Membrane fatty acids as pacemakers of animal metabolism.

The recent discovery that the fatty acid composition of tissue phospholipids varies in a systematic manner among species has lead to the proposal that membrane fatty acid composition is an important determinant of the metabolic rate characteristic for each species. Endotherms (mammals and birds) have a basal metabolic rate (BMR) that is several times that of ectotherms and have more polyunsaturated membranes. In both birds and mammals, as species size increases there is a decrease in mass-specific BMR and a decrease in membrane polyunsaturation. Membrane-associated processes are significant components of BMR and important membrane proteins operate at much faster rates in species with high BMR than in those with low BMR. A series of "species-crossover" experiments show that the rate of this molecular activity is largely due to the nature of the membrane bilayer surrounding these membrane proteins such that polyunsaturated membranes are associated with fast membrane-associated processes. It is suggested that this influence is due to the physical properties that such polyunsaturated membranes possess. This has been called the membrane pacemaker theory of metabolism and provides a framework to understand factors such as the influence of diet on metabolism. It is noted that in the rat membrane fatty acid composition is a regulated parameter being more influenced by the balance between n-3 and n-6 polyunsaturates in the diet than it is by general diet content of saturated, monounsaturated and total polyunsaturated fats.