Hofmeister effect of anions on calcium translocation by sarcoplasmic reticulum Ca²⁺-ATPase

The occurrence of Hofmeister (specific ion) effects in various membrane-related physiological processes is well documented. For example the effect of anions on the transport activity of the ion pump Na⁺, K⁺-ATPase has been investigated. Here we report on specific anion effects on the ATP-dependent Ca²⁺ translocation by the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA). Current measurements following ATP concentration jumps on SERCA-containing vesicles adsorbed on solid supported membranes were carried out in the presence of different potassium salts. We found that monovalent anions strongly interfere with ATP-induced Ca²⁺ translocation by SERCA, according to their increasing chaotropicity in the Hofmeister series. On the contrary, a significant increase in Ca²⁺ translocation was observed in the presence of sulphate. We suggest that the anions can affect the conformational transition between the phosphorylated intermediates E₁P and E₂P of the SERCA cycle. In particular, the stabilization of the E₁P conformation by chaotropic anions seems to be related to their adsorption at the enzyme/water and/or at the membrane/water interface, while the more kosmotropic species affect SERCA conformation and functionality by modifying the hydration layers of the enzyme.