Purified factor XII has a higher specific activity than the parent molecule in plasma

The specific clot promoting activity of factor XII (F XII) in plasma samples from 50 healthy adults was between 30 and 48 U/mg, whereas the specific activity of purified F XII ranged from 55 to 66 U/mg. This difference was neither due to partial proteolytic activation during purification of F XII nor to the influence of plasma protease inhibitors. Purified F XII showed normal size and charge, as demonstrated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing, respectively. The increase of the specific F XII activity during the purification process mainly occurred after anion exchange chromatography on DEAE-Sephadex and after the final gel filtration step. Upon dextran sulfate activation, proteolytic cleavage of F XII and generation of kallikrein-like amidolytic activity was faster in F XII deficient plasma containing purified F XII than in F XII deficient plasma containing a corresponding amount of pooled normal plasma (NHP). The binding to kaolin was similar for both, purified F XII and plasma F XII. In conclusion, purification alters the properties of F XII in an unknown way, resulting in an increased specific clot promoting activity.