Highly sensitive and stable phosphatidylserine liposome aggregation assay for annexins

Annexins are a gene family of Ca²⁺-dependent membrane binding proteins which interact specifically with acidic phospholipids. We describe here details of highly sensitive and precise assays for annexins I and V, utilizing turbidometric analysis of phosphatidylserine liposome aggregation. In the case of annexin I, the new assay is 7-fold more sensitive than the conventional chromaffin granule aggregation assay in terms of threshold for detection and the rate of increase of initial absorbance is 15-fold greater. Annexin V, which binds but does not aggregate liposomes, can be assayed on the basis of inhibition of Ca²⁺-dependent liposome aggregation. Further comparative advantages of the assay include lower expense and increased shelf life of the liposome reagent.