What Is L-Methionine-¹³C₅,¹⁵N?

L-Methionine-¹³C₅,¹⁵N is a stable, non-radioactive isotopologue of the essential sulfur-containing amino acid L-methionine. In this compound, all five carbon positions (¹³C₅) and the alpha-amino nitrogen (¹⁵N) carry heavy isotopes, resulting in a molecule with a nominal mass increase of +6 Da compared to the natural abundance form. Because ¹³C and ¹⁵N are stable and non-radioactive, this compound can be handled under standard laboratory conditions without specialist radiation safety infrastructure.

Methionine occupies a central position in cellular biochemistry. It is the universal initiator of protein synthesis, a precursor to S-adenosylmethionine (SAM)—the principal biological methyl donor—and a key node in the transsulfuration pathway leading to cysteine and glutathione biosynthesis. Full labeling at both carbon and nitrogen positions maximizes the analytical discrimination between labeled and unlabeled pools, enabling precise isotope ratio measurements by mass spectrometry.

Key Applications in Research

Metabolic Flux Analysis (MFA) and Stable Isotope-Resolved Metabolomics (SIRM): By supplying cells or organisms with L-Methionine-¹³C₅,¹⁵N as the sole methionine source, researchers can track the incorporation of label into downstream metabolites including SAM, S-adenosylhomocysteine (SAH), homocysteine, cystathionine, cysteine, and glutathione. The distinct +6 Da mass shift allows clear separation of labeled species from background in LC-MS experiments, enabling quantitative mapping of one-carbon and transsulfuration pathway fluxes.

SILAC-Based Quantitative Proteomics: Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC) relies on the metabolic incorporation of heavy amino acids into newly synthesized proteins. L-Methionine-¹³C₅,¹⁵N can serve as the heavy methionine channel in SILAC workflows, enabling accurate relative and absolute quantification of protein expression changes between experimental conditions by comparing heavy-to-light peptide ratios in MS data.

Epigenetics and Methylation Research: Because methionine is the obligate precursor of SAM—the methyl group donor for DNA, RNA, and histone methyltransferases—isotopically labeled methionine enables researchers to measure the dynamics of cellular methylation reactions. Incorporation of ¹³C and ¹⁵N from labeled methionine into the methyl groups of methylated nucleotides or amino acid residues can be detected by high-resolution mass spectrometry, providing mechanistic insights into epigenetic regulation.

Experimental Considerations and Best Practices

Cell Culture Labeling: For complete isotopic incorporation in cell culture experiments, methionine-free base medium should be used, supplemented exclusively with L-Methionine-¹³C₅,¹⁵N at the physiologically relevant concentration (typically 100–200 µM for mammalian cell culture, though this should be confirmed for the specific cell line and experimental design). Labeling duration sufficient for steady-state isotopic enrichment is typically several doubling times for SILAC, or shorter pulse periods for flux experiments.

Sample Preparation and MS Analysis: Quench metabolism rapidly upon harvest (e.g., with cold methanol/water mixtures or liquid nitrogen) to preserve isotopic labeling patterns. For metabolomics, protein precipitation followed by LC-MS analysis on a high-resolution instrument (e.g., Orbitrap or Q-TOF) is recommended. Isotopologue distribution analysis software should be used to correct for natural isotope abundance contributions and to extract metabolic flux information.

Storage and Handling: Store lyophilized or solid L-Methionine-¹³C₅,¹⁵N at –20 °C, protected from moisture and light. Prepare stock solutions in water or dilute HCl; avoid repeated freeze-thaw cycles to preserve isotopic and chemical integrity. Confirm purity and isotopic enrichment by NMR or LC-MS before use in quantitative experiments.

Comparison with Related Isotope Standards

Several methionine isotopologue standards exist, each suited to different experimental objectives. Singly labeled variants such as L-Methionine-¹³C₅ (carbon-only label, +5 Da) or L-Methionine-¹⁵N (nitrogen-only label, +1 Da) offer narrower mass shifts and may be preferred when multiplexing with other labeled amino acids. The dual-labeled L-Methionine-¹³C₅,¹⁵N provides the largest mass shift (+6 Da) among common methionine isotopologues, maximizing separation from the M+0 through M+5 natural isotope envelope and reducing spectral overlap in complex biological matrices.

For researchers requiring internal quantification standards rather than metabolic tracers, deuterium-labeled methionine analogs (e.g., L-Methionine-d3) are sometimes used; however, deuterium labels can introduce chromatographic retention time shifts relative to the unlabeled compound, a limitation that does not apply to ¹³C/¹⁵N labels. This makes L-Methionine-¹³C₅,¹⁵N the preferred choice for LC-MS-based quantitative workflows where co-elution of labeled and unlabeled species is required.