Phosphorylation of Ca2+-ATPase by inorganic phosphate in water-organic solvent media: dielectric constant and solvent hydrophobicity contribution.

The effect of organic solvents on the phosphorylation of the Ca2+-dependent ATPase of sarcoplasmic reticulum by inorganic phosphate in the absence of a calcium gradient was investigated. Kinetic analysis of the reaction in water and water-organic solvent media according to a bireactant scheme shows no correlation between changes in kinetic parameters and the dielectric constant of the mixed solvents. The pronounced increase in equilibrium levels of phosphoenzyme in water-solvent mixtures is attributed to changes in the water activity of the medium.