Supplementation with glutathione (GSH) is incapable of increasing cellular glutathione (GSH) since the glutathione (GSH) concentration found in the extracellular environment is much lower than that found intracellularly by about a thousand-fold. This large difference means that there is an insurmountable concentration gradient that prohibits extracellular glutathione entering cells and it is only inside the cell where glutathione performs its essential functions.

Gamma-Glutamylcysteine (GGC) is not subject to such a concentration gradient as it occurs in human plasma in the range of 1 – 5 µM ^{[2, 3]} and intracellularly at 5 – 10 µM ^[4]. The intracellular concentration of gamma-glutamylcysteine (GGC) is generally low allowing it (GGC) to diffuse into the cell. Once inside the cell it (GGC) rapidly bonds to glycine to form glutathione (GSH). This second and final reaction step in glutathione (GSH) biosynthesis is catalyzed by the activity of the ATP dependent glutathione synthase (GS) enzyme. Although currently unproven, gamma-glutamylcysteine (GGC) may be the pathway intermediate of glutathione transportation in multicellular organisms ^{[5, 6]}.

A human clinical study in healthy, non-fasting adults demonstrated that orally administered gamma-glutamylcysteine (GGC) can significantly increase lymphocyte glutathione (GSH) levels indicating systemic bioavailability, validating the therapeutic potential of gamma-glutamylcysteine (GGC) ^[7]. Gamma-glutamylcysteine (GGC) is also capable of being a powerful antioxidant in its own right as well ^[8-10].

Since the production of cellular gamma-glutamylcysteine (GGC) in humans slows down with age, as well as during the progression of many chronic diseases, it has been postulated that supplementation with gamma-glutamylcysteine (GGC) could offer health benefits. Other benefits of gamma-glutamylcysteine (GGC) supplementation may extend to situations where glutathione (GSH) has been acutely lowered below optimum such as following strenuous exercise, and during trauma or episodes of poisoning.

Several review articles have been published regarding the therapeutic potential of gamma-glutamylcysteine (GGC) to replenish glutathione (GSH) in age related ^[11] and chronic disease states such as Alzheimer’s disease ^[12].

As gamma-glutamylcysteine (GGC) has become commercially available several researchers have reported invitro, animal and human studies investigating a potential therapeutic role for gamma-glutamylcysteine (GGC) in both the reduction of oxidant stress-induced damage in tissues including the brain ^{[13, 14]} and as a treatment for sepsis ^[15].

References

1. Orlowski, M. and A. Meister, The gamma-glutamyl cycle: a possible transport system for amino acids. Proc Natl Acad Sci U S A, 1970. 67(3): p. 1248-55.
2. Meister, A. and M.E. Anderson, Glutathione. Annu Rev Biochem, 1983. 52: p. 711-60.
3. Anderson, M.E. and A. Meister, Transport and direct utilization of gamma-glutamylcyst(e)ine for glutathione synthesis. Proceedings of the National Academy of Sciences of the United States of America., 1983. 80(3): p. 707-11.
4. Mårtensson, J., Method for determination of free and total glutathione and γ-glutamylcysteine concentrations in human leukocytes and plasma. Journal of Chromatography B: Biomedical Sciences and Applications, 1987. 420(0): p. 152-157.
5. Wu, G., et al., Glutathione metabolism and its implications for health. Journal of Nutrition, 2004. 134(3): p. 489-92.
6. Stark, A.A., et al., The role of gamma-glutamyl transpeptidase in the biosynthesis of glutathione. Biofactors, 2003. 17(1-4): p. 139-49.
7. Zarka, M.H. and W.J. Bridge, Oral administration of γ-glutamylcysteine increases intracellular glutathione levels above homeostasis in a randomised human trial pilot study. Redox Biology, 2017. 11: p. 631-636.
8. Quintana-Cabrera, R. and J.P. Bolanos, Glutathione and gamma-glutamylcysteine in the antioxidant and survival functions of mitochondria. Biochemical Society Transactions, 2013. 41: p. 106-110.
9. Quintana-Cabrera, R., et al., γ-Glutamylcysteine detoxifies reactive oxygen species by acting as glutathione peroxidase-1 cofactor. Nat Commun, 2012. 3: p. 718.
10. Nakamura, Y.K., M.A. Dubick, and S.T. Omaye, γ-Glutamylcysteine inhibits oxidative stress in human endothelial cells. Life Sciences, 2011(0).
11. Ferguson, G. and W. Bridge, Glutamate cysteine ligase and the age-related decline in cellular glutathione: The therapeutic potential of γ-glutamylcysteine. Archives of Biochemistry and Biophysics, 2016. 593: p. 12-23.
12. Cao, P., et al., Therapeutic approaches to modulating glutathione levels as a pharmacological strategy in Alzheimer’s disease. Curr Alzheimer Res, 2015. 12(4): p. 298-313.
13. Le, T.M., et al., gamma-Glutamylcysteine ameliorates oxidative injury in neurons and astrocytes in vitro and increases brain glutathione in vivo. Neurotoxicology, 2011. 32(5): p. 518-25.
14. Braidy, N., et al., -glutamylcysteine (GGC)-mediated upregulation of glutathione levels can ameliorate toxicity of natural beta-amyloid oligomers in primary adult human neurons, in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association. 2013, Elsevier. p. P854.
15. Yang, Y., et al., γ-glutamylcysteine exhibits anti-inflammatory effects by increasing cellular glutathione level. Redox Biology, 2019. 20: p. 157-166.

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We have already discounted the most apparent strategy in one of our articles in which we discuss why taking glutathione itself will not increase its concentration inside the cell. So, let’s move on to the most often quoted myth that the amino acid cysteine is in limited supply in the body. We are aware that cysteine is one of the three building blocks that make up glutathione, but is there any evidence to suggest that we may be low on cysteine? And, regardless, would taking cysteine be effective in increasing cellular glutathione (GSH)? On initial observation, the principle behind the theory of cysteine deficiency being a cause of low glutathione (GSH) appears reasonably sound, but it is not that simple.

The first question is relatively easy to answer. The fact is that our diet usually contains plenty of cysteine and the other sulphur containing amino acid called methionine which can be easily converted into cysteine in the liver ^[1]. For example, the typical American diet supplies much more than the recommended required quantity of cysteine ^[2]. We can, therefore, rule out a cysteine deficiency. But would taking a cysteine supplement such as N-acetylcysteine (NAC) increase our cellular glutathione (GSH)? Unfortunately, it is not that easy, otherwise none of the chronic diseases attributed to low glutathione (GSH) would be so prevalent.

Cysteine, unlike most other amino acids, is extremely unstable and rapidly autoxidizes to cystine which is the oxidized disulphide form. It has exceedingly low solubility, and it will not be absorbed Cysteine, unlike most other amino acids, is extremely unstable and rapidly autoxidizes to cystine which is the oxidized disulphide form of cysteine. Cystine has exceedingly low solubility, and it will not be absorbed from the GI tract. Additionally, this cysteine autoxidation reaction, catalyzed by transition metal ions, generates oxygen free radicals and hydrogen peroxide. In high concentrations, this may result in cellular toxicity ^[3-6] and has the potential to be neurotoxic ^[7]. Our cells have adapted to this potential toxicity by storing cysteine in the form of glutathione (GSH) ^[8], which is far more stable to oxidation. We can therefore consider glutathione to be a safe storage for cysteine. It is important to note that consuming cysteine as part of our usual diet will never exceed the threshold to become toxic.

In summary, taking a cysteine supplement is of little use to increase glutathione (GSH) because our body tightly regulates both the storage and production of cysteine and any excess consumed is broken down into more stable byproducts. There is a notable exception which relates to acute glutathione (GSH) depletion due to acetaminophen (paracetamol) overdose as we shall see.

By far the most studied cysteine supplement is the cysteine prodrug N-Acetylcysteine (NAC). Several human clinical studies have determined the bioavailability of NAC. Orally delivered NAC undergoes extensive first-pass metabolism resulting in about 90% loss by enzymatic deacetylation to form cysteine in the small intestine ^[9]. As we have seen, this mainly gets converted to cystine and is of little use in healthy individuals or those suffering from a chronic undersupply of glutathione (GSH) due to aging or disease. Our notable exception is the observation in several studies that NAC is highly effective in elevating glutathione (GSH) under conditions where there has been a dramatic (acute depletion) drop in intracellular glutathione (GSH) levels, for instance as is the case in acetaminophen overdose. Here, the sharp decline in glutathione (GSH) levels, especially in the liver, to almost zero is effectively counteracted by NAC ^[10]. It immediately supplies available cysteine for repletion of glutathione and thus, recovery from toxicity. Unfortunately, this is where NAC has gained a false reputation as a go-to drug if low glutathione (GSH) is suspected. While immensely helpful indeed, it does not address our problem of supplementing glutathione (GSH) in cases of gradual depletion such as chronic illness or just simply getting older.

In contrast, diseases in which there is a prolonged and chronic decrease in glutathione (GSH) do not respond well to NAC treatment. An example of this is the situation that occurs in HIV/AIDS patients who experience a persistent drop in tissue glutathione (GSH) levels. In a clinical trial of AIDS patients were treated with 1.8 g/day of NAC for two weeks with the glutathione (GSH) status monitored in plasma and lymphocytes. During the treatment, no significant increase in glutathione (GSH) was observed ^[11]. Similar disappointing observations of HIV patients supplemented with NAC were also made by ^[12] and ^[13]. NAC has been tried in numerous chronic diseases with similarly disappointing results including cystic fibrosis protection against contrast-induced nephropathy and thrombosis ^[14].

The tight negative feedback control that glutathione (GSH) exerts on the first of two enzymes responsible for glutathione synthesis, GCL, can explain this phenomenon. This enzyme has the task of combining the amino acids glutamate and cysteine to form gamma-glutamylcysteine (GGC), which is used to produce glutathione (GSH) by the GS enzyme. As long as cellular glutathione is above a level considered to be adequate, which is called homeostasis, GCL is inhibited from making gamma-glutamylcysteine (GGC), no matter how much cysteine is available. However, when intracellular glutathione (GSH) is well below this homeostatic level, GCL is no longer inhibited and can actively utilize the cysteine supplied by NAC supplements. Several researchers have come to the same conclusion when trying to explain this fact ^[15]. Negative feedback controls exist as part of many of our body’s processes to tightly regulate certain functions, for example, our body temperature.

Supplementing with cysteine to increase cellular glutathione (GSH) is therefore of little use except in a few severe and limited cases mainly used in clinical settings.  The causes of glutathione (GSH) depletion in chronic diseases and how to effectively, rapidly and safely augment cellular glutathione (GSH) is now well understood.

References

1. Courtney-Martin, G., R.O. Ball, and P.B. Pencharz, Sulfur amino acid metabolism and requirements. Nutrition Reviews, 2012. 70(3): p. 170-175.
2. Lang, C.A., The impact of glutathione on health and longevity. Journal of Anti Aging Medicine, 2001. 4(2): p. 137-144.
3. Nath, K.A. and A.K. Salahudeen, Autoxidation of cysteine generates hydrogen peroxide: cytotoxicity and attenuation by pyruvate. American Journal of Physiology, 1993. 264(2): p. F306-F314.
4. Harman, L.S., C. Mottley, and R.P. Mason, Free radical metabolites of L-cysteine oxidation. Journal of Biological Chemistry, 1984. 259(9): p. 5606-11.
5. Viña, J., et al., The effect of cysteine oxidation on isolated hepatocytes. Biochem. J., 1983. 212(1): p. 39-44.
6. Wang, X.F. and M.S. Cynader, Pyruvate Released by Astrocytes Protects Neurons from Copper-Catalyzed Cysteine Neurotoxicity. The Journal of Neuroscience, 2001. 21(10): p. 3322-3331.
7. Janáky, R., et al., Mechanisms of L-Cysteine Neurotoxicity. Neurochemical Research, 2000. 25(9): p. 1397-1405.
8. Aoyama, K., M. Watabe, and T. Nakaki, Regulation of Neuronal Glutathione Synthesis. Journal of Pharmacological Sciences, 2008. 108(3): p. 227-238.
9. Olsson, B., et al., Pharmacokinetics and bioavailability of reduced and oxidized N-acetylcysteine. European Journal of Clinical Pharmacology, 1988. 34(1): p. 77-82.
10. Yang, R.K., et al., Prolonged treatment with N-acetylcystine delays liver recovery from acetaminophen hepatotoxicity. Critical Care, 2009. 13(2).
11. Witschi, A., et al., Supplementation of N-acetylcysteine fails to increase glutathione in lymphocytes and plasma of patients with AIDS. AIDS Research & Human Retroviruses, 1995. 11(1): p. 141-3.
12. Akerlund, B., et al., Effect of N-acetylcysteine(NAC) treatment on HIV-1 infection: a double-blind placebo-controlled trial. European Journal of Clinical Pharmacology., 1996. 50(6): p. 457-61.
13. Nakamura, H., H. Masutani, and J. Yodoi, Redox imbalance and its control in HIV infection. Antioxidants & Redox Signaling, 2002. 4(3): p. 455-64.
14. Rushworth, G.F. and I.L. Megson, Existing and potential therapeutic uses for N-acetylcysteine: The need for conversion to intracellular glutathione for antioxidant benefits. Pharmacology & Therapeutics, 2014. 141(2): p. 150-159.
15. Nielsen, H.B., et al., N-acetylcysteine does not affect the lymphocyte proliferation and natural killer cell activity responses to exercise. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, 1998. 275(4): p. R1227-R1231.

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Gamma-glutamylcysteine (GGC) has the advantage of being able to diffuse into cells where it is effectively converted to glutathione (GSH) by an enzymatic reaction which simply adds the amino acid glycine. Unlike cysteine prodrugs, such as NAC, which only increase cellular glutathione (GSH) following severe depletion such as in acetaminophen (paracetamol) overdose, gamma-glutamylcysteine (GGC) does so without limitations. This ability to increase glutathione (GSH) regardless of the current state of cellular glutathione (GSH) makes gamma-glutamylcysteine (GGC) unique in the vast supplement market touting effective ways to do so. The body’s biochemistry clearly indicates and research confirms that consuming gamma-glutamylcysteine (GGC) is the only way to ensure healthy glutathione (GSH) levels for all individuals including those suffering from chronic disease and inflammation.

Reference

1. Zarka, M.H. and W.J. Bridge, Oral administration of γ-glutamylcysteine increases intracellular glutathione levels above homeostasis in a randomised human trial pilot study. Redox Biology, 2017. 11: p. 631-636.

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Supplementation with glutathione will not increase cellular glutathione (GSH) for one simple reason. In the tissues of your body there are two very different environments. One is the fluid inside your cells (intracellular, about 70% of the total fluids) and the other is the fluid outside your cells (extracellular, about 30% of total fluids).

The intracellular environment, which is bound by your cellular membranes, is where most of the essential reactions such as protein synthesis and energy production occur.  Many of these reactions generate free radicals that can, if not controlled (neutralized), cause damage inside your body (oxidative stress). To counter this, your cellular glutathione (GSH) levels must be maintained at an optimal concentration (homeostasis) and in a tightly controlled (regulated) manner.

The extracellular environment, such as the plasma of your blood, allows the transportation of nutrients to the cells and removal of waste products which in turn are processed in the liver and kidneys. The glutathione (GSH) concentration found in this environment (micromolar) is much lower than that found intracellularly (millimolar) by about a thousand-fold. This large concentration difference means that there is an insurmountable concentration gradient that prohibits extracellular glutathione (GSH) entering cells.

So, when you take a glutathione supplement orally or by injection, it will ‘hang around’ in the extracellular environment but will not be able to enter cells to combat oxidative stress.

But what happens to all of that extracellular glutathione (GSH)?  Well, glutathione (GSH) is made up of some valuable amino acids and cells have a system of scavenging them before they are recycled. Most cells in the body have an outer membrane bound enzyme (gamma-glutamyltransferase)^[1] which starts this process by breaking glutathione (GSH) down to its constituent amino acids (protein building blocks).

Several researchers have confirmed this thesis. For example a group of researchers^[2] concluded that “dietary glutathione (GSH) is not a major determinant of circulating glutathione (GSH), and it is not possible to increase circulating glutathione (GSH) to a clinically beneficial extent by the oral administration of a single dose of 3g of glutathione (GSH)”. Similarly, authors [3] of the first double-blind, randomized, placebo-controlled clinical trial of oral glutathione (GSH) supplementation performed in healthy adult humans concluded that “despite the biochemical plausibility, optimal dose, recommended timing of administration and appropriate choice of outcome measures, no significant changes were observed in oxidative stress biomarkers or erythrocyte glutathione (GSH) concentrations following 4 weeks of oral (2 x 500mg daily) glutathione (GSH) supplementation”.

Gamma-glutamylcysteine (GGC), on the other hand, does not have this concentration gradient problem. It is found in roughly the same low concentration both intracellularly and extracellularly. Once ingested however, the extracellular gamma-glutamylcysteine (GGC) concentration increases and therefore it can easily diffuse through the cell membrane to the inside of cells. Once inside, it is immediately used to produce glutathione (GSH) ^[4].

In summary, the problem in aging and most chronic disease is that our cells lose the ability to make enough gamma-glutamylcysteine (GGC) which results in insufficient glutathione (GSH) production. Glutathione supplements cannot enter cells as there is an insurmountable concentration gradient across the cell membrane and, although there are well understood membrane transporters for transporting glutathione from the inside to the outside of the cell, there are no known transporters for transporting extracellular glutathione to inside the cell. However, Gamma-glutamylcysteine (GGC) does get transported into the cell ^{[5, 6]}.

References

1. Franco, R., et al., The central role of glutathione in the pathophysiology of human diseases. Archives Of Physiology And Biochemistry, 2007. 113(4-5): p. 234-258.
2. Witschi, A., et al., The systemic availability of oral glutathione. European Journal of Clinical Pharmacology, 1992. 43(6): p. 667-669.
3. Allen, J. and R.D. Bradley, Effects of oral glutathione supplementation on systemic oxidative stress biomarkers in human volunteers. J Altern Complement Med, 2011. 17(9): p. 827-33.
4. Wu, G., et al., Glutathione metabolism and its implications for health. Journal of Nutrition, 2004. 134(3): p. 489-92.
5. Zarka, M.H. and W.J. Bridge, Oral administration of γ-glutamylcysteine increases intracellular glutathione levels above homeostasis in a randomised human trial pilot study. Redox Biology, 2017. 11: p. 631-636.
6. Le, T.M., et al., gamma-Glutamylcysteine ameliorates oxidative injury in neurons and astrocytes in vitro and increases brain glutathione in vivo. Neurotoxicology, 2011. 32(5): p. 518-25.

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Chronic inflammation fundamentally means a constant excess of free radicals or reactive oxygen species (ROS) inside the cells. While these ROS continue to exist inside cells, the damage continues resulting in chronic disease. Maintenance of healthy levels of antioxidants is essential to reduce the risk of being exposed to excess free radicals. By far, the most abundant antioxidant produced by cells is glutathione (GSH).

Apart from the aging process and chronic disease, glutathione (GSH) also plays an important role in the health of the body’s immune system, resistance to cancer and infectious diseases as well as in sports recovery.

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