Nonsubstrate thrombin-binding capacities
Fibrinogen Naples I (Bβ A68T) is characterized by defective thrombin binding and fibrinopeptide cleavage at the fibrinogen substrate site in the E domain. We evaluated the fibrinogen of three homozygotic members of this kindred (II.1, II.2, II.3) who have displayed thrombophilic phenotypes and two heterozygotic subjects (I.1, I.2) who were asymptomatic. Electron microscopy of Naples I fibrin networks showed relatively wide fiber bundles, probably due to slowed fibrin assembly secondary to delayed fibrinopeptide release. We evaluated 125I-thrombin binding to the fibrin from subjects I.1, I.2, II.1, and II.2 by Scatchard analysis with emphasis on the high-affinity site in the D domain of fibrin(ogen) molecules containing a γ chain variant termed γ′. Homozygotic subjects II.1 and II.2 showed virtually absent low-affinity binding, consistent with the Bβ A68T mutation, whereas heterozygotes I.1 and I.2 showed only moderately reduced low-affinity binding. The homozygotes also showed impaired high-affinity thrombin binding, whereas that of the heterozygotes was nearly the same as normal. Genomic sequencing of the γ′ coding sequence (I.2, II.2), ELISA measurements of two γ′ chain epitopes (L2B, γ′409–412, and IF10, γ′417–427) (I.2, II.1, II.2, II.3), and mass spectrometry of Naples I fibrinogen (II.2) showed no differences from normal, thus indicating that there were no abnormal structural modifications of the γ′ chain residues in Naples I fibrinogen. However, thrombin reportedly utilizes both of its available exosites for binding to high- and low-affinity sites on normal fibrin, suggesting that binding is cooperative. Thus, reduced high-affinity thrombin binding to homozygotic Naples I fibrin may be related to the absence of low-affinity binding sites.