Annexins are a family of calcium-dependent phospholipid-binding proteins. They are abundant in the eukaryotic kingdom. Though structurally well investigated the in vivo function of the annexins is still unclear. They definitively belong to a family of ubiquitous cytoplasmic proteins involved in signal transduction. All annexins have been shown to have putative binding site for protein kinases C but only annexin V would possess a potential pseudo-substrate site. Thus annexin V seems to modulate the activity of some PKCs on their substrates. Annexin V was found to play a major role in matrix vesicle-initiated cartilage calcification as a collage-regulated calcium channel (6). Annexin V binds to procoagulant phospholipids (Vascular anticoagulant alpha) with high affinity. Annexin V’s preferential binding partner is phosphatidylserine (PS). PS is predominantly located in membrane leaflets, which face the cytosol. However, recent findings show that each cell type has the molecular machinery to expose PS at ist cell surface. This machinery is activated during the execution of apoptosis. Once PS is exposed at the cell surface it exhibits procoagulant and proinflammatory activities. Annexin V will bind to the PS-exposing apoptotic cell and can inhibit the procoagulant and proinflammatory activities of the dying cell. These findings together with the presence of Annexin V in the extracellular space depict a novel pathophysiological significance for Annexin V in vivo. Total endogenous annexin V can be detected with the present ELISA which will offer further insights in the pathophysiological role of Annexin V.
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