Leucine is one of the standard amino acids that make up proteins, and it also occurs on its own, unattached, in foods, fermentation samples, and cell culture media. A lab reports it as part of a wider profile, not in isolation. Leucine is one of the branched-chain amino acids, alongside isoleucine and valine. One method point matters here in particular: leucine and isoleucine are different compounds, and a method that reports them separately has to separate them. This page states that requirement without quoting a performance figure for it.
Chemistry in plain terms
Standard references classify leucine as an essential amino acid for humans, meaning it must come from food rather than being made by the body in sufficient amounts. Its side chain is a branched hydrocarbon group, which is why it is grouped with isoleucine and valine as a branched-chain amino acid. PubChem lists L-leucine as a standard proteinogenic amino acid, built into protein chains as well as present free in solution. Both forms, bound and free, are real, and they answer different questions.
Leucine and isoleucine have the same overall composition but a different arrangement: the branch sits in a different place. A panel therefore cannot treat them as one combined result if it claims to report each one. Separation is a requirement of the method, not an optional extra.
Where it occurs
Leucine occurs as a building block in proteins across foods, feeds, and biological materials. Free leucine also occurs in cell culture and spent media, fermentation liquids, protein hydrolysates, and food and supplement products, where a free profile describes the sample as it is.
This page gives no typical level for leucine in any material. Levels vary by recipe, organism, process, and sampling point; no verified figure is available. Measure your sample rather than borrowing a number.
Free vs total for leucine
A free leucine result measures leucine already dissolved and unattached in the sample as received, with no hydrolysis step. That is usually the relevant number for a question about the sample as used or as eaten: what is in the medium now, or what free profile a product presents.
A total result is measured after acid hydrolysis, the step that breaks protein chains apart and releases bound amino acids. It therefore includes free leucine plus leucine bound inside protein. This page gives no free-to-total ratio for any material, because that ratio is a property of the specific sample.
Leucine has no special hydrolysis conversion of the kind that affects asparagine and glutamine, whose amide groups are lost during acid hydrolysis and which are therefore reported together with aspartic acid and glutamic acid respectively. The general total-panel caution still applies: a hydrolysed measurement describes the sample after the method has acted on it, not a photograph of its native state.
Method notes: how leucine is measured here
At OpenChemWorks, leucine is measured as part of a panel using AQC pre-column derivatization, the Waters AccQ-Tag approach, with HPLC-UV detection. The amino acids are chemically tagged, separated on the HPLC column, and quantified against calibrated standards. AABA, short for alpha-aminobutyric acid, is added at a known amount as the internal standard. AABA is the standard, added by the lab; leucine is the analyte, measured from your sample. Different compounds, different roles.
For leucine, the two must be separated for each to be reported on its own. That is stated as a method requirement only. No resolution, recovery, or detection-limit figure is quoted, because no verified figure from this laboratory's validated method is available.
Which panel includes leucine
Leucine is included in AA-1, the free amino acid panel: 19 amino acids plus ammonia, measured by AQC HPLC-UV. Cysteine is excluded from AA-1 and needs a separate AA-3 workup. Leucine is also part of a total result under AA-T, where the sample is hydrolysed first and leucine is reported on that total basis.
Prices are only these: AA-1 costs $150 per sample for liquids, $180 per sample for powders ($150 plus $30 powder preparation), and $150 flat for first-time customers, with $125 per sample at 25 or more samples per month. AA-T, total amino acid analysis, costs $230 per sample. The pilot is $250 for 5 samples, credited in full against a first contract signed within 90 days.
OpenChemWorks, in Davis, California, is validating now and reserving pilot slots. The lab is not GLP, GMP, or ISO/IEC 17025 accredited, and no turnaround is promised here. Email info@openchemworks.com with your sample description and whether your question is about the sample as it is, or its protein as a whole.
Frequently asked questions
Is leucine the same as isoleucine?
No. Leucine and isoleucine are different compounds with the same overall composition arranged differently. A panel that reports each one has to separate them.
Is AABA the same as leucine?
No. AABA, alpha-aminobutyric acid, is the internal standard added by the lab for quantification. Leucine is a different compound, and it is the analyte measured from your sample.
Do I need free or total leucine?
Free answers a question about the sample as received. Total answers a question about protein composition after hydrolysis, and includes the free fraction.
Is leucine reported on its own?
At OpenChemWorks it is reported as part of the AA-1 free panel or the AA-T total panel, alongside the other panel analytes.
References
- PubChem — L-Leucine (CID 6106) — Identity reference for the amino acid named on this page.
- Waters — Derivatization of Amino Acids Using Waters AccQ-Tag Chemistry — Method background for AQC derivatization and HPLC-UV amino acid analysis.
- British Journal of Nutrition (2023) — Asparagine and glutamine lose an amide residue during acid hydrolysis and are measured with aspartate and glutamate respectively.
- OpenChemWorks — Pricing — Published service and pricing copy (2026).
OpenChemWorks Laboratory · Reviewed by the laboratory director, PhD chemist · Published October 29, 2026