The Possible Residual Initial Polymerisation Site in Fibrin Plasmic Fragment D-Dimer.
The stable plasmic fragment of fibrin, D-dimer, again becomes susceptible to plasmin digestion if calcium ions are removed. Pragressive cleavage af the C-terminal end of the γ chain occurs releasing a peptide containing the cross link and any biologically attached (F XIII: transglutaminase) dansyl cadaverine. The cleaved peptide binds tightly to the parent malecule but further plasmin digestion can be inhibited by calcium ions. D-dimer has two and D monomer a single high affinity calcium-specific binding sites (Kd 1χ10-5M). The farmation of a noncavalently-linked D-dimer (NCDD) can be shown by the molecular size on malecular sieving and by the retention of molar volume by the dansyl-cadaverine labelled but cleaved D-dimer using fluorescence polarisation. Some partial dissociation occurs at 5°C but reassaciation occurs on return to 35° C. Antibodies to D-dimer have a 5-fold lesser affinity for fibrinogen and D-monomer with our without fragment E present but NCDD gradually loses affinity with extent of cleavage and in time. The interactions of D-dimer with the E domain on fibrinogen and NDSK are dependent on the intact calcium-stabilised γ chain. The inter-D interaction maintaining NCDD could be the residual initial polymerisation site of fibrinogen.