Modulation of the kinetic characteristics of the sarcoplasmic reticulum ATPase by membrane fluidity.

(Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum has been reconstituted with dipalmitoylphosphatidylcholine, and the activating effect of ATP and Ca2+ on this enzyme has been studied at different temperatures. It has been found that two kinetic forms of the enzyme are interconverted at about 31 degrees C, and this is possibly related to a phase change in the phospholipid which is more directly associated with the protein. Above 31 degrees C the enzyme is less dependent on ATP activation at high ATP concentrations but shows positive cooperativity for Ca2+ activation. On the other hand, below 31 degrees C, the reconstituted enzyme is more dependent on ATP for activation at high ATP concentrations than the purified ATPase and does not show cooperativity for Ca2+ activation.