Most free amino acids are stable enough that a result reflects the sample. A few are not: glutamine breaks down to pyroglutamate and ammonia, and tryptophan and cysteine oxidize. This reference covers what matters for each analyte we measure.
Most free amino acids are chemically stable in solution, but a few change on their own, and those changes show up in results. Glutamine is the main one: it breaks down to pyroglutamate and ammonia. This page covers what matters analytically for each analyte in AA-1 (19 amino acids plus ammonia), AA-2 (the extended panel) and AA-3 (cysteine/cystine, on request).
How the panel is measured
We derivatize free amino acids with AQC, separate them by reversed-phase HPLC on a C18 column and detect them by UV at 260 nm. AQC reacts with primary and secondary amino groups 1, so proline and hydroxyproline are measured alongside the primary amino acids. Ammonia also reacts and gives its own peak 14. The chemistry is on the AQC derivatization page.
Glutamine: cyclization to pyroglutamate
Free glutamine breaks down without enzymes to pyroglutamate (pyrrolidonecarboxylic acid) and ammonia 3,4. The rate depends on pH, temperature and the anions present 3. Phosphate and bicarbonate speed it up, and at a fixed phosphate concentration the rate rises with pH 3. Glutamine bound in a peptide is stable, which is why some media use alanyl- or glycyl-glutamine dipeptides 3.
In your results, degradation shows up as lower glutamine and higher ammonia 3. Glutamate does not rise, and pyroglutamate is not in our panel. Acid hydrolysis converts glutamine to glutamic acid 2, so total amino acid methods cannot report glutamine separately. See free versus total amino acids.
Asparagine: deamidation is mainly a peptide reaction
Asparagine deamidation matters most in peptides and proteins. There it proceeds through a succinimide intermediate to aspartyl and isoaspartyl products 6. A review of amino acids in cell culture notes that free asparagine and aspartate may not degrade in solution 4. Protein residues, by contrast, deamidate in a pH-dependent way 4. Acid hydrolysis converts asparagine to aspartic acid 2.
In culture, asparagine changes are mainly metabolic. Cells deaminate asparagine as well as glutamine, releasing ammonia 7. In one CHO batch culture, asparagine ran out almost simultaneously with glutamine and cystine 8.
Tryptophan, methionine and tyrosine
Tryptophan degrades in cell culture media at elevated temperature and under light, mostly through oxidation 9. Products include kynurenine and N-formylkynurenine, and tryptophan was the main contributor to media browning in that study 9. Acid hydrolysis destroys tryptophan outright 2.
Methionine can oxidize, notably during acid hydrolysis 2. Tyrosine has the lowest water solubility of the amino acids used in media, about 0.54 g per kg of water 4.
Cysteine and cystine: AA-3 on request
Cysteine oxidizes to cystine easily in vitro, and copper and iron accelerate the reaction 10. The cysteine/cystine balance in a sample can therefore shift before analysis. Published methods measure them as derivatives, after reduction and alkylation or after oxidation to cysteic acid 5. That is why cysteine/cystine is a separate service, AA-3, with a reduction–alkylation workup; see pricing.
Where ammonia comes from
Ammonia in a sample has several sources, and the profile alone cannot tell them apart. In mammalian cell culture it rises mainly from consumption of glutamine and other amino acids 11. Chemical breakdown of glutamine adds more 3. In fermentations, lactic acid bacteria that break down arginine by the arginine deiminase pathway release ammonia and ornithine 12. In grape juice, ammonium and primary amino acids are the major nitrogen sources for yeast 13.
The lab contributes too. An ammonia peak is always present in AQC derivatization, varying with ammonia in the water and air 14. Reagent blanks, described on our quality control page, exist partly for this reason. For a worked cell-culture example, see reading an amino acid profile and our cell culture media page.
The extended panel (AA-2)
AA-2 adds five amino compounds that matter in specific sample types.
- GABA is made from glutamate by glutamate decarboxylase. Many GABA-producing lactic acid bacteria have been isolated from cheese, yogurt, kimchi and fermented soy products 15.
- Ornithine comes from arginine. Arginase splits arginine into ornithine and urea, and the bacterial arginine deiminase pathway also yields ornithine 12. Some lactic acid bacteria decarboxylate ornithine to putrescine 16.
- Citrulline is formed from arginine by certain wine lactic acid bacteria. With urea, it is a precursor of ethyl carbamate 17, which makes it relevant to wine and brewing.
- Taurine (2-aminoethanesulfonic acid) comes mainly from meat, milk, dairy products and seafood; food plants contain little 18.
- Hydroxyproline occurs predominantly in collagen and is used as a marker of collagen content 19. Collagen methods hydrolyze the sample first 19; AA-2 measures free hydroxyproline only.
