Role of Phospholipids in the Calcium-dependent ATPase of the Sarcoplasmic Reticulum

Three types of partially purified ATPase enzymes having different phospholipid contents and compositions have been prepared: (a) an enzyme whose phospholipid moiety has been replaced predominantly by dioleoyl lecithin (DOL-enzyme), with about the same phospholipid content as the original sarcoplasmic reticulum, (b) dipalmitoyl lecithin-replaced enzyme whose phospholipid content is 30% of that of DOL-enzyme (DPL-enzyme), and (c) a partially delipidated enzyme with about the same phospholipid content as DPL-enzyme but with the original sarcoplasmic reticulum phospholipid composition (del-enzyme). The temperature dependence of Ca*+ -activated ATPase activity of these preparations showed clearcut differences; with DOL-enzyme there was no appreciable break in the Arrhenius plot in the 3-40” range; DPL-enzyme showed a break at 29”, and del-enzyme and sarcoplasmic reticulum one at 18”. Transition temperatures obtained from ESR studies with the use of spin-labeled stearic acid incorporated into the membranes agreed with those derived from ATPase assays. Thermodynamic analysis of the ATP hydrolysis rates shows that DPL-enzyme has considerably larger values of activation enthalpy and activation entropy below the transition temperature (29”) than those of the other preparations, while all enzyme preparations show similar free energies of activation. The ESR data show that below their transition temperatures DPL-enzyme, and to a lesser degree del-enzyme, have a strongly restricted motion of their phospholipid molecules as compared with either DOL-enzyme or sarcoplasmic reticulum. Studies on the formation and decomposition of phosphoenzyme have been carried out with the three types of ATPase preparations. At O”, the rate of inorganic phosphate liberation is 8 times lower in DPL-enzyme than in del-enzyme with little difference in the steady state level of phosphoenzyme. In DOL-enzyme, the level of phosphoenzyme and the rate of inorganic phosphate liberation are 1.8 and 3.5 times higher than the corresponding values obtained with del-enzyme. Addition of ADP to the phosphorylated intermediate of DPL-enzyme induces a fast reversal of the phosphorylation reaction. These results indicate that the physical state of the phospholipid molecules associated with the enzyme affects the decomposition of phosphoenzyme, with little effect on the phosphorylation reaction and its reversal.