A number of changes in cell structure occur during apoptosis. One major change is the loss of membrane phospholipid asymmetry, with translocation of phosphatidylserine (PS) from the inner leaflet of the phospholipid bilayer to the external surface.1 Annexin V (Mr 36-kDa), a member of the annexin family of calcium-dependent phospholipid binding proteins, has a high affinity for PS-containing phospholipid bilayers.2,3 When PS is exposed on the extracellular face of a cell membrane, annexin V binds with an affinity of approximately 7 nM.4 Fluorochrome conjugates of annexin V can be used to monitor changes in cell membrane phospholipid asymmetry,5-8 thereby providing a convenient tool for detection of apoptotic cells. ApoScreen Annexin V is a labeled annexin V that can be used in conjunction with propidium iodide to evaluate subpopulations of cells along the apoptotic pathway. ApoScreen detection reagents can also be used to analyze changes in membrane asymmetry in platelets5 and erythrocytes.8 Analysis can be performed on a flow cytometer equipped with a single laser using excitation at 488 nm. Alternatively, the cells can be examined with a fluorescence microscope equipped with appropriate filters.
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