NEUTRAL ORGANIC SOLUTE EFFECTS ON THE ACTIVITY OF THE PLASMA-MEMBRANECA2-ATPASE()

We have compared effects of dimethylsulfoxide (Me(2)SO) and two polyol s on the Ca2+-ATPase purified from human erythrocytes. As studied unde r steady-state conditions over a broad solute concentration range and temperature, Me(2)SO, glycerol, and xylitol do not inhibit the Ca2+-AT Pase activity; this Is In contrast to numerous other organic solutes t hat we have investigated. Under specific experimental conditions, Me(2 )SO (but not glycerol) substantially increases Ca2+-ATPase activity, s uggesting a possible facilitation of enzyme oligomerization. The activ ation is more pronounced at low Ca2+ concentrations. In contrast to gl ycerol, Me(2)SO shows no protective effect on enzyme structure as asse ssed by determining residual Ca2+-ATPase activity after exposing the e nzyme 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 o n the Ca2+-ATPase activity we believe that Me(2)SO activates the Ca2+- ATPase by indirect water-mediated interactions.