SIGNIFICANCE OF IGG FOR THE ACTIVITY OF FACTOR-XII MEASURED IN HUMAN PLASMA

The plasma level of factor XII (FXII) was measured in samples from hea lthy young men. The activated contact factor was assayed as prekallikr ein activator (PKA), as S-2222 amidase, and in radial immunodiffusion tests. By removing the bulk of IgG on protein G columns before the act ivation procedure, the functional activities increased to about 135%. In such test preparations, PAGE immunoblot experiments with polyclonal antibodies against FXII showed, in addition to FXIIa (80 kD), a doubl e band with a molecular weight of about 46 kD. This protein could also be detected with a light-chain-specific monoclonal antibody to FXII, but not with such an antibody directed against its heavy chain. The 46 -kD band was also observed in plasma deficient in FXII. The amidase as says indicated that the minor part of FXIIa was present in some kind o f association with another protease. To obtain a correct estimation of total FXIIa in the amidase assays a sufficiently high level of FXI wa s required compared to that of FXII. The PKA assays were generally car ried out with a prekallikrein (PK) substrate containing IgG. By replac ing this substrate by PK free from IgG additional PKA activity was obs erved, the activity appearing also in plasma deficient in FXII.