L-Carnitine

L-Carnitine is used as a defined substrate and pathway probe in mitochondrial bioenergetics. Typical work includes carnitine palmitoyltransferase activity assays, acylcarnitine profiling by tandem mass spectrometry, and fatty-acid oxidation flux in cells or isolated mitochondria. It also serves in models of carnitine shuttle deficiency, in skeletal muscle energetics work, and as a reference standard in the acylcarnitine methods behind newborn screening for fatty-acid oxidation disorders. No claim is made about supplementation in humans.

L-Carnitine is a quaternary ammonium compound, (R)-3-hydroxy-4-(trimethylazaniumyl)butanoate, with a molecular weight of 161.20 g/mol. Unlike the other compounds in this catalogue it is not a peptide. It is endogenously biosynthesised in the liver and kidneys from lysine and methionine through a four-step pathway proceeding via trimethyllysine and gamma-butyrobetaine, the final hydroxylation being catalysed by gamma-butyrobetaine dioxygenase.

Its central role in metabolic research is as the obligate carrier in the carnitine shuttle, the mechanism by which long-chain fatty acids cross the inner mitochondrial membrane. Long-chain acyl-CoA cannot pass that membrane directly. Carnitine palmitoyltransferase 1, sitting on the outer mitochondrial membrane, transfers the acyl group onto carnitine to form acylcarnitine; carnitine-acylcarnitine translocase moves it across the inner membrane; carnitine palmitoyltransferase 2 regenerates acyl-CoA on the matrix side, where beta-oxidation proceeds. CPT1 is the rate-limiting step of the entire pathway and is allosterically inhibited by malonyl-CoA, which is what couples fatty acid oxidation to fatty acid synthesis.

Cellular uptake depends on OCTN2, the sodium-dependent transporter encoded by SLC22A5. Loss-of-function mutations in that gene cause primary systemic carnitine deficiency, and that genetic model is a large part of why the carnitine shuttle is as well characterised as it is. A second research literature concerns the microbial metabolism of dietary carnitine rather than carnitine itself: Koeth and colleagues (Nat Med, 2013) reported that gut microbiota convert carnitine to trimethylamine, which is oxidised hepatically to trimethylamine-N-oxide, and examined the association of that metabolite with atherosclerosis in animal models. It is noted here because it is a genuine part of the compound's research picture rather than a footnote to it.

L-Carnitine is supplied here as a ready-to-use solution at 600mg/ml in a 10ml vial. It is not a licensed medicine in the UK in this presentation and is not supplied for human consumption, veterinary use, or any therapeutic purpose.