Colorimetric phospholipid determination with erythrosin B.

Binding of phospholipid vesicles to erythrosin B results in a shift of the absorption maximum of the dye from 528 to 549 nm. This effect was employed to develop a simple, rapid, and sensitive quantification method for unilaminar phospholipid vesicles. At room temperature the color development of the phospholipid-dye complex at 549 nm is essentially complete in 5 min and only a slight decrease is observed in the following hours. The pH optimum of 4.5 for the assay is related to the tight binding at this pH (Kd = 3.6 micrograms/ml). A maximal binding of one erythrosin to seven phospholipid molecules is found. The sensitivity of the assay is high for zwitterionic phospholipids (e.g., 0.023 delta A549 x ml/micrograms for dioleoyl phosphatidyl choline) and lower for anionic phospholipids. The assay depends on the size of the phospholipid liposomes, indicating the importance of the phospholipid surface area for binding.