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 mam-malian fetus, however, carbohydrates rather than lipids appear to serve as the primary source of energy (4, 5). This difference in energy metabo-lism 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-hydroxy-butyrate) (6), intensive investigation was under-taken 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 acti-vated long chain fatty acyl groups from the extra-mitochondrial 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 demon-strated whereby long chain fatty acylcarnitine is formed. As acylcarnitine esters, the long chain fatty acids can be translocated across the mito-chondrial membrane, which is relatively imperme-able to acyl CoA molecules. The transesterifica-tion reaction is catalyzed by a long chain fatty acyl CoA-carnitine transferase, which has been