Effect of avidin binding on EP217609 direct inhibition of thrombin. Titrations of the decrease in substrate hydrolysis activity (100μM S2366) of ∼ 1nM thrombin as a function of added EP42675 (circles) or EP217609 (triangles) in the absence (closed symbols) or presence (open symbols) of 50nM avidin. The effects of doubling the avidin or S2366 concentration in EP217609 titrations is shown by the open and closed inverted triangles, respectively. Titrations were fit by the equation for tight binding to obtain K_I values.