L-Carnitine (Injectable)

L-Carnitine (3-hydroxy-4-trimethylammonio-butanoate) is a small, water-soluble quaternary ammonium compound synthesised in the liver and kidney from lysine and methionine and obtained in the diet mainly from red meat. It is not a peptide. Its defining biochemical role is as the obligatory carrier for long-chain fatty acids across the inner mitochondrial membrane: carnitine palmitoyltransferase 1 (CPT1) on the outer membrane transfers the acyl group from coenzyme A to carnitine, the carnitine–acylcarnitine translocase exchanges acylcarnitine into the matrix, and CPT2 regenerates acyl-CoA for β-oxidation [1]. Carnitine also buffers the mitochondrial acetyl-CoA pool by forming acetylcarnitine, which keeps free CoA available for pyruvate dehydrogenase [5]. This product is a sterile liquid formulation of L-carnitine (the biologically active L-enantiomer) offered in 600 mg and 1200 mg vials as a research reagent.

Transport and tissue distribution

Plasma carnitine enters cells through the high-affinity sodium-dependent transporter OCTN2 (SLC22A5); loss-of-function mutations in this transporter cause primary carnitine deficiency, a disorder characterised by impaired fatty-acid oxidation in muscle and heart [1]. Skeletal muscle holds more than 95% of the body pool and turns it over slowly, which is why acute increases in plasma carnitine do not translate quickly into higher muscle content [3][5].

What the human and animal literature reports

• Muscle carnitine loading. In a 24-week study in 14 healthy men, oral L-carnitine taken with carbohydrate (to stimulate insulin-mediated OCTN2 transport) raised muscle carnitine content by about 21% and shifted substrate use during exercise, whereas carnitine alone did not raise muscle content [3]. Earlier work by the same group showed that hyperinsulinaemia is required for muscle carnitine uptake [5].
• Body-weight meta-analyses. A 2016 meta-analysis of nine randomised trials reported a pooled body-weight difference of about −1.3 kg with carnitine supplementation, with the effect diminishing over time [2]. A 2020 systematic review of 37 trials reported a similar small effect that was largely confined to participants with overweight or obesity and to lower doses [4]. Both note substantial heterogeneity.
• Gut-microbial metabolism. In mice and humans, dietary L-carnitine is converted by intestinal microbes to trimethylamine, which is oxidised in the liver to trimethylamine-N-oxide (TMAO); in the 2013 Nature Medicine study, chronic carnitine feeding in mice accelerated atherosclerosis in a microbiota-dependent manner, and plasma TMAO was associated with cardiovascular events in a human cohort [6].

Related forms

Only the L-enantiomer is biologically active; D-carnitine is not transported efficiently by OCTN2 and competitively inhibits uptake of the L-form, which is why racemic material is unsuitable for metabolic work [1]. Acetyl-L-carnitine and propionyl-L-carnitine are short-chain acylcarnitine esters that are interconverted with free carnitine by carnitine acetyltransferase; they are separate reagents with their own literature and should not be confused with the free base supplied here.

Why it is used as a laboratory reagent

L-carnitine is a routine component of fatty-acid oxidation assays (as the co-substrate for CPT1 in isolated mitochondria or permeabilised cells), a supplement in hepatocyte and cardiomyocyte culture media, a standard in acylcarnitine profiling by tandem mass spectrometry for inborn-error screening, and a tool compound in OCTN2 transport studies [1]. A sterile, pre-dissolved solution avoids the weighing and pH adjustment otherwise needed for the hygroscopic free base.

What the research does not establish

The clinical literature does not establish L-carnitine as an effective agent for weight reduction in the general population; the pooled effects are small, heterogeneous and derived mostly from oral supplementation studies. The parenteral pharmaceutical form (levocarnitine injection) is an approved medicine only for specific carnitine-deficiency states and end-stage renal disease, and those indications, doses and safety data do not transfer to a research reagent. The TMAO findings raise an unresolved question about long-term cardiovascular effects of carnitine exposure. This solution is supplied for in-vitro laboratory work only, has not been evaluated by the FDA, and no statement here describes a benefit in a person or animal.

Format. Clear, colourless sterile aqueous solution of L-carnitine in a sealed multi-dose glass vial, supplied at 600 mg or 1200 mg per vial. Concentration, pH and excipients are stated on the certificate of analysis; no reconstitution is required.

Storage. Store at controlled room temperature (15–25°C), protected from light; do not freeze. Once the stopper has been punctured, keep at 2–8°C and use within the laboratory’s validated in-use period. Discard if the solution becomes cloudy or discoloured.

Dilution. The solution is miscible with water and standard aqueous buffers. If a working dilution is needed, the reconstitution calculator can be used in dilution mode to compute final concentration.

Handling. Gloves and eye protection. L-carnitine is hygroscopic and mildly basic in concentrated form. For in-vitro laboratory research only — not for human or veterinary use.