Autoactivation of FXII proteins. A total of 25 μL of FXII proteins (400 nmol/L), 25 μL of twofold serial dilutions of DS 500 and 50 μL of buffer yielding final concentrations of 50 mmol/L Tris-HCl, 50 mmol/L NaCl, 0.1% (wt/vol) Tween-20, pH 7.8, were incubated for 60 minutes at 37°C. Thereafter, 50 μL of assay mixture containing S2302 were added and the increase in absorbance at 405 nm was immediately recorded. Results are expressed as percentage of total FXII activity and represent the mean of two experiments. Symbols: FXII (•), rFXII-▵19 (▴).