Key findings
- Transport across the apical membrane of enterocytes and brush-border membrane vesicles was sodium-dependent, voltage-dependent and pH-dependent, with a Km of 26 to 31 micromolar and a one-to-one sodium to carnitine stoichiometry. [1]
- Inhibition studies in the same preparations ranked the competing substrates, with unlabeled L-carnitine above D-carnitine, acetyl-L-carnitine and gamma-butyrobetaine, which in turn exceeded palmitoyl-L-carnitine and betaine, while alanine, histidine and choline had no significant effect. [1]
- Structural requirements for inhibiting the human transporter, established in L6 cells stably overexpressing it, were a permanently positively charged nitrogen atom plus a carboxyl, nitrile or ester group joined by a two to four atom linker. [2]
- Crystal structures of two carnitine acyltransferases place carnitine and coenzyme A in deep channels on opposite sides of the catalytic histidine, and show that a single residue in the acyl binding pocket shifts substrate preference between short and medium chain acyl groups. [3][4]
Identity and structure
- Chemical name
- 3-hydroxy-4-(trimethylazaniumyl)-butanoate [5]
- Transporter
- OCTN2, the gene product of SLC22A5, a high-affinity sodium-dependent carnitine transporter with high renal expression [2][6]
- Stereochemistry
- The L form and the D form are separate materials; both were tested as competing substrates in the cited transport work, as was acetyl-L-carnitine [1]
- Form as supplied
- Solid; highly hydrophilic
Mechanism as studied
The transport work describes a concentrative, sodium-coupled carrier. In isolated enterocytes the molecule was held against a five-to-one gradient and in membrane vesicles uptake overshot the equilibrium value, with transport dependent on sodium, membrane voltage and pH and a one-to-one sodium to carnitine stoichiometry, matching the properties of OCTN2, whose transcript the authors localized to the cells lining the intestinal villus. [1]
Work in engineered and native cell systems then defines what the transporter recognizes and what regulates it. In L6 cells stably overexpressing the human transporter, inhibitors required a permanently charged nitrogen and an anionic or polar group on a short linker; in differentiating human monocytes, transporter transcription was driven by a cytokine-activated mTOR and STAT3 axis; and in C2C12 myotubes a range of AMPK-activating compounds inhibited uptake rather than raising it. [2][6][7]
The crystal structures address the other half of carnitine biochemistry, the transfer of acyl groups. Carnitine and coenzyme A bind in deep channels on opposite sides of the catalytic histidine, and the authors suggest carnitine itself assists catalysis by stabilizing the oxyanion of the reaction intermediate; comparison between the acetyl and the octanoyl transferase shows a larger hydrophobic pocket in the latter and a single residue whose mutation switches chain-length preference. [3][4]
Research findings
Ex vivo
- System
- Isolated chicken enterocytes and brush-border membrane vesicles, with in situ hybridization in small intestine
- Measured
- Steady-state gradient, uptake overshoot, sodium, voltage and pH dependence, Km, stoichiometry, inhibitor rank order and transporter transcript localization
- Reported
- Enterocytes maintained a five-to-one gradient and vesicle uptake overshot equilibrium 2.5-fold; transport was sodium, voltage and pH dependent with a Km of 26 to 31 micromolar and one-to-one stoichiometry, and the transcript appeared only in cells lining the villus. [1]
In vitro
- System
- L6 cells stably transduced to overexpress the human OCTN2 transporter, with in silico prediction of inhibitor structures
- Measured
- Transporter expression at transcript, protein and functional level, and the structural requirements of inhibitors
- Reported
- Expression rose at all three levels, and inhibition required a constantly positively charged nitrogen atom together with a carboxyl, nitrile or ester group connected by a two to four atom linker. [2]
In vitro
- System
- C2C12 skeletal muscle myotubes expressing all three OCTN transporters
- Measured
- Uptake of L-carnitine under insulin and under a panel of AMPK-activating compounds, with an AMPK inhibitor rescue and saturation curve analysis
- Reported
- Insulin did not raise uptake at 100 nanomolar and raised it modestly at 150 nanomolar; every AMPK activator tested significantly inhibited uptake, the AMPK inhibitor gave only a partial rescue, and the caffeine effect was not competitive. [7]
In vitro
- System
