The dietary regulation of stearyl coenzyme A desaturase activity and membrane fluidity in the rat aorta.

Numerous studies have demonstrated that alterations in membrane composition or fluidity are often associated with alterations in the properties of membrane-bound enzymes. In order to obtain membranes of varying fluidity, rats were fed diets that were either fat-free or supplemented with 15% safflower oil, and two properties associated with aorta and liver microsomal membranes were selected for study: stearyl CoA desaturase activity, and fluidity as monitored by fatty acid composition and microviscosity (measured by fluorescence depolarization). If fluidity directly modulates desaturase activity, one would predict that a low fluidity would stimulate the desaturase activity. Ten times more desaturase activity is present in aorta microsomes from rats on a fat-free diet than in microsomes from rats on a safflower oil supplemented diet. However, on the fat-free diet, these aorta microsomes were more fluid than those of rats fed safflower oil supplemented diet. The fluidity of liver microsomal membranes was not altered in response to diet, despite significant changes in desaturase enzyme content. The contrasting evidence presented here suggests that no correlation exists between desaturase enzyme activity and membrane fluidity in the two tissues studies. We have demonstrated that the aorta has appreciable capacity to desaturate stearyl CoA and that dietary manipulation causes significant changes in aorta membrane fluidity that may be of sufficient magnitude to affect the overall metaboism of aorta cells.