THE DIMERIC A-ALPHA CHAIN COMPOSITION OF DYSFIBRINOGENEMIC MOLECULES WITH MUTATIONS AT A-ALPHA-16

In the last stage of fibrinogen synthesis, two A alpha-B beta-gamma ha lf-molecules are disulfide linked in their N-terminal regions to form a dimeric fibrinogen molecule. It is not known whether intracellular h epatocyte assembly of fibrinogen half-molecules occurs randomly or is a directed process. One analysis based on partitioning of coagulable c omponents of fibrinogen from a heterozygous dysfibrinogenemic subject having a mutation at the thrombin cleavage site (Fibrinogen Louisville , A alpha 16 R-->H), suggested that only homodimeric molecules contain ing two normal fibrinopeptides A (FPA, FPA) or two abnormal fibrinopep tides A (FPA, FPA*) were present in plasma, implying that fibrinogen dimer assembly is directed. The same type of analyses on Fibrinogen Bi rmingham (A alpha 16 R-->H) indicated that there were heterodimers as well as homodimers, suggesting that fibrinogen dimer assembly is rando m. To examine this question more directly, the composition of fibrinog en molecules from seven dysfibrinogenemic families with either R-->C ( four) or R-->H (three) A alpha 16 mutations was determined. Following treatment with Atroxin to release normal FPA from fibrinogen, N-termin al disulfide knot ('N-DSK') cleavage fragments were prepared and subse quently separated by SDS-PAGE to resolve 'N-DSK' components with two F PA'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 A alpha chains, yielded a heterodim eric 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 o f Acr chain are produced, both Aa chain alleles are expressed and the resulting fibrinogen dimers are assembled randomly.