Comparison with FVIIIa stability of WT and K1813A mutant. FVIII WT or mutant (1 nM) were incubated with thrombin (30 nM) before measuring FVIII activity at the indicated times in a 1-stage clotting assay. The symbols used are defined in Figure 1. FVIIIa activity was expressed as the fold change of the initial value and was plotted as a function of incubation time. The experiments were performed 3 times, and the average and standard deviation calculations are shown. The data were fitted to the formula given in Equation 2.