The properties of the binding of annexin V to variously composed phospholipid 
vesicles have been studied by applying a recently developed EPR method, using an 
annexin V spin label. By this approach, this protein is seen to bind to acidic 
phospholipid-containing vesicles, as reported, thus confirming the reliability 
of the method. In addition, binding of this annexin to cardiolipin-containing 
vesicles has been studied in more depth, and the protein has been shown to have 
a distinct affinity for this phospholipid. As a cardiolipin-rich natural 
membrane system, mitochondrial membranes and mitoplasts from rat liver were 
considered, and a strong binding of AV to these membranes was observed. Having 
compared this binding with that to phospholipid vesicles, cardiolipin-rich 
microdomains in the mitochondrial membranes are proposed as the putative 
mitochondrial binding sites for annexin V.