During apoptosis, cells expose phosphatidylserine at the cell surface. Annexin V is a phospholipid-binding protein, which in the presence of calcium ions binds selectively and with high affinity to phosphatidylserine. It displays very low affinity for phospholipid species such as phosphatidylethanolamine, sphingomyelin and phosphatidylcholine. This binding profile makes annexin V a powerful and selective tool for the detection of apoptotic cells. In several papers, the successful use of annexin V for the measurement of apoptosis of various cell types by flow cytometry has been described (1-3).
The APOPTEST™-FITC kits include fluorescein-conjugated annexin V and the dye, propidium iodide, thus allowing separate identification of viable, apoptotic, and necrotic cells.