- 661W murine photoreceptor cell line
- Measured
- Saturability and Km of uptake, sensitivity to extracellular pH and sodium, transporter transcript levels and inhibition by known carnitine transporter substrates
- Reported
- Uptake was saturable with a Km of 5.5 millimolar and sensitive to pH and sodium, but transcripts for the known carnitine transporters were very low and their substrates did not inhibit uptake, so the profile matched no known carnitine transporter. [5]
In vitro
- System
- Human monocytes differentiated to macrophages with GM-CSF
- Measured
- Sodium-dependent uptake kinetics, SLC22A5 and SLC38A2 transcript and protein levels, and mTOR and STAT3 activation
- Reported
- Uptake rose sharply on differentiation, resolving into a high-affinity component near 4 micromolar attributed to OCTN2 and a low-affinity component above 10 millimolar attributed to system A, with transporter transcription driven by mTOR-activated STAT3. [6]
Structural
- System
- Crystal structures of murine carnitine acetyltransferase alone and in complex with carnitine or with coenzyme A
- Measured
- Domain fold, active site location and substrate binding geometry
- Reported
- The two domains share a backbone fold and the active site lies at their interface, with carnitine and coenzyme A in deep channels on opposite sides of the catalytic His343; the authors suggest carnitine stabilizes the oxyanion of the reaction intermediate. [3]
Structural
- System
- Crystal structures of mouse carnitine octanoyltransferase alone and in complex with octanoylcarnitine, at resolutions to 2.0 angstrom, with site-directed mutants
- Measured
- Acyl group binding pocket geometry and the effect of single-residue substitutions on substrate preference
- Reported
- A larger hydrophobic pocket binds the octanoyl group in an extended conformation, mutation of Gly-553 shifts preference between short and medium chain acyl groups, and mutagenesis indicated Met-335 is important for catalysis. [4]
Handling for in-vitro work
- Solvent in the cited work
- Uptake assays were run in aqueous buffer with defined sodium content and pH, both of which changed the measured transport in the cited work [1][5]
- Related forms
- D-carnitine, acetyl-L-carnitine, palmitoyl-L-carnitine and gamma-butyrobetaine are distinct materials that competed with this one in the cited transport assays [1]
- Storage
- Solid material kept dry, dark and cold; solutions prepared fresh and used promptly
Open questions
- One cited cell line took up the molecule by a carrier whose profile matched none of the known carnitine transporters, so the cited work does not establish a single uptake route across cell types.
- The intestinal transport work used chicken tissue and the acyltransferase structures used murine enzymes, so the cited work does not establish that either result transfers unchanged to another species.
- The cited work does not report an identity or purity method that would distinguish the L form from the D form or from an acylated form in a supplied material.
Lot records
Check the record for the exact material you order. A published paper and a batch certificate answer different questions.
- RV-24-0016-2 ↗L-Carnitine · 99.10% HPLC2026-09-22
- RV-24-0016-1 ↗L-Carnitine · 99.10% HPLC2026-09-16
References
- Durán JM, Peral MJ, Calonge ML, et al. Functional characterization of intestinal L-carnitine transport. The Journal of membrane biology. 2002.
- Todesco L, Bur D, Brooks H, et al. Pharmacological manipulation of L-carnitine transport into L6 cells with stable overexpression of human OCTN2. Cellular and molecular life sciences : CMLS. 2008.
- Jogl G, Tong L Crystal structure of carnitine acetyltransferase and implications for the catalytic mechanism and fatty acid transport. Cell. 2003.
- Jogl G, Hsiao YS, Tong L Crystal structure of mouse carnitine octanoyltransferase and molecular determinants of substrate selectivity. The Journal of biological chemistry. 2005.
- Ohno Y, Otsuka Y, Nohara M, et al. Characterization of an L-Carnitine Transport System in Murine Photoreceptor Cell Line. Biological & pharmaceutical bulletin. 2017.
- Ingoglia F, Visigalli R, Rotoli BM, et al. Human macrophage differentiation induces OCTN2-mediated L-carnitine transport through stimulation of mTOR-STAT3 axis. Journal of leukocyte biology. 2017.
- Shaw A, Jeromson S, Watterson KR, et al. Multiple AMPK activators inhibit l-carnitine uptake in C2C12 skeletal muscle myotubes. American journal of physiology. Cell physiology. 2017.
Publication records fetched from PubMed on 2026-09-20. Profile text reviewed 2026-09-20.