Lipid metabolism in the newborn heart.

In recent years long chain fatty acids have been shown to be the principal metabolic fuel of the adult heart (1-3). For the tissues of the mammalian fetus, however, carbohydrates rather than lipids appear to serve as the primary source of energy (4, 5). This difference in energy metabolism related to maturation suggested that the heart of the newborn, in contrast to that of the adult, might be unable to utilize long chain fatty acids and perforce must rely on glucose as the major substrate for energy production. After the observation that the rate of long chain fatty acid oxidation in tissues was enhanced by carnitine (y-trimethyl ammonium 18-hydroxybutyrate) (6), intensive investigation was undertaken to elucidate the mechanism of action of this compound. Carnitine, a normal constituent of many tissues, is especially abundant in myocardium (7, 8). From the evidence obtained in several laboratories (9, 10), it has been proposed that carnitine effects a stimulation of long chain fatty acid oxidation by functioning as a carrier of activated long chain fatty acyl groups from the extramitochondrial cytoplasm to the intramitochondrial sites of fatty acid oxidation. In this operation, a reversible transesterification between long chain fatty acyl CoA and carnitine has been demonstrated whereby long chain fatty acylcarnitine is formed. As acylcarnitine esters, the long chain fatty acids can be translocated across the mitochondrial membrane, which is relatively impermeable to acyl CoA molecules. The transesterification reaction is catalyzed by a long chain fatty acyl CoA-carnitine transferase, which has been