Comparison of the physicochemical properties of fragment D derivatives of fibrinogen and fragment D-D of cross-linked fibrin.

The molecular weight of Fragment D derivatives obtained from plasmic digests of fibrinogen and cross-linked fibrin was determined by equilibrium sedimentation and compared with the summated molecular weight of their polypeptide chains observed after electrophoresis of reduced protein in sodium dodecyl sulfate polyacrylamide gels. The measured molecular weight of Fragment D (Stage 2) of fibrinogen is 103 500, which is compatible with a molecule containing only one each of the Aalpha (13 000), Bbeta (43 000) and gamma (39 000) chain remnants. Fragment D-D of cross-linked fibrin has a molecular weight of 189 000, compatible with a molecule containing one isopeptide-bound gamma-gamma chain (80 000) and two each of Bbeta (43 000) and Aalpha (13 000) chain remnants. The NH2-terminal amino acid residues of the Fragment D derivatives were measured quantitatively using a thioacetic-thioglycolic acid method, and molar quantities were calculated on the basis of the molecular weights determined by equilibrium sedimentation. Fragment D preparations obtained from Stage 2 and Stage 3 digests of fibrinogen have 3 mol of NH2-terminal amino acids per molecule, while Fragment D-D has seven. These data support the view that two Fragment D molecules, each of three polypeptide chains, are derived by plasmic degradation from each fibrinogen molecule, and that an isopeptide-bound, six chain Fragment D-D molecule is released from cross-linked fibrin by plasmin. Equlibrium sedimentation measurement of the molecular weights of Fragment X (Stage 1 and Stage 2) and Fragment Y are 265 000 and 148 000, respectively. These values are compatible with asymmetric cleavages of Fragment X to Fragments Y and D (Stage 2), and of Fragment Y to Fragments D (Stage 2) and E, and with a fibrinogen model in which the two halves are joined by disulfide bonds only in the amino-terminal regions.