[The specificity of modulation of sarcoplasmic reticulum Ca(2+)-ATPase by transmembrane Ca2+ gradient]

We have previously reported that transmembrane Ca2+ gradient-mediated changes in lipid fluidity could modulate the conformation and enzyme activity of sarcoplasmic reticulum (SR) Ca(2+)-ATPase. The aim of this paper is to explore the specificity of transmembrane Ca2+ gradient-mediated modulation of SR Ca(2+)-ATPase. The results showed that such specificity exhibited in two aspects: 1. The modulation could not be ascribed to transmembrane potential resulted from the transmembrane Ca2+ gradient, Dissipation of transmembrane potential by FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone) could not affect the activity of SR Ca(2+)-ATPase. 2. Transmembrane Sr2+ gradient had little effect on the enzyme activity of SR Ca(2+)-ATPase. A significant difference between the effect of transmembrane Ca2+ and Sr2+ gradient on the lipid fluidity was detected in the middle region of bilayer of Ca(2+)-ATPase incorporated proteoliposomes using a set of n-AS [n-(9-anthroyloxy) fatty acids] fluorescence polarization probes. It is known that Ca2+ binding domain of SR Ca(2+)-ATPase is just located in the middle region of bilayer, hence it may be deduced that possibly, membrane lipids are involved in transmembrane Ca2+ gradient-mediated modulation of Ca(2+)-ATPase.