Conditions Affecting the Molecular Weight of Factor VIII.
By purifying Factor VIII while maintaining physiological concentrations of calcium we have recently demonstrated that about 50% of the procoagulant activity is in a very low molecular weight (VLHW) form not associated with the carrier (VIII: RAG). The remainder is carrier associated and elutes at Vo as a high molecular weight (HMW) compound upon Sepharose 6B chromatography. Reduction of the calcium concentration by increasing the amount of citrate added to heparin results in decreasing the ratio of VLMW:HMW from 1:1 in pure heparin to 1:5 in pure citrate. If citrate is replaced with the more strongly chelating EDTA no VLMW is detectable in the plasma. It has also been found that most of the biochemical purification techniques which have been previously used to prepare Factor VIII for study actually result in the aggregation of this VLMW with the carrier to produce the high molecular weight form. This includes: cryoprecipitation, precipitation by polyethylene glycol and storage -80°C. As well, the VLMW material will self-associate upon freezing to produce an aggregate with a molecular weight of 106. However, this material does not cross-react with rabbit antibody directed against VIII: RAG. The data indicate that many of the previously reported biochemical characteristics, including molecular weights, actually describe species which are artifacts of the isolation process rather than those of the physiologically occuring Factor VIII.