Electron microscopic studies of plasmic degradation products of fibrinogen. Implications for the disulfide structure of fibrinogen.

Fibrinogen, coagulable plasmic derivatives (Fragments X) and Fragments Y, D and E were studied by negative staining electron microscopy. Fragment X obtained from Stage 1 digests and fibrinogen were both globular, while Fragment X of Stage 2 digests appeared as a nodular filament. The Stage 1 and Stage 2 Fragment X preparations had approximately the same molecular weight, but could be differentiated by several subtle differences in polypeptide chain structure. Fragments Y and D were also filamentous, although shorter than Fragment X (Stage 2), and Fragment E appeared as a small, compact or folded filament. These results agree with the concept that fibrinogen consists of a strand of nodules connected by thin strands, folded into a compact, spherical shape. The molecule opens up when stabilizing bonds are disrupted or liberated by plasmin. The data are compatible with a fibrinogen molecule in which the two halves are linked by a single locus of disulfide bonds at the amino terminus and with the asymmetric hypothesis of plasmic degradation to Fragments X, Y, D and E.