Membrane fluidity is not an important physiological regulator of the (Ca2+-Mg2+)-dependent ATPase of sarcoplasmic reticulum.

The (Ca2+-Mg2+)-ATPase from sarcoplasmic reticulum has been reconstituted into phospholipid bilayers of different fatty acyl chain length and degree of unsaturation, and ATPase activity was found to vary about 10-fold. Order parameters for bilayers of these lipids measured using the ESR probe 5-doxylstearic acid varied from 0.5 to 0.6 at 37 degrees C. There was no correlation between phospholipid order parameter and ATPase activity, and we conclude that phospholipid structure is a more important determinant of protein activity than is membrane fluidity.