COMPARISON OF 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 human 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 was 103,500, which was compatible with a molecule containing only one each of the A.alpha. (13,000), B.beta. (43,000) and .gamma. (39,000) chain remnants. Fragment D-D of crosslinked fibrin had a molecular weight of 189,000 compatible with a molecule containing 1 isopeptide-bound .gamma.-.gamma. chain (80,000) and 2 each of B.beta. (43,000) and A.alpha. (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 had 3 mol of NH2-terminal amino acids per molecule, while Fragment D-D had 7. Two Fragment D molecules, each of 3 polypeptide chains, were apparently derived by plasmic degradation from each fibrinogen molecule, and that an isopeptide-bound, 6 chain Fragment D-D molecule was released from cross-linked fibrin by plasmin. Equilibrium 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 were 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 2 halves were joined by disulfide bonds only in the amino-terminal regions.