Cysteine is a sulfur-containing amino acid. Cystine is its oxidised form, made when two cysteine units link through a sulfur-sulfur bond. The key fact comes first: cysteine is excluded from the AA-1 free panel. AA-1 covers 19 amino acids plus ammonia, and cysteine is not one of them. It requires a separate AA-3 workup, because its chemistry changes during handling in ways the other panel amino acids do not. Do not order AA-1 expecting a cysteine result it will not give.
Chemistry in plain terms
Cysteine carries a reactive sulfur group. In air and in solution, that group readily bonds with the same group on another cysteine, forming cystine. This is oxidation. Under reducing conditions it reverses, splitting cystine back into cysteine. A sample therefore holds a shifting balance between the two forms, depending on how it was taken, stored, and prepared.
A preparation set up for stable amino acids can transform cysteine before measurement unless it first locks the form in place. The exclusion is not lower priority; it is recognition that cysteine work needs different handling from the start. Cystine is also less soluble than many amino acids in plain aqueous conditions. Both points are stated qualitatively here: no solubility, recovery, or stability figure is given, because none was verified.
Where cysteine and cystine occur
Cysteine occurs in proteins, where cystine cross-links help hold protein shape. Hair, wool, and other keratin-rich materials are familiar examples. Cysteine is also part of glutathione, a molecule involved in normal cellular redox balance. It appears in foods, supplements, fermentation media, and cell culture media as a protein component, an added ingredient, or a breakdown product.
The useful question is rarely just whether cysteine is present. It is whether you need free cysteine, cystine, a combined total after a dedicated preparation, or protein cysteine after hydrolysis. Each is a different measurement. Name the form you need, in writing, before testing.
Free vs total for cysteine and cystine
A free measurement asks what is already unbound in the sample as received. A total measurement asks what hydrolysis releases from protein. For this analyte, neither fits the routine panels without qualification.
Free: AA-1 does not report cysteine. An AA-1 report with no cysteine line is not evidence that a sample contains no cysteine. Absence from the panel is not absence from the sample. If free cysteine or cystine is your question, ask about the separate AA-3 workup, and what forms it reports on what basis, before ordering.
Total: AA-T, at $230 per sample, uses acid hydrolysis before measurement. Harsh acid hydrolysis is known to affect sulfur amino acids, and cysteine and cystine need special handling for reliable total work. This page makes no recovery claim for cysteine under AA-T and does not present AA-T as a cysteine method. If total cysteine is your number, say so up front, so the lab can state in writing whether a suitable workup is available. No conversion or destruction percentage is given, because none was verified.
Method notes: AQC AccQ-Tag HPLC-UV
AA-1 uses AQC (AccQ-Tag) pre-column derivatization with HPLC-UV: amino acids are tagged, separated by high-performance liquid chromatography, and measured by ultraviolet detection against calibrated standards. That suits the 19 panel amino acids. Cysteine's oxidation to cystine is the kind of pre-measurement change a separate workup is designed to control, for example by fixing the form during preparation.
No AA-3 performance numbers are stated here: no detection limit, precision, or recovery figure. Ask for the reporting basis, forms reported, and documented caveats for your matrix before committing a sample. For complex matrices, matrix-effect qualification, checking distortion with a known spike in your sample, may be relevant and is priced on inquiry.
Which panel includes it
AA-1: cysteine is excluded. Panel: 19 amino acids plus ammonia by AQC HPLC-UV. Prices: $150 per sample for liquids, $180 for powders ($150 plus $30 powder preparation), $150 flat for first-time customers, $125 at 25 or more samples per month. None of those prices buys a cysteine result.
AA-T: $230 per sample. Do not assume it gives a reliable stand-alone cysteine figure without a written statement for your sample type.
AA-3: the separate cysteine and cystine workup. No AA-3 price appears in the approved price list, so none is stated. Ask for a quote naming the forms reported and preparation used.
Pilot: $250 for 5 samples, credited in full against a first contract signed within 90 days. Do not assume a pilot covers AA-3 unless confirmed in writing.
OpenChemWorks is validating now and reserving pilot slots. It is not GLP, GMP, or ISO/IEC 17025 accredited. No turnaround is promised here.
Frequently asked questions
Why is cysteine not on my AA-1 report?
It is excluded by design. Its oxidation to cystine needs a separate AA-3 preparation. A missing line means not measured, not zero.
Are cysteine and cystine the same?
No. Cystine is the oxidised pair of two linked cysteine units. A report should name which form, or which combined basis, it uses.
Can I add cysteine to AA-1?
No add-on price is published. AA-3 is a separate workup, quoted separately.
Does total analysis solve this?
Not automatically. Acid hydrolysis also affects cysteine and cystine. Ask how total cysteine would be handled for your matrix.
What should I write when I enquire?
Sample type, the form you need (free, total, cysteine, cystine, or combined), and what decision the number feeds. Email info@openchemworks.com.
References
- Waters — Derivatization of Amino Acids Using Waters AccQ-Tag Chemistry — Method background for AQC derivatization and HPLC-UV amino acid analysis.
- OpenChemWorks — Pricing — Published service and pricing copy (2026).
OpenChemWorks Laboratory · Reviewed by the laboratory director, PhD chemist · Published October 26, 2026