How Glutathione is Formed
The sulfhydryl (thiol) group (SH) of cysteine is responsible for the chemical properties of the whole GSH molecule (L-gamma-glutamyl-Lcysteinylglycine). As systemic availability of oral GSH is negligible in man and because there is no evidence for transport of GSH into cells, GSH has to be synthesized intracellularly. Though the inflow of cysteine, glutamate, and glycine (components of GSH) may prove somewhat limiting under selected circumstances, numerous observations have shown that cysteine tends to be the rate limiting event in GSH synthesis.
However, free cysteine does not represent an ideal delivery system: it is toxic and spontaneously oxidized.
On the other hand, cysteine present as cystine (two cysteines linked by a disulfide bond) released during digestion in the gastrointestinal tract is more stable than the free amino acid: the disulfide bond is pepsin- and trypsin-resistant, but may be split by heat and mechanical stress.
Thus, cystine travels safely in the body and is promptly reduced to the two cysteine molecules on cell entry.
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