Description
PrymaLab · Research Use Only
L-Carnitine 500mg
The mirror image is not inert. It competes.
L-carnitine is a 161 dalton zwitterion, not a peptide, and the catalogues that file it under peptides are wrong about that. The chemistry that actually matters here is older and simpler than any receptor argument. It is chirality.
Specification Table
| Property | Value |
|---|---|
| Compound | L-carnitine, also written levocarnitine |
| Article class | A small zwitterionic metabolite. Not a peptide and not a protein |
| CAS number | 541-15-1 |
| Molecular formula | C7H15NO3 |
| Molecular weight | Approximately 161.2 |
| Structure | A quaternary ammonium group and a carboxylate on the same small molecule |
| Net charge | Zero at physiological pH. Both charges are permanent and internal |
| Chiral centre | One, at carbon 3 |
| Biologically active form | L only |
| D enantiomer | Competes for the same transporter and depletes tissue L-carnitine |
| Racemate | DL-carnitine. Withdrawn from use for the reason above |
| Endogenous status | Made in the body from lysine and methionine, and also obtained from diet |
| Cellular role | Carries long chain fatty acids across the inner mitochondrial membrane |
| Vial | 500mg |
| Storage | Sealed, cool and dry. Extremely hygroscopic |
Why Is L-Carnitine in a Peptide Catalogue?
Because it is injectable, endogenous and sold to the same buyers. None of those makes it a peptide.
A peptide is a chain of amino acid residues joined head to tail through amide bonds. This molecule has one nitrogen, three carbons between the charged ends, and no chain at all.
It is built from lysine and methionine in the body. That is probably where the confusion started, though a molecule made from amino acids is not a peptide any more than a brick wall is a clay pit.
The reason this is worth stating rather than shrugging at is that the handling rules in the rest of this catalogue do not transfer.
There is no methionine to oxidise, no asparagine to deamidate, no disulfide to reduce, no aggregation. All of those are properties of chains.
What this molecule has instead is a stereocentre and an appetite for water, and those two account for almost everything that goes wrong with it.
What Does the D Enantiomer Actually Do?
The naive assumption is that the wrong enantiomer is dead weight. Half your material does nothing, you use twice as much, and the arithmetic works out.
That is not what happens here.
D-carnitine is taken up by the same transporter as the L form, which means it competes for entry into the tissues that need the L form most.
Once inside it occupies the acyltransferase machinery without doing the job. The reported outcome in the stereopharmacology literature is depletion of tissue L-carnitine, not dilution of it.
The racemate was used and then withdrawn, and the reports of muscle weakness that drove the withdrawal resolved when the L form was given.
So a preparation contaminated with the D form is worse than a preparation containing less L-carnitine. It is a preparation containing an active antagonist that competes with the material you actually wanted.
That changes what a certificate has to establish. On most articles in this catalogue an impurity dilutes the result. Here one specific impurity reverses it.
The awkward part is that ordinary chromatography does not separate enantiomers. They have identical mass, identical polarity and identical behaviour on a standard column, so a purity figure of 99 percent by area is entirely consistent with a fifty-fifty racemate.
Confirming chirality needs a chiral column or optical rotation, and neither appears on a routine certificate unless somebody asked.
What Does the Carnitine Shuttle Do?
Long chain fatty acids cannot cross the inner mitochondrial membrane on their own.
The carnitine shuttle is the transport system that gets them across. An enzyme on the outer face attaches the fatty acid to carnitine, a translocase moves the resulting ester through, and a second enzyme on the inner face releases the fatty acid where beta-oxidation happens.
Carnitine then returns to the outer face and does the whole thing again, which is why a small pool can move a great deal of substrate.
That is the whole mechanism and it has been settled biochemistry for decades, which puts this molecule in a different position from most of the catalogue. There is no mechanism debate to have.
The open questions are about supply rather than about pathway. Whether adding more carnitine to a system that already has some changes flux through that shuttle depends entirely on whether carnitine was limiting in the first place, and in most tissues under most conditions it is not.
