Studies on the procoagulant activity of human amniotic fluid. II. The role of factor VII

Partially purified Amniotic Fluid Factor (AFF) is found to directly activate human factor X, with a Km for factor X of 0.37 +/- 0.06 M. Added phospholipid has only a slight effect on the activation at low concentrations, and inhibits the reaction at higher concentrations. Both DIPF and PMSF inhibit AFF factor X activation. Although added phospholipid is not required for AFF-activity, phospholipase C rapidly destroys it, indicating the presence of intrinsic phospholipid. Phospholipase C treated AFF releases factor VII activity, which leads to the conclusion that AFF is in fact a thromboplastin: factor VII complex. Both AFF and a human brain thromboplastin factor VII complex prepared in vitro were inhibited by Zn++ ion, while human brain thromboplastin alone is not. AFF is markedly larger than the human brain thromboplastin-factor VII complex as judged by gel filtration.