Competition of thrombin activation of factor VIII wild type by factor VIII R372H. Mixtures of factor VIII wild type (10 nM) and variable concentrations of R372H were reacted with thrombin (0.04 nM) for 1 minute. After the addition of hirudin, factor VIIIa was reacted with factor IXa (20 nM) in the presence of phospholipid vesicles (10 μM). Factor Xa generation was initiated by addition of factor X (300 nM). Factor Xa generated in the absence of R372H (100% level) was approximately 160 nM/min. Data were corrected for the amount of factor Xa generated from factor VIII R372H. Initial rates of factor Xa generation were plotted as a function of R372H and fitted to a noncompetitive inhibition equation by nonlinear least squares regression. Experiments were performed at least 3 separate times and average values are shown.