Glutathione is the cell’s main line of antioxidant defence, present at concentrations high enough to be measured in millimolar rather than the vanishing amounts most signalling molecules turn up in. Your cells make a great deal of it, continuously.
The whole story hangs off one sulphur atom.
Structure
Three amino acids: glutamate, cysteine and glycine. The unusual part is how the first two are joined, through a bond that ordinary peptide-cleaving enzymes cannot break, which is a large part of why the molecule survives long enough to be useful.
The cysteine carries a free thiol group, and that sulphur is where everything happens.
Reduced and oxidised
Two glutathione molecules can donate their thiol hydrogens and link to each other, forming the oxidised dimer. The ratio between reduced and oxidised glutathione is one of the standard readouts of a cell’s redox state, which makes this molecule a measuring instrument as much as a participant.
What we supply is the reduced form. That is what the certificate assay refers to.
Why solutions are prepared fresh
The same free thiol that makes glutathione useful makes it eager to oxidise on contact with air. A solution left standing will shift towards the oxidised form, and your reduced glutathione concentration is no longer what you calculated.
Which is why the standing advice is to prepare fresh rather than store, and to work reasonably briskly once the vial is open. Not a fragile compound, just an eager one.
Handling and solubility
Freely soluble in water, giving an acidic solution. White powder. Sealed vial at minus twenty, kept dry.
What to check on your certificate
The assay figure, which tells you the reduced glutathione content specifically. Appearance. Then the batch reference against the vial.
Regulatory position in the UK
Supplied as a laboratory reagent for in vitro research.