A design that adds carnitine without establishing that carnitine was the constraint is testing nothing in particular.
Those systems are narrower than the marketing suggests.
Why Is the Hygroscopicity a Practical Problem?
This is the part that catches people who come to it from peptide work, where a hygroscopic powder is an inconvenience.
L-carnitine is deliquescent. It does not merely take up moisture, it takes up enough to dissolve in it, and an open container in a humid room will produce a syrup rather than a damp powder.
The consequence for weighing is direct and it runs one way. Mass measured on material that has absorbed water is mass of carnitine plus mass of water, and the error runs in one direction only.
On a 500mg article that error can be several percent within minutes of opening.
Weigh fast, weigh dry, and where accuracy matters, dry the material first and record that you did.
A second consequence: material that has taken up water and been resealed will cake, and a caked lot cannot be subsampled evenly.
Neither of these is exotic. They are the ordinary handling rules for a deliquescent solid, and they are simply not the rules the rest of this catalogue teaches.
What Should the Certificate Show?
Chirality, by a chiral method or by optical rotation, with the figure stated. Everything else on that document is secondary.
Identity by nuclear magnetic resonance or mass spectrometry.
Purity by chromatography with the method named, and with an explicit note that the method does not resolve enantiomers.
Water content by Karl Fischer titration, which on a deliquescent solid is a real number rather than a formality.
Residual solvents, since the synthetic routes to this molecule use them.
Lot number and manufacturing date.
A certificate reporting 99 percent purity and nothing about chirality has told you that the material is carnitine. It has not told you which one.
What Does 500 Milligrams Support?
At 161.2 daltons, 500 milligrams works out at about 3.1 millimoles of l-carnitine.
That is an enormous molar quantity by the standards of this catalogue. A 10 milligram peptide vial at 2000 daltons holds 5 micromoles, so this article contains roughly six hundred times as many molecules as a typical peptide vial.
The reason is size. A small molecule packs far more molecules into the same mass, and anything sold by weight in this range will behave that way.
Practically, 3.1 millimoles dissolved in 10 millilitres of water gives a 310 millimolar stock, which is close to the solubility ceiling and far above anything an assay needs.
Working concentrations in cell systems generally sit in the high micromolar to low millimolar range, so a 500 milligram vial is a very long supply for in vitro work and a short one for anything at animal scale.
The mismatch is worth working out before ordering rather than after, because the deliquescence problem means an opened container has a clock on it that a sealed one does not.
A second point on the arithmetic. If chirality is unconfirmed and the material turns out to be racemic, the molar quantity of the active form is half of what the label implies and the other half is competing with it.
That is the one place where the chirality question stops being an analytical nicety and becomes a concentration error of a factor of two, running in the wrong direction.
Why Buy It If the Body Makes It?
A fair question about l-carnitine, and it applies to several other articles in this catalogue too.
Carnitine is synthesised from lysine and methionine through a pathway that ends in the liver and kidney, and it is also absorbed from diet, with meat the main source.
Most tissues do not make their own l-carnitine. Skeletal muscle, which holds the large majority of the body pool, imports all of it through a transporter.
That import step is where the interesting questions live, because a transporter can be saturated, competed for, or expressed at different levels between tissues and between individuals.
It is also where the D enantiomer does its damage to l-carnitine supply, since competition for a transporter is competition for the only route in.
For a research article the practical framing is that this compound is useful where the transport step or the tissue pool is the variable under study, and much less useful as a general addition to a system that has plenty.
There is a class of genuine transporter deficiency where the pool is low and supply matters a great deal, and that literature is where most of the solid work on this molecule sits.
Reading that literature is more informative than reading anything written about l-carnitine and exercise, which is a large body of work with a small effect size and a replication record I would not want to build on.
How Should the Vial Be Handled?
Store sealed, cool and dry, and keep the desiccant in the container rather than discarding it with the packaging.
Bring the container to room temperature before opening it, because cold glass in a warm room collects water and this compound will take all of it.
