Neutral organic solute effects on the activity of the plasma membrane Ca2+-ATPase

We have compared effects of dimethylsulfoxide (Me2SO) and two polyols on the Ca(2+)-ATPase purified from human erythrocytes. As studied under steady-state conditions over a broad solute concentration range and temperature, Me2SO, glycerol, and xylitol do not inhibit the Ca(2+)-ATPase activity; this is in contrast to numerous other organic solutes that we have investigated. Under specific experimental conditions, Me2SO (but not glycerol) substantially increases Ca(2+)-ATPase activity, suggesting a possible facilitation of enzyme oligomerization. The activation is more pronounced at low Ca2+ concentrations. In contrast to glycerol, Me2SO shows no protective effect on enzyme structure as assessed by determining residual Ca(2+)-ATPase activity after exposing the enzyme to thermal denaturation at 45 degrees C. Under these conditions several other organic solutes strongly enhance the denaturating effect of temperature. Because of the temperature dependence of its effect on the Ca(2+)-ATPase activity we believe that Me2SO activates the Ca(2+)-ATPase by indirect water-mediated interactions.