Prothrombin activation determined by Western blotting. The activation of prothrombin in diluted whole plasma and Western blotting were performed as described in the Materials and Methods. The location and size of molecular weight standards is indicated on the left. The activation reaction was stopped at the time in seconds indicated at the top of the figure. Prothrombin, a glycoprotein is seen corresponding to the molecular weight marker 97.2 kD. The lane marked thr is purified thrombin and serves as a marker for this product. The band just above the 50 kD marker that appears after activation is prethrombin-1 and the decreased amount seen in the proband (P) is consistent with the decreased thrombin activity in this sample. C indicates control plasma. The lower molecular weight bands seen primarily at 600 seconds correspond to the activation peptides, fragment 1, and fragment 2. The highest molecular weight band in this unreduced sample corresponds to cross-reacting human IgG present in the plasma. The other bands present both before and after activation are not identified.