The long-chain acylcarnitine (LCAC) products of CPT1 are transported across the inner mitochondrial membrane by carnitine acylcarnitine translocase and then converted back to LCACoAs by carnitine palimitoyltransferase

Journal of Lipid Research Volume 55, 2014 635 Copyright © 2014 by the American Society for Biochemistry and Molecular Biology, Inc. L -Carnitine is a conditionally essential nutrient that serves as a substrate for a family of acyltransferase enzymes that catalyze the interconversion of acyl-CoAs and acylcarnitines. Unlike their acyl-CoA precursors, acylcarnitines can be transported across cellular membranes. Accordingly, carnitine is best known for its obligatory role in shuttling long-chain acyl-CoAs (LCACoAs) from the cytoplasm into the mitochondrial matrix for fatty acid oxidation, a function that is mediated by the outer mitochondrial membrane enzyme, carnitine palmitoyltransferase 1 (CPT1). The long-chain acylcarnitine (LCAC) products of CPT1 are transported across the inner mitochondrial membrane by carnitine acylcarnitine translocase and then converted back to LCACoAs by carnitine palimitoyltransferase 2 (CPT2), also localized to the inner membrane. By contrast, carnitine acetyltransferase (CrAT) resides in the mitochondrial matrix and has strong preference for short-chain acyl-CoA (SCACoA) intermediates of fatty acid, glucose, and amino acid catabolism. Thus, CrAT facilitates traffi cking and effl ux of carbon intermediates from the mitochondrial compartment to other cellular and extracellular sites. Recent animal studies have established important roles for L -carnitine and CrAT in regulating glucose homeostasis and mitochondrial substrate switching ( 1 ). By converting acetyl-CoA to acetylcarnitine, CrAT not only buffers the mitochondrial acetyl-CoA pool but also regenerates free CoA, both of which infl uence the activities of several oxidative enzymes. Carnitine supplementation promotes Abstract Carnitine acetyltransferase (CrAT) is a mitochondrial matrix enzyme that catalyzes the interconversion of acetyl-CoA and acetylcarnitine. Emerging evidence suggests that this enzyme functions as a positive regulator of total body glucose tolerance and muscle activity of pyruvate dehydrogenase (PDH), a mitochondrial enzyme complex that promotes glucose oxidation and is feedback inhibited by acetyl-CoA. Here, we used tandem mass spectrometry-based metabolic profi ling to identify a negative relationship between CrAT activity and muscle content of lipid intermediates. CrAT specifi c activity was diminished in muscles from obese and diabetic rodents despite increased protein abundance. This reduction in enzyme activity was accompanied by muscle accumulation of long-chain acylcarnitines (LCACs) and acyl-CoAs and a decline in the acetylcarnitine/ acetyl-CoA ratio. In vitro assays demonstrated that palmitoyl-CoA acts as a direct mixed-model inhibitor of CrAT. Similarly, in primary human myocytes grown in culture, nutritional and genetic manipulations that promoted mitochondrial infl ux of fatty acids resulted in accumulation of LCACs but a pronounced decrease of CrAT-derived shortchain acylcarnitines. These results suggest that lipidinduced antagonism of CrAT might contribute to decreased PDH activity and glucose disposal in the context of obesity and diabetes. —Seiler, S. E., O. J. Martin, R. C. Noland, D. H. Slentz, K. L. DeBalsi, O. R. Ilkayeva, J. An, C. B. Newgard, T. R. Koves, and D. M. Muoio. Obesity and lipid stress inhibit carnitine acetyltransferase activity. J. Lipid Res. 2014. 55: 635–644.