**3.4. Reduced glutathione**

Glutathione is present in two forms in the body; in a "reduced" or an "oxidized" form. The reduced form or "L" type glutathione is the most active form and is found in healthy cells. Reduced glutathione is responsible for all vital biological activity /function of glutathione in the body. In normal healthy cells the oxidized glutathione is quickly recycled back to its active reduced state and the majority of glutathione in the body is present in its reduced form. Reduced glutathione is a tripeptide composed of the amino acids glutamine, cysteine, and glycine (gamma-glutamylcysteinylglycine, GSH).

*3.4.2. The role of GSH-enzymes in the metabolism of arachidonic acid*

of some PGs, particularly PGEs [114].

thione depletion.

insufficient in ASD [11].

activated glutaminase.

**3.5. Gluten vs. Glutamate (glutamic acid) vs. Glutamine**

The tissue content of GSH is normally very high, in some tissues concentration up to 5 mM is found. The function of GSH is often tissue protective, and GSH plays a central role as a cofactor in numerous enzyme reactions. GSH-peroxidase (GSH-Px) is one of the GSH-enzymes located in the circulation almost exclusively in the red cells, various GSH-transferases that have peroxidise-like activity and bind chemicals, and γ-glutamyl transferase that reflects the liver function and is involved in the transport of amino acids across the cell membrane. GSH is also consumed by some cytochromes, most notably cytochrome P-450. Several steps in the metabolism of arachidonic acid may be normally regulated by GSH-enzymes [113]. An early observation was that GSH may function as a chemical cofactor or coenzyme in the formation

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The findings of several studies indicate that alterations of cellular methylation capacity, antioxidant defence, and oxidative stress contribute to the pathophysiology of autism. An imbalance in intracellular levels of GSH and GSSG could provide a