L-Carnitine

L-Carnitine, also called levocarnitine, is a small molecule of 161 daltons carrying both a permanently charged quaternary trimethylammonium group and a carboxylate. It is therefore a zwitterion across the entire practical pH range, with no neutral form available, and this dictates almost everything about how it behaves in solution and on a chromatography column.

It is biosynthesised from lysine and methionine and functions in the carnitine shuttle, the system that moves long-chain fatty acyl groups across the inner mitochondrial membrane. Carnitine palmitoyltransferase 1 forms the acylcarnitine on the outer membrane, a translocase exchanges it inward, and carnitine palmitoyltransferase 2 regenerates the acyl-CoA on the inner face.

A second function follows from the first. Because acyl groups can be parked reversibly on carnitine, the molecule acts as a buffer for the cellular acyl-CoA pool, and the resulting acylcarnitine profile is a widely used readout of fatty acid oxidation status in metabolic research.

Enzyme kinetics of the palmitoyltransferase system in isolated mitochondria constitute the classical literature, with the inhibition of the outer-membrane enzyme by malonyl-CoA the regulatory mechanism that connects fatty acid synthesis to fatty acid oxidation.

Acylcarnitine profiling by tandem mass spectrometry is now the most widespread application of this chemistry in research, since the pattern of chain lengths accumulating in a cell or tissue reports on where in beta-oxidation a block lies. Rodent models and cultured cell systems examining substrate handling make up the remainder of the literature.

The compound has no ultraviolet chromophore. It contains no aromatic ring and no conjugated system, so it is effectively invisible to the ultraviolet detector fitted to most HPLC systems, and a purity figure from an ultraviolet method has to be treated with care unless the method states what was actually detected. Practical analysis uses refractive index, evaporative light scattering or charged aerosol detection, mass spectrometry, or pre-column derivatisation to introduce a chromophore.

Enantiomeric purity is the specification that matters most for this compound. The D-enantiomer has an identical formula, an identical mass and an identical ultraviolet profile, and it is not merely inactive: it is reported to inhibit carnitine acyltransferases, so its presence has a direct effect rather than a diluting one. Detecting it requires chiral chromatography or optical rotation, neither of which appears on a routine purity analysis unless specifically requested.

Chromatographically, the permanent charge means poor retention on a standard reversed-phase column, so hydrophilic interaction or ion-pairing methods are the appropriate choice.