Measurement of flipase activity and PS exposure.

(A) The fluorescence of cells loaded with NBD-PS for 1 hour (curve a), followed by back-extraction with BSA (curve b), or NEM-treated RBCs loaded with NBD-PS followed by back-extraction (curve c). (B) Two cell populations were mixed prior to labeling: 20% of the cells were PS-exposing after NEM treatment and loading with Ca⁺⁺followed by removal of ionophore and Ca⁺⁺, and 80% were normal control RBCs. These cells were labeled with PE-SA/biotin-AV, and subsequently incubated with NBD-PS, followed by back-extraction with BSA.