were present in plasma, implying that fibrinogen dimer assembly is directed. The same type of analyses on Fibrinogen Birmingham (Aα16 R→H) indicated that there were heterodimers as well as homodimers, suggesting that fibrinogen dimer assembly is random. To examine this question more directly, the composition of fibrinogen molecules from seven dysfibrinogenemic families with either R→C (four) or R→H (three) Aα16 mutations was determined. Following treatment with Atroxin to release normal FPA from fibrinogen, N-terminal disulfide knot (‘N-DSK’) cleavage fragments were prepared and subsequently separated by SDS-PAGE to resolve ‘N-DSK’ components with two FPA ★ 's (N-DSK homodimer), one FPA ★ (des A N-DSK heterodimer), or no FPA's (des AA N-DSK homodimer). Fibrinogen from subjects whose molecules contained both normal and abnormal Aα chains, yielded a heterodimeric des A N-DSK derivative, as well as smaller amounts of homodimeric N-DSK and des AA N-DSK. These results indicate that when both types of Aα chain are produced, both Aα chain alleles are expressed and the resulting fibrinogen dimers are assembled randomly.