Activation of purified recombinant FXII proteins by kallikrein. FXII recombinant proteins (125 nmol/L) were activated with kallikrein (12.5 nmol/L) as described in Materials and Methods in the presence (filled symbols) or in the absence (open symbols) of DS500. At several time intervals, the reaction was stopped by adding 50 μL of assay mixture containing SBTI and S-2302, and the increase in absorbance at 405 nm was immediately recorded. Results are expressed as the percentage of the maximum amount of FXIIa present in the wells after full activation and represent the mean ± SE of three experiments. The absence of standard error bars indicates that the variation was too little to portray visually. Symbols: plasma FXII (•), rFXII (▪), rFXII-▵19 (▴).