Fatty acid oxidation in human and rat heart. Comparison of cell-free and cellular systems.

Oxidation rates of palmitate and activities of the mitochondrial marker enzymes cytochrome c oxidase and citrate synthase have been determined in homogenates, isolated mitochondria and slices of human and rat heart and in calcium-tolerant rat cardiac myocytes. Homogenates and mitochondria from rat heart showed a 6- and 2.5-fold higher palmitate oxidation rate than the corresponding preparations from human heart. From the palmitate oxidation rates and cytochrome c oxidase and citrate synthase activities as parameters, the mitochondrial protein contents of human and rat heart were calculated to be about 18 and 45 mg/g wet weight, respectively. Based on citrate synthase activities, the fatty acid oxidation rates were about the same in homogenates and isolated mitochondria, much lower in myocytes and lowest in slices. In the cellular systems the palmitate molecule was more completely oxidized than in homogenates or isolated mitochondria. Fatty acid oxidation rates were concentration-dependent in slices, but not with myocytes. With the cellular systems, palmitate oxidation was synergistically stimulated by the addition of carnitine, coenzyme A and ATP to the incubation medium. This stimulation could be attributed only partly to an increased oxidation in damaged cells.