Article Title: Elevated Prothrombin Results in Clots with an Altered Fiber Structure: a Possible Mechanism of the Increased Thrombotic Risk Running title: Elevated prothrombin alters clot structure

Individuals with elevated prothrombin levels are at increased risk of venous thrombosis. To understand the mechanism behind this observation, we studied the effect of prothrombin concentration on thrombin generation and fibrin clot structure. The pattern of thrombin generation was directly related to the prothrombin level at all concentrations tested. From 0 to 300% of normal plasma levels of prothrombin, increasing the prothrombin concentration increased the initial rate, peak and total amount of thrombin generated. Importantly, fibrin clot structure was also affected by the prothrombin concentration. Fibrin clots made from prothrombin concentrations less than 10% of plasma levels were weak and poorly formed. Fibrin clots made at 10 to 100% of plasma levels of prothrombin had similar fiber structures (mass/length ratio; μ). However, the fiber mass/length ratio decreased with increasing prothrombin levels greater than 100% of plasma levels, in a dose-dependent manner. These results suggest that increased levels of prothrombin alter thrombin generation and clot structure. Specifically, elevated prothrombin levels produce clots with reduced fibrin mass/length ratios compared to normal clots. We hypothesize that this alteration in fibrin clot structure is an important determinant of the risk of thrombosis.