Annexin V Conjugates
Some of the earliest apoptotic changes occur at the cell surface. The early recognition of apoptotic cells by phagocytic cells, the significant loss of water leading to cell shrinkage, and the maintenance of intact cell membranes, despite the cell surface blebbing observed in many cell types, indicate significant changes have occurred in the plasma membrane. One of the better understood cell surface modifications is exposure of phosphatidylserine (PS). Normally, PS is confined to the inner layer of the lipid bilayer. This asymmetric distribution is maintained in normal cells by the action of specific proteins. After induction of apoptosis under the influence of translocases, the PS is flipped from the inner to the outer bilayer, rendering the molecule available for detection. Annexin V is a blood clotting factor that exhibits a high calcium-dependent specificity for PS binding. The coupling of annexin to fluorescein, or biotin, generates a direct, rapid, and simple method for the detection of apoptosis on unfixed cells.