Work quickly once it is open, then close it again immediately.
Dissolve in water rather than in an organic solvent. A permanently charged zwitterion is very soluble in water and poorly soluble in most organics.
Prepared solutions are stable at neutral pH and do not need the freeze-thaw discipline a peptide stock does.
Filter-sterilise if a sterile solution is needed, rather than autoclaving it.
Record lot, chirality as stated on the certificate, water content, the diluent and the date.
Chirality goes at the top of that list, and it is the one field that will be missing from most paperwork you receive for this article.
Published Literature
Selected references on the stereochemistry of this molecule, on the transport system it belongs to and on the comparative work in animals that established the enantiomer problem.
- Bremer J. Physiological Reviews. 1983;63(4):1420-1480. DOI: 10.1152/physrev.1983.63.4.1420
- Evans AM, Fornasini G. Clinical Pharmacokinetics. 2003;42(11):941-967. DOI: 10.2165/00003088-200342110-00002
- Kudo N, Miyauchi K, Kawashima Y. Nihon Yakurigaku Zasshi. 2006;127(1):43-46. PMID: 16493928
- Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Pharmaceutical Research. 2010;27(4):544-575. DOI: 10.1007/s11095-009-0045-6
Frequently Asked Questions
Is L-carnitine a peptide?
No. It is a 161 dalton zwitterion with one nitrogen and no amide backbone. It is made in the body from lysine and methionine, which is probably where the mislabelling started.
What is a zwitterion?
A molecule carrying a permanent positive and a permanent negative charge at the same time, so the net charge is zero. Here the positive is a quaternary ammonium and the negative is a carboxylate.
Is the D enantiomer inactive?
No, and that is the point. It uses the same transporter as the L form, occupies the same enzymes without doing the work, and depletes tissue stores of the active form.
What happened with the racemate?
DL-carnitine was used and then withdrawn. Reports of muscle weakness resolved when the L form was given instead, which is what a competitive depletion would predict.
Will chromatography detect the wrong enantiomer?
Not on a standard column. Enantiomers have identical mass and polarity, so 99 percent by area is entirely consistent with a fifty-fifty racemate.
How is chirality confirmed?
By a chiral column or by optical rotation. Neither appears on a routine certificate unless someone asks for it.
What is the carnitine shuttle?
The transport system that carries long chain fatty acids across the inner mitochondrial membrane. Carnitine picks up the fatty acid on the outside, releases it on the inside, and then goes back for another one.
Does adding carnitine increase flux through that shuttle?
Only if carnitine was the limiting factor, and in most tissues under most conditions it is not. A design that adds it without establishing that is testing nothing in particular, which is a waste of an experiment.
What does deliquescent mean?
It takes up enough water from the air to dissolve in it. An open container in a humid room gives a syrup, not a damp powder.
How does that affect weighing?
Directly, and in one direction. Absorbed water is counted as compound, so every figure is an overestimate. On a 500mg article that is several percent.
What solvent should be used?
Water. A permanently charged zwitterion dissolves readily in water and poorly in most organic solvents.
What is the one certificate field that matters most?
Chirality. A 99 percent purity figure with no chirality statement tells you the material is carnitine without telling you which one.
Compliance Statement
L-carnitine is sold exclusively for laboratory research use. It is not a drug, food, or cosmetic product, and it is not a dietary product of any kind. It is not approved by the FDA or any comparable authority for human or veterinary use, it is a small metabolite rather than a peptide and the handling conventions used elsewhere in this catalogue do not apply to it, the D enantiomer competes with the active form rather than being inert so a preparation of unstated chirality cannot be interpreted, standard chromatography does not separate the two forms and a purity percentage alone leaves the question open, and no article in this range is offered for any human or veterinary purpose. This product is not intended to diagnose, treat, cure, or prevent any disease. It must not be given to humans or animals. Purchase is restricted to qualified researchers and institutions operating within applicable laws. All handling is the responsibility of the purchasing laboratory.
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