Membrane lipid fluidity and its effect on the activation energy of membrane-associated enzymes.

1. The fatty acid composition of mitochondrial membranes from sheep and rats was altered by feeding these animals diets which were rich in unsaturated fatty acids. Changes in membrane lipid fluidity resulting from the altered membrane lipid composition were assessed by determining the upper temperature limit of the disorder-order transition (Tf) and the Arrhenius activation energy (Ea) of succinate oxidase. 2. After feeding the unsaturated fatty acid-rich diet to sheep the Ea, in the temperature range above Tf, increased from 8 to 63 kJ . mol-1 while Tf decreased from 32 to 15 degrees C. Rats fed an unsaturated fatty acid-rich diet exhibited an increase in Ea from 17 to 63 kJ . mol-1 and a decrease in Tf from 23 to 4 degrees C. 3. This decrease in Tf was related to an increase in the ratio of linoleic acid to stearic acid in the membrane lipid. Tf was not related to the proportion of unsaturated fatty acids in the membrane lipids, although an increase in unsaturation usually led to a decrease in Tf. 4. The results show that membrane lipid fluidity has a direct influence on the conformation of the active site of some membrane-associated enzymes, with the result that such enzymes display a higher Ea when the membrane lipids are comparatively more fluid. The increase in Ea of membrane-associated enzymes which accompanies changes in the physical state of membrane suggests that some proteins may phase separate with the more fluid lipids at low temperatures.