Continuous analysis of the mechanism of activated transbilayer lipid movement in platelets.

Dithionite reduction of fluorescent (NBD) phospholipids was used as the basis of a continuous assay of transbilayer lipid movement to the cell surface during platelet activation. This assay reveals that virtually all previously internalized phosphatidylserine passes through the external leaflet of the membrane within 90 s after activation with Ca2+ and ionophore or with thrombin and thapsigargin. We demonstrate that this lipid scrambling is reversible, bidirectional, and insensitive to the lipid headgroup. Prolonged activation gradually results in inactivation of the scramblase. The assay also reveals that activation of the scrambling activity is sensitive to the sulfhydryl reagent pyridyldithioethylamine, suggesting the involvement of a protein in the process of activated transbilayer lipid scrambling.