Quick reference
| Analyte | Abbr. | Panel | Why it matters analytically |
|---|---|---|---|
| Alanine | Ala | AA-1 | Nitrogen sink in cell culture 11; can switch from production to consumption 8 |
| Arginine | Arg | AA-1 | Source of urea, ornithine and citrulline in fermentations 12,17 |
| Asparagine | Asn | AA-1 | Becomes aspartic acid on acid hydrolysis 2; can be depleted in CHO culture 8 |
| Aspartate | Asp | AA-1 | Common lab contaminant 5; receives asparagine after hydrolysis 2 |
| Glutamate | Glu | AA-1 | Common lab contaminant 5; substrate for GABA 15 |
| Glutamine | Gln | AA-1 | Cyclizes to pyroglutamate and ammonia in solution 3; becomes glutamic acid on hydrolysis 2 |
| Glycine | Gly | AA-1 | Among the most common contaminants; dust raises it 2,5 |
| Histidine | His | AA-1 | Decarboxylated to histamine by some lactic acid bacteria 16 |
| Isoleucine | Ile | AA-1 | Same formula as leucine (C6H13NO2) 20,21, so chromatography must separate them |
| Leucine | Leu | AA-1 | Same formula as isoleucine 20,21; essential in human nutrition 22 |
| Lysine | Lys | AA-1 | Decarboxylated to cadaverine in some fermented foods 16 |
| Methionine | Met | AA-1 | Can oxidize, notably during acid hydrolysis 2 |
| Phenylalanine | Phe | AA-1 | Precursor of 2-phenylethylamine 16; essential in human nutrition 22 |
| Proline | Pro | AA-1 | Secondary amine that AQC derivatizes 1; barely used by yeast in winemaking 13 |
| Serine | Ser | AA-1 | Common contaminant; dust raises it 2,5; partly destroyed by acid hydrolysis 2 |
| Threonine | Thr | AA-1 | Partly destroyed by acid hydrolysis 2; essential in human nutrition 22 |
| Tryptophan | Trp | AA-1 | Degrades with heat, light and oxidation 9; destroyed by acid hydrolysis 2 |
| Tyrosine | Tyr | AA-1 | Lowest water solubility of media amino acids 4; precursor of tyramine 16 |
| Valine | Val | AA-1 | One of nine amino acids classed as essential in human nutrition 22 |
| Ammonia | NH3 | AA-1 | From glutamine breakdown, metabolism and lab background 3,11,14 |
| Cysteine/cystine | Cys | AA-3 | Oxidizes readily 10; measured after reduction–alkylation |
| γ-Aminobutyric acid | GABA | AA-2 | Made from glutamate by lactic acid bacteria in fermented foods 15 |
| Ornithine | Orn | AA-2 | Arginine breakdown product 12; precursor of putrescine 16 |
| Citrulline | Cit | AA-2 | Formed from arginine by wine lactic acid bacteria; ethyl carbamate precursor 17 |
| Taurine | Tau | AA-2 | Found in meat, dairy and seafood; scarce in food plants 18 |
| Hydroxyproline | Hyp | AA-2 | Collagen marker; mostly protein-bound, so free Hyp is only part of it 19 |
References
- Fast and Sensitive Quantification of AccQ-Tag Derivatized Amino Acids and Biogenic Amines by UHPLC-UV Analysis from Complex Biological Samples — A. Guba et al., Metabolites, 2022.
- 〈1052〉 Biotechnology-Derived Articles—Amino Acid Analysis — United States Pharmacopeia, harmonized text, official 2018.
- L-Glutamine in Cell Culture — MilliporeSigma, technical article.
- Amino acids in the cultivation of mammalian cells — A. Salazar, M. Keusgen and J. von Hagen, Amino Acids, 2016.
- UPLC Amino Acid Analysis Solution System Guide, Rev. B — Waters Corporation, 2007.
- Kinetics and Mechanisms of Deamidation and Covalent Amide-Linked Adduct Formation in Amorphous Lyophiles of a Model Asparagine-Containing Peptide — M.P. DeHart and B.D. Anderson, Pharmaceutical Research, 2012 (abstract).
- Responses of CHO-DHFR cells to ratio of asparagine to glutamine in feed media: cell growth, antibody production, metabolic waste, glutamate, and energy metabolism — L.-X. Zhang et al., Bioresources and Bioprocessing, 2016.
- mAb production kinetics in CHO batch culture: exploring extracellular and intracellular dynamics — A. Avilan Garzon et al., Frontiers in Bioengineering and Biotechnology, 2025.
- Degradation Products of Tryptophan in Cell Culture Media: Contribution to Color and Toxicity — A. Schnellbaecher et al., International Journal of Molecular Sciences, 2021.
- L-Cysteine in Cell Culture — MilliporeSigma, technical article.
- Transient ammonia stress on Chinese hamster ovary (CHO) cells yield alterations to alanine metabolism and IgG glycosylation profiles — B.F. Synoground et al., Biotechnology Journal, 2021 (author manuscript via NSF Public Access Repository).
- Ethyl Carbamate in Fermented Food Products: Sources of Appearance, Hazards and Methods for Reducing Its Content — M.Yu. Shalamitskiy et al., Foods, 2023.
- Yeast assimilable nitrogen — S.M. Weeks and P.A. Henschke, The Australian Wine Research Institute (from Australian & New Zealand Wine Industry Journal), 1999.
- What is the source of the NH3 peak in the AccQ•Tag derivatization reaction? — Waters Corporation, Knowledge Base.
- Production of Gamma-Aminobutyric Acid from Lactic Acid Bacteria: A Systematic Review — Y. Cui et al., International Journal of Molecular Sciences, 2020.
- Biogenic Amine Production by Lactic Acid Bacteria: A Review — F. Barbieri et al., Foods, 2019.
- Ethyl Carbamate Preventative Action Manual — U.S. Food and Drug Administration, 1997 (web page updated 2024).
- Taurine is absent from amino components in fruits of Opuntia ficus-indica — H.S.M. Ali, A.S. Al-Khalifa and H. Brückner, SpringerPlus, 2014.
- Determination of Hydroxyproline in Meat and Meat Products — MilliporeSigma, protocol.
- Leucine — National Institute of Standards and Technology, NIST Chemistry WebBook.
- L-Isoleucine — National Institute of Standards and Technology, NIST Chemistry WebBook.
- Amino acids — MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, reviewed 2025.
OpenChemWorks Laboratory · Reviewed by the laboratory director, PhD chemist · Published September 23, 2026