Increased electrical stability of hepatic mitochondrial lipid membranes in hyperthyroidism

Thyroid hormones regulate the velocity of lipid and protein metabolism in the body by determining both the quantitative and the qualitative composition of these main components of biomembranes [9, ii]. With a fall in the thyroid hormone level the degree of unsaturation of fatty acids in the mitochondria of the heart and liver increases, and the point of phase transition in lipids isolated from these organelles is shifted considerably toward the region of lower temperatures [i0, ii]. Administration of thyroxine into hypothyroid or normal animals caused the opposite changes [i0, ii]. A tendency for the viscosity of the mitochondrial membrane lipids to decrease in hypothyroidism and to increase in the presence of an excess of circulating thyroid hormone has been discovered [i]. In hypothyroidism the activation energy of substrate oxidation processes by mitochondria has been shown to be increased.