Effects of organic solvents, methylamines, and urea on the affinity for Pi of the Ca2+-ATPase of sarcoplasmic reticulum

The Ca2+-ATPase of sarcoplasmic reticulum can be phosphorylated by Pi, forming an acylphosphate residue at the catalytic site of the enzyme. In a previous report (de Meis, L., Alves, E., and Martins, O.B. (1980) Biochemistry 19, 4252-4261), it was shown that organic solvent such as dimethyl sulfoxide and glycerol cause a decrease in the apparent Km for Pi. In this report it is shown that a similar effect is obtained with the methylamines glycine betaine and trimethylamine N-oxide. The apparent Km value for Pi in totally aqueous medium and in the presence of either 6.4 M glycerol, 1.4 M dimethyl sulfoxide, 0.4 M trimethylamine N-oxide, or 1 M glycine betaine were found to be respectively 2.85, 0.52, 0.52, 0.81, and 0.93 mM at pH 6.2 and greater than 10.0, 1.08, 2.53, 3.05, and 2.05 mM at pH 7.5. In contrast to the effect of methylamines, urea caused an increase in the apparent Km for Pi. When mixed in the appropriate concentration ratio, the effect of either organic solvent or methylamines is cancelled by urea.