A FLUORESCENCE STUDY OF A23187 INTERACTION WITH PHOSPHOLIPID-VESICLES

The fluorescence of the ionophore A23187 [calcimycin] was monitored in suspensions of egg yolk phosphatidycholine (EYPC) and dipalmitoyl phosphatidylcholine (DPPC) vesicles. Both the protonated form of A23187 and the Ca2+ complex exhibit fluorescence enhancement when extracted into a hydrophobic environment. Measurements of fluorescence intensity vs. lipid concentration were used to establish lower limits to the lipid/water partition coefficients. Values obtained in this way were .gtorsim. 50 ml water/mg phosphatidylcholine. Quenching of A23187 fluorescence by the spin labels 5NMS (methyl ester of 5-nitroxy stearate), 12NMS, 16NMS and TEMPO stearamide in EYPC and DPPC vesicles was also investigated. In EYPC all the labels yielded fairly linear Stern-Volmer plots, with TEMPO stearamide quenching about 1/2 as strong as the other probes. Quenching in DPPC was generally much stronger than in EYPC, but 12 NMS and 16NMS gave hyperbolic Stern-Volmer plots, apparently due to clustering of the labels. In all the cases the protonated form of A23187 was quenched approximately twice as efficiently as the Ca2+ complex, possibly due to a longer fluorescence lifetime for the former. Calculations based on measured spectral properties were performed which indicate that the Foerster transfer mechanism extends the nitroxides'' quenching range to .simeq. 10 .ANG.