Reactions of activated factor X-phosphatide mixtures in vitro and in vivo

The composition, surface properties, and capacity of lipid particles to bind activated factor X can be correlated both with the influence of various phosphatide combinations on activated factor X activity in vitro and on the intensity and duration of the thrombogenic stimulus as measured by a standard bioassay for thrombus formation. The measurable activity of activated factor X in vitro increased as a linear function of lipid concentration from 0 to 40 micro moles/liter. The effectiveness of the lipids examined was in the following decreasing order: phosphatidyl serine-phosphatidylcholine mixture, "cephalin", and phosphatidyl serine alone. An increase in the duration of hypercoagulability with increasing lipid concentration was also observed, but, with regard to the three lipid fractions tested, the in vivo system appeared to be more discriminatory than in vitro assays. Lipid mixtures containing phosphatidyl serine with either phosphatidylcholine, phosphatidyl ethanolamine, or cetyltrimethylammonium bromide markedly enhanced the in vitro activity of activated factor X. Phosphatidic acid-phosphatidylcholine mixtures had a similar but smaller effect, and phosphatidyl ethanolamine-phosphatidylcholine mixtures were inert. The duration of the hypercoagulability was similarly related to the composition of the phosphatide infused. In mixtures containing phosphatidyl serine, the surface charge density of the lipid particles and the binding of the activated factor X activity to lipid showed some correlations with the in vitro activated factor X assay and with the intensity and duration of the thrombogenic stimulus. These data suggest that the catalytic effect of phosphatides on prothrombin activation and their role in the retardation of in vivo compensatory mechanisms directed against circulating activated factor X, are dependent on the affinity of activated factor X for the lipid surface.