Acetyl-L-carnitine, ALCAR or ALC, is an acetylated form of L-carnitine. It is naturally produced by the body, although it is often taken as a dietary supplement. Acetylcarnitine is broken down in the blood by plasma esterases to carnitine which is used by the body to transport fatty acids into the for breakdown.
Carnitine is both a nutrient and made by the body as needed; it serves as a substrate for important reactions in which it accepts and gives up acyl groups. When it accepts one acyl-group, acetyl-L-carnitine is the product; other acylcarnitines include propionyl-L-carnitine, isovaleryl-L-carnitine, butyryl-L-carnitine, hydroxybutyryl-L-carnitine, hexanoyl-L-carnitine, ortanoyl-L-carnitine, decanoyl-L-carnitine, palmitoyl-L-carnitine, stearoyl-L-carnitine, and acetoacetyl-L-carnitine.
Acetylcarnitine (ALCAR) is the most abundant naturally occurring derivative and is formed in the reaction:
where the acetyl group displaces the hydrogen atom in the central hydroxyl group of carnitine. Coenzyme A (CoA) plays a key role in the Krebs cycle in , which is essential for the production of ATP, which powers many reactions in cells; acetyl-CoA is the primary substrate for the Krebs cycle, once it is de-acetylated, it must be re-charged with an acetyl-group in order for the Krebs cycle to keep working.
Most cell types appear to have transporters to import carnitine and export acyl-carnitines, which seems to be a mechanism to dispose of longer-chain moieties; however many cell types can also import ALCAR.
Within cells, carnitine plays a key roles in importing acyl-coA into mitochondria; the acyl-group of the acyl-CoA is transferred to carnitine, and the acyl-carnitine is imported through both mitochondrial membranes before being transferred to a coA molecule, which is then beta oxidized to acetyl-CoA. A separate set of enzymes and transporters also plays a buffering role by eliminating acetyl-CoA from inside mitochondria created by the pyruvate dehydrogenase complex that is in excess of its utilization by the Krebs cycle; carnitine accepts the acetyl moiety and becomes ALCAR, which is then transported out of the mitochondria and into the cytosol, leaving free coA inside the mitochondria ready to accept new import of fatty acid chains. ALCAR in the cytosol can also form a pool of acetyl-groups for coA, should the cell need it.