Entry of [(1,2‐¹³C₂)acetyl]‐l‐carnitine in liver tricarboxylic acid cycle and lipogenesis

Abstract

The biochemical pathways involved in acetyl-l-carnitine utilization were investigated in conscious, freely moving rats by 13 C NMR spectroscopy. Following 4-h [(1,2-13 C 2 )acetyl]-l-carnitine infusion in fasted animals, the free carnitine levels in serum were increased, and an efflux of unlabelled acetyl-l-carnitine from tissues was observed. [(1,2-13 C 2 )Acetyl]-l-carnitine was found to enter biosynthetic pathways in liver, and the acetyl moiety was incorporated into both cholesterol and 3-hydroxybutyrate carbon skeleton. In accord with the entry of [(1,2-13 C 2 )acetyl]-l-carnitine in the mitochondrial acetylCoA pool associated with tricarboxylic acid cycle, the 13 C label was also found in liver glutamate, glutamine, and glutathione. The analysis of the 13 C-labelling pattern in 3-hydroxybutyrate and cholesterol carbon skeleton provided evidence that the acetyl-l-carnitine-derived acetylCoA pool used for ketone bodies synthesis in mitochondria was homogeneous, whereas cholesterol was synthesized from two different acetylCoA pools located in the extra-and intramitochondrial compartment, respectively. Furthermore, cholesterol molecules were shown to be preferentially synthesized by the metabolic route involving the direct channelling of CoA-activated mitochondria-derived ketone bodies into 3-hydroxy-3-methylglutarylCoA pathway, prior to equilibration of their acyl groups with extramitochondrial acetylCoA pool via acetoacetylCoA thiolase.Keywords: acetyl-l-carnitine; NMR spectroscopy; carbon-13; cholesterol; lipogenesis.Acetyl-l-carnitine is a major esterified carnitine congener in most mammalian tissues [1,2]. It is synthesized by carnitine O-acetyltransferase reaction (EC 2.3.1.7), which reversibly converts acetylCoA and free carnitine to acetyl-l-carnitine and CoASH without ATP utilization. Carnitine O-acetyltransferase activity was reported to be present almost in all tissues [3,4], and in the liver, it was found in peroxisomes, mitochondria, and microsomes [5,6].Previous investigations on acetyl-l-carnitine metabolism have focused on its role in energy-producing processes [7,8]. Evidence has been also presented that supports the conclusion that the acetylcarnitine/carnitine system buffers the CoASH/acetylCoA ratio in the mitochondrial matrix [9].To date, however, no detailed study has investigated both the role of acetyl-l-carnitine in acetyl groups trafficking between intracellular organelles and its involvement in oxidative and biosynthetic processes in vivo.As a product of peroxisomal b-oxidation, acetyl-l-carnitine may be shuttled out from peroxisomes to the cytosol [10,11], while the acetylCoA synthesized in mitochondria by glucose or fatty acid oxidation may be transported to the cytosolic compartment as acetyl-l-carnitine both in liver [12,13] and heart myocytes [14]. Studies on exogenous administration of ([1-14 C]acetyl)-l-carnitine in 24-h fasted mice reported that most acetyl moiety was oxidized in mitochondria and radioactivity was released as 14 CO 2 [15]. Moreover, the liposoluble fraction o...

Daily exercise increases hepatic fatty acid oxidation and prevents steatosis in Otsuka Long-Evans Tokushima Fatty rats

“…Increased acetyl carnitine levels in muscle and plasma have been found in obesity models or after chronic high-fat feedings (21,41). Although not directly confirmed, previous reports indicate that the products of incomplete oxidation (acetyl CoA and acetyl-carnitine) may also be directed to fatty acid biosynthesis pathways (2). In findings similar to ours in liver, Koves et al also found that exercise training increased the flux of both ␤-oxidation and TCA cycle improving the rate of complete fatty acid oxidation to CO 2 in skeletal muscle, providing evidence that exercise training can enhance the complete oxidation of lipids in both liver and skeletal muscle.…”

Carnitine Insufficiency Caused by Aging and Overnutrition Compromises Mitochondrial Performance and Metabolic Control

“…In carnitine-supplemented animals, urinary excretion of total acylcarnitines increased 130-fold. Still, a large proportion of the circulating acylcarnitines are retained by renal reabsorption (56), implying that these molecules might be destined for other metabolic fates (57)(58)(59). Interestingly, acetylcarnitine supplements have proven therapeutically effective in models of neuronal dysfunction.…”

Carnitine acyltransferases and their influence on CoA pools in health and disease

“…Another study found that carnitine supplementation increased the levels of dopamine, epinephrine, and serotonin particularly in regions rich in cholinergic neurons (Juliet et al, 2003). In a careful NMR study, the [ 13 C] label from [(1,2-[ 13 C](2))acetyl]-L L -carnitine was also found in liver glutamate, glutamine, and glutathione (Aureli et al, 1999) in accord with entry into the mitochondrial acetyl-CoA pool associated with the tricarboxylic acid cycle. In brain, after injection of [1-14 C]acetyl-L L -carnitine, 60% of the radioactivity was recovered as CO 2 consistent with energy generation.…”