Cation binding to brain plasma membranes: An evaluation of the use of anionic fluorescent probes

Cation binding to brain plasma membranes has been studied using anionic sulfonate fluorescent probes. Ion affinity sequences follow the order Mg²⁺ > Ca²⁺ ? K⁺ > Cs⁺ > Na⁺ > Li⁺. The order of effectiveness, in increasing probe fluorescence, is the reverse of the affinity sequence for ions of the same charge. The affinity orders for erythrocyte membranes and dipalmitoyl lecithin are Mg²⁺ > Ca²⁺ ? Cs⁺ > K⁺ > Na⁺ > Li⁺ and Mg²⁺ > Ca²⁺ ? Li⁺ > Na⁺ > K⁺ > Cs⁺. These sequence variations are related to the differences in the nature of the ion binding sites. Heterogeneity in ion binding sites is demonstrated. Evidence is presented for the role of proteins in binding hydrophobic probes. The problem of separating specific conformational effects on ion binding from nonspecific charge neutralization effects is discussed. Pyrene excimer fluoresence rules out the possibility of extensive changes in mobility in the lipid phase on cation binding. Tetrodotoxin has been shown to inhibit Li⁺-, Na⁺-, and K⁺-induced fluorescence enancements of 1-anilino-8-naphthalene sulfonate bound to brain membranes.