CL- REGULATES THE STRUCTURE OF THE FIBRIN CLOT

The differences between coarse and fine fibrin clots first reported by Ferry have been interpreted in terms of nonspecific ionic strength ef fects for nearly 50 years and have fostered the notion that fibrin pol ymerization is largely controlled by electrostatic forces. Here we rep ort spectroscopic and electron microscopy studies carried out in the p resence of different salts that demonstrate that this long-held interp retation needs to be modified. In fact, the differences are due entire ly to the specific binding of Cl- to fibrin fibers and not to generic ionic strength or electrostatic effects. Binding of Cl- opposes the la teral aggregation of protofibrils and results in thinner fibers that a re also more curved than those grown in the presence of inert anions s uch as F-. The effect of Cl- is pH dependent and increases at pH > 8.0 , whereas fibers grown in the presence of F- remain thick over the ent ire pH range from 6.5 to 9.0. From the pH dependence of the Cl- effect it is suggested that the anion exerts its role by increasing the pK(a ) of a basic group ionizing around pH 9.2. The important role of Cl- i n structuring the fibrin clot also clarifies the role played by the re lease of fibrinopeptide B, which leads to slightly thicker fibers in t he presence of Cl- but actually reduces the size of the fibers in the presence of F-. This effect becomes more evident at high, close to phy siological concentrations of fibrinogen. We conclude that Cl- is a bas ic physiological modulator of fibrin polymerization and acts to preven t the growth of thicker, stiffer, and straighter fibers by increasing the pK(a) of a basic group. This discovery opens new possibilities for the design of molecules that can specifically modify the clot structu re by targeting the structural domains responsible for Cl- binding to fibrin.