Human factor VIII: a calcium-linked protein complex

The possible role of Ca2+ as an essential constituent part of the human factor VIII complex has been investigated by stability studies, metal determinations, and gel filtration experiments. In citrated plasma, the factor VIII coagulant activity (VIII:C) deteriorated during storage in a biphasic manner. Collection of blood in heparin, instead of chelating anticoagulants, or neutralization of citrate by addition of CaCl2 to heparinized citrate phosphate dextrose (CPD) plasma rendered VIII:C noticeably stable. At physiologic levels of ionized calcium, VIII:C was almost completely stable during incubation of plasma for 6 hr at 37 degrees C. The influence of other divalent ions was also studied. Highly purified factor VIII complex was subjected to atomic absorption spectrophotometric analysis and found to contain about 1.0 mole calcium per 220,000 daltons. This intrinsic calcium could be readily removed by EDTA. When heparin plasma and CPD plasma were chromatographed on Sepharose CL-6B at 37 degrees C, all the factor-VIII-related activities eluted together as large protein complexes. In contrast, factor VIII coagulant antigen (VIII:CAg) and factor-VIII-related antigen (VIIIR:Ag) were completely dissociated upon exposure to EDTA. From these observations it is concluded that human factor VIII circulates in normal plasma as a calcium-linked protein complex.