pH-dependent denaturation of thrombin-activated porcine factor VIII

Thrombin-activated porcine factor VIII (fVIIIaIIa) is a stable, active, 160-kDa heterotrimer at concentrations exceeding 2 x 10(-7) M in 0.7 M NaCl, 0.01 M histidine Cl, 5 mM CaCl2, pH 6.0, at 4 degrees C or 20 degrees C. Two of the subunits, fVIIIA1 and fVIIIA2, are derived from the heavy chain of the plasma-derived, heterodimeric fVIII precursor. The third subunit, fVIIIA3-C1-C2, is derived from the fVIII light chain. We now find that fVIIIaIIa undergoes a sharp decline in coagulant activity between pH 7 and 8. At pH 7.5, the activity of fVIIIaIIa at 3 x 10(-7) M decays within a few hours to a stable level that is approximately 70% of the value at pH 6.0, whereas at pH 8.0, greater than 99% of the activity is lost. The activity cannot be restored by readjusting the pH to 6.0. The loss of activity at pH 8.0 coincides with dissociation of the fVIIIA2 subunit since an inactive fVIIIA1/A3-C1-C2 heterodimer can be isolated by Mono S high performance liquid chromatography. After prolonged incubation at pH 8.0, the fVIIIA1 subunit also dissociates. The free fVIIIA2 fragment appears to be poorly soluble which may explain the irreversible loss of activity. Analytical velocity sedimentation of the pH-inactivated fVIIIaIIa preparation also is consistent with dissociation and precipitation of the fVIIIA2 fragment. We propose that denaturation of fVIIIaIIa by pH-dependent subunit dissociation may provide a major mechanism of inactivation of fVIIIaIIa under physiologic conditions.