Confirmation of mendelian properties of heterodimeric fibrinogen molecules in a heterozygotic dysfibrinogenemia, "fibrinogen Amarillo," using gprphoresis to differentiate semifibrin molecules from fibrinogen and fibrin

Evidence that heterodimers exist in the fibrinogen Matsumoto II (gamma308N-->K) proband and participate in fibrin fiber formation

INTRODUCTION

In this report, we established the gamma308K/N fibrinogen-secreting Chinese hamster ovary (CHO) cell line, which is an artificially heterozygous, Matsumoto II (M-II; gamma308K-->N) type of dysfibrinogen, to indirectly demonstrate the existence of heterodimeric molecules in propositus plasma and the participation of these molecules in fibrin fiber formation.

MATERIALS AND METHODS

We co-transfected the gamma-chain of gamma308K- and gamma308N-coding vectors into CHO cells expressing Aalpha- and Bbeta-chains and selected the clones by utilizing the unique electrophoretic mobility of the variant gamma-chain of gamma308K. Although sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis under reducing conditions showed that the amount of the variant gamma-chain was slightly less than the amount of normal gamma-chain in recombinant gamma308K/N fibrinogen, we judged that our selected clone was still a useful model of the M-II individual.

RESULTS AND CONCLUSION

Functional analyses demonstrated that thrombin-catalyzed fibrin polymerization decreased in the following order: gamma308N, gamma308K/N, an equimolar mixture of gamma308K with gamma308N. The difference in the polymerization curves between gamma308N and gamma308K/N is highly similar to the difference between plasma fibrinogen from a normal control and the M-II proband. In addition, experimental results using the equimolar mixture indicated that gamma308K is able to polymerize into fibrin fibers and does not inhibit the gamma308N polymerization. In conclusion, our results indirectly demonstrated that gamma308K/N fibrinogen is the mixture of normal homodimers, heterodimers, and variant homodimers, and all of these can participate in the fibrin fiber formation.

Allosteric effects potentiating the release of the second fibrinopeptide A from fibrinogen by thrombin

Fibrin formation depends on the release of the two N-terminal fibrinopeptides A (FPA) from fibrinogen, and its formation is accompanied by an intermediate, alpha-profibrin, which lacks only one of the FPA. In this study, we confirm that the maximal levels of alpha-profibrin found over the course of thrombin reactions with human fibrinogen are only half of what would be expected if the first and second FPA were being released independently with equal rate constants. The rapidity of release of the fibrinopeptides by thrombin had been shown to depend on an allosteric transformation that is induced when Na(+) binds to a site defined by the 215-227 residues of thrombin, a transformation that results in the exposure of its fibrinogen-binding exosites transforming the thrombin from a slow to a fast acting form toward fibrinogen. When choline was substituted for sodium to transform thrombin to its slow form, the maximal levels of alpha-profibrin rose to those expected for independent release of the two FPA. Thus, it is only the fast thrombin that releases the second FPA fast, and that fast release only occurs when both FPA are present because of a partial coupling of its release with that of the first FPA. The release of the FPA from purified alpha-profibrin with the first FPA already missing is no faster than the release of any FPA. Surprisingly, we also found that slow thrombin became increasingly transformed to a fast form in the absence of sodium when the fibrinogen was elevated to high concentrations. This potentiation by concentrated fibrinogen also occurs with the recombinant mutant thrombin (Y225P), which is otherwise slow in both the presence and absence of Na(+). The potentiation of thrombin by fibrinogen must be short-lived so that the thrombin reverts to its slow acting form in the interim among encounters with other fibrinogen molecules in dilute fibrinogen solutions lacking Na(+), whereas at high fibrinogen concentrations the thrombin encounters other molecules before it reverts back to the slow form.