Synthetic membranes of phosphatidylcholine require inclusion of at least 5% phosphatidylserine (Ptd-L-Ser) to form binding sites for factor VIII. The relatively high requirement for Ptd-L-Ser suggests that stimulated platelets may contain another membrane constituent that enhances expression of factor VIII-binding sites. We report that phosphatidylethanolamine (PE), which is exposed in concert with Ptd-L-Ser in the course of platelet stimulation, induces high affinity binding sites for factor VIII on synthetic membranes containing 1-15% Ptd-L-Ser. The affinity of factor VIII for binding sites on membranes of Ptd-L-Ser/PE/phosphatidylcholine in a 4:20:76 ratio was 10.2 ± 3.5 nM with 180 ± 33 phospholipid molecules/site. PE did not induce binding sites on membranes of 4% Ptd-D-Ser, indicating that the induced binding sites require the correct stereochemistry of Ptd-L-Ser as well as PE. Egg PE and dimyristoyl-PE were equivalent for inducing factor VIII-binding sites, indicating that hexagonal phase-inducing properties of PE are not important. We conclude that PE induces high affinity factor VIII-binding sites on membranes with physiologic mole fractions of Ptd-L-Ser, possibly including those of stimulated platelets.

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