Metabolism of acetyl‐l‐carnitine for energy and neurotransmitter synthesis in the immature rat brain

Abstract

Acetyl-L-carnitine (ALCAR) is an endogenous metabolic intermediate that facilitates the influx and efflux of acetyl groups across the mitochondrial inner membrane. Exogenously administered ALCAR has been used as a nutritional supplement and also as an experimental drug with reported neuroprotective properties and effects on brain metabolism. The aim of this study was to determine oxidative metabolism of ALCAR in the immature rat forebrain. Metabolism was studied in 21 day old rat brain at 15, 60 and 120 minutes after an intraperitoneal injection of [2-13C]acetyl-L-carnitine. The amount, pattern, and fractional enrichment of 13C-labeled metabolites were determined by ex vivo 13C-NMR spectroscopy. Metabolism of the acetyl moiety from [2-13C]ALCAR via the tricarboxylic acid (TCA) cycle led to incorporation of label into the C4, C3 and C2 positions of glutamate (GLU), glutamine (GLN) and GABA. Labeling patterns indicated that [2-13C]ALCAR was metabolized by both neurons and glia; however, the percent enrichment was higher in GLN and GABA than in GLU, demonstrating high metabolism in astrocytes and GABAergic neurons. Incorporation of label into the C3 position of alanine, both C3 and C2 of lactate, and the C1 and C5 positions of glutamate and glutamine demonstrated that [2-13C]ALCAR was actively metabolized via the pyruvate recycling pathway. The enrichment of metabolites with 13C from metabolism of ALCAR was highest in alanine C3 (10%) and lactate C3 (9%), with considerable enrichment in GABA C4 (8%), GLN C3 (~4%) and GLN C5 (5%). Overall, our 13C-NMR studies reveal that the acetyl moiety of ALCAR is metabolized for energy in both astrocytes and neurons and the label incorporated into the neurotransmitters glutamate and GABA. Cycling ratios showed prolonged cycling of carbon from the acetyl moiety of ALCAR in the TCA cycle. Labeling of compounds formed from metabolism of [2-13C]ALCAR via the pyruvate recycling pathway was higher than values reported for other precursors and may reflect high activity of this pathway in the developing brain. This is, to our knowledge, the first study to determine the extent and pathways of ALCAR metabolism for energy and neurotransmitter biosynthesis in the brain.

Novel Mitochondrial Targets for Neuroprotection

“…This mechanism of action might be particularly important during reperfusion after cerebral ischemia, when the mitochondrial redox state is hyperoxidized, ROS production is elevated, and there are increased markers of oxidative molecular modification (Perez-Pinzon et al, 1999;Fiskum et al, 2004). There is evidence for mitochondrial metabolism of both ketone bodies, e.g., b-hydroxybutyrate, and acetyl-L-carnitine after acute brain injury, which is associated with a reduction in oxidative stress and neuroprotection (Rosenthal et al, 1992;Liu et al, 1993;Prins et al, 2005;Scafidi et al, 2010Scafidi et al, , 2011. While there is less evidence that neuroprotection by exogenous pyruvate is a consequence of its oxidative metabolism, support for this mechanism comes from recent measurements of brain slice O 2 consumption indicating that exogenous pyruvate significantly elevates endogenous respiration (Schuh et al, 2011).…”

Functional Expression of Organic Ion Transporters in Astrocytes and Their Potential as a Drug Target in the Treatment of Central Nervous System Diseases

“…In addition to carnitine, acetyl-L-carnitine, which is another endogenous OCTN2 substrate, is also known to be a physiologically multi-talented molecule, not simply an energy source. It can be used for amino acid synthesis, 46) and acts as a nutraceutical with anti-oxidative effects. 47,48) Consistently, acetyl-L-carnitine is expected to ameliorate several brain disease conditions, such as depression 49) and ischemia.…”

Neuroprotection by Acetyl-<i>L</i>-Carnitine after Traumatic Injury to the Immature Rat Brain

“…There are no published studies, however, that have tested neuroprotection by ALCAR in the injured immature brain, where the pathophysiology is significantly different from that of the mature brain [2,35,36,37]. We have recently demonstrated that following systemic administration, the acetyl moiety of 13 C-labeled ALCAR is metabolized for energy via the TCA cycle in astrocytes and neurons within the forebrains of 21- to 22-day-old rats [38]. Furthermore, 13 C from ALCAR was incorporated into γ-aminobutyric acid (GABA), glutamate and glutamine, which are formed subsequent to TCA cycle metabolism [38].…”