Sites of D-domain interaction in fibrin-derived D dimer

We have examined the plasmic digestion products of fibrin formed in the presence of dansylcadaverine, the fluorescent D dimer, to determine whether they are held together not only by the cross-link region on the gamma chain but also by other interactions on the D domain. Antibodies to the D dimer reacted 8X more strongly with sites on the D dimer (purified or in the presence of E) than with sites on fibrinogen or plasmin-digested fibrinogen. The reactivity of this surface site was lost when the gamma chain was cleaved by plasmin after the molecule had been destabilized by the removal of calcium ions, thus breaking the covalent linkage of the homodimer. The noncovalent D dimer retained its dimeric structure by the criteria of molar volume, measured by fluorescence polarization, and molecular sieving. The noncovalently attached, cross-link-containing peptide bound tightly to the parent molecules at higher temperatures but rotated more freely below 15 degrees C, and could be lost from the parent molecules without destroying the dimeric structure. We therefore propose that the forces maintaining the dimeric structure of the noncovalently joined molecule are not solely located at the gamma-chain cross-link region. These other sites on the D domain are therefore candidates for the initial fibrinogen polymerization site and may also have a role in fibrinogen half-molecule assembly.