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Glutathione (GSH)

Cystine is the preferred precursor for glutathione synthesis

Glutathione (GSH) is a tripeptide - a chain of three amino acids - cysteine, glycine, and glutamic acid - found in human cell tissue.

Cysteine is potentially toxic and catabolized in the bodyGlutathione exists in the reduced form (GSH), and maintains a balance with its oxidized form (GSSG) - a disulfide. GSH is our body's Master Antioxidant.

Cysteine - as a free amino acid - is potentially toxic and is spontaneously catabolized or destroyed in the gastrointestinal tract and blood plasma.

Hence it does not represent an ideal delivery system to the cell.

However, when it is present as a cysteine=cysteine dipeptide - called cystine - (two cysteine molecules linked by a disulfide bond) it is more stable than the free amino acid cysteine.

Cystine is the preferred precursor for glutathione

The di-peptide, cystine, travels safely through the digestive tract and blood and is promptly broken up or reduced to the two cysteine molecules when it enters the cell.

How is Glutathione manufactured?

Intracellular synthesis of glutathione with cysteine and glutamateGlutathione is synthesized inside your cells from the three amino acid precursors, L-glutamate, L-cysteine, and L- glycine.

Glutathione synthesis is a two-step process involving the enzymes gamma-glutamylcysteine synthetase and GSH synthetase.

The first reaction is the rate-limiting step and is effectively inhibited by GSH feedback.

This means the reaction is controlled by its own end-product and slows down once enough glutathione is synthesised. The second reaction is not subject to negative feedback by GSH.

When GSH is consumed or used up in the neutralisation of harmful oxyradicals, this feedback inhibition is lost.

So now, the availability of L-cysteine as a precursor determines how much glutathione is synthesised by the cell.

The amount of cysteine available can hence become the rate-limiting factor for the manufacture of glutathione.

If cysteine levels drop, the body will convert methionine, another amino acid, to cysteine, but then other systems run out of methionine, which is needed for making protein.

Macrophages (immune "scavenger" cells) and Astrocytes (a category of brain cells that influence the activity of neurons) prefer Cystine for manufacturing glutathione, while Lymphocytes and Neurons prefer cysteine.

Optimizing glutathione levels in macrophages and astrocytes with cystine allows these cells to provide cysteine to lymphocytes and neurons directly upon demand.


The systemic availability of oral glutathione
Witschi A, Reddy S, Stofer B, Lauterburg BH. [Eur J Clin Pharmacol. 1992;43(6):667-9.]
Because of hydrolysis of glutathione by intestinal and hepatic gamma-glutamyltransferase, dietary glutathione is not a major determinant of circulating glutathione, and it is not possible to increase circulating glutathione to a clinically beneficial extent by the oral administration of a single dose of glutathione.

Also see:

From Cell to Super-Cell - with Glutathione

Glutathione (GSH) - Master Antioxidant and Cellular Detoxifier

Glutathione (GSH) & the Immune System - Your Lifeline to Health

Un-denatured Whey Protein - The Best Way to Raise Glutathione Levels

Diagram of Glutathione synthesis Courtesy of Michael W. King, Professor IU School of Medicine and IU Center for Regenerative Biology and Medicine, Terre Haute, IN. 47809

Diagram of Glutathione disulfide Courtesy of Michael W. King, Professor IU School of Medicine and IU Center for Regenerative Biology and Medicine, Terre Haute, IN. 47809

Diagrams of Cysteine and Cystine molecules courtesy of Cystinuria.org


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