Formation and disappearance of an endogenous uncoupling factor during swelling and contraction of mitochondria

Abstract

During swelling of rat-liver mitochondria which occurs spontaneously or is induced by Ca 2+ or thyroxine, direct analytical measurements show a parallel intramitochondrial formation of U factor, a heat-stable, isooctane-soluble uncoupling and swelling agent of fatty acid nature. On the other hand, mitochondrial swelling induced by glutathione or by phosphate is not accompanied by increased formation of U factor. Serum albumin, which is a “trap” for U factor by virtue of its ability to bind fatty acids, prevents swelling induced by Ca 2+ and thyroxine, but not produced by GSH and phosphate. These findings therefore indicated that Ca 2+ and thyroxine accelerate mitochondrial swelling indirectly, by accelerating the enzymic formation of U factor from a precursor lipid. Stimulation of U factor formation is given by Ca 2+ in sonicated mitochondria, but not by l-thyroxine. When swollen mitochondria are contracted again by ATP, U factor rapidly disappears at a rate which parallels the rate of contraction and ceases when contraction ceases. While there is an enormous increase in the level of U factor during swelling, experiments with [ 14C]oleate as an indicator of U factor show that oxidative removal of oleate occurs simultaneously. On addition of ATP, which produces contraction of mitochondria and disappearance of U factor, [ 14C]oleate rapidly disappears. Most of th [ 14C]oleate is rapidly incorporated into the phosphatidic acid and cephalin fractions of mitochondrial lipids. These findings thus show that enzymic formation and removal of U factor are closely geared to mictochondrial swelling and contraction and that accumulation of U factor may be cause of swelling in the presence of Ca 2+ and thyroxine.