Studies on the ultrastructure of fibrin lacking fibrinopeptide B (ß-fibrin)

Abstract

Release of fibrinopeptide B from fibrinogen by copperhead venom procoagulant enzyme results in a form of fibrin (ß-fibrin) with weaker self-aggregation characteristics than the normal product (aß-fibrin) produced by release of fibrinopeptides A (FPA) and B (FPB) by thrombin. We investigated the ultrastructure of these two types of fibrin as well as that of ß-fibrin prepared from fibrinogen Metz (Aa16 Arg ? Cys), a homozygous dysfibrinogenemic mutant that does not release FPA. At 14°C and physiologic solvent conditions (0.15 mol/L of NaCl, 0.015 mol/L of Tris buffer pH 7.4), the turbidity (350 nm) of rapidly polymerizing aß-fibrin (thrombin 1 to 2 U/mL) plateaued in <6 min and formed a 'coarse' matrix consisting of anastomosing fiber bundles (mean diameter 92 nm). More slowly polymerizing aß-fibrin (thrombin 0.01 and 0.001 U/mL) surpassed this turbidity after =60 minutes and concomitantly developed a network of thicker fiber bundles (mean diameters 118 and 186 nm, respectively). Such matrices also contained networks of highly branched, twisting, 'fine' fibrils (fiber diameters 7 to 30 nm) that are usually characteristic of matrices formed at high ionic strength and pH. Slowly polymerizing ß-fibrin, like slowly polymerizing aß-fibrin, displayed considerable quantities of fine matrix in addition to an underlying thick cable network (mean fiber diameter 135 nm), whereas rapidly polymerizing ß-fibrin monomer was comprised almost exclusively of wide, poorly anastomosed, striated cables (mean diameter 212 nm). Metz ß-fibrin clots were more fragile than those of normal ß-fibrin and were comprised almost entirely of a fine network. Metz fibrin could be induced, however, to form thick fiber bundles (mean diameter 76 nm) in the presence of albumin at a concentration (500 µmol/L) in the physiologic range and resembled a Metz plasma fibrin clot in that regard. The diminished capacity of Metz ß-fibrin to form thick fiber bundles may be due to impaired use or occupancy of a polymerization site exposed by FPB release. Our results indicate that twisting fibrils are an inherent structural feature of all forms of assembling fibrin, and suggest that mature ß-fibrin or aß-fibrin clots develop from networks of thin fibrils that have the ability to coalesce to form thicker fiber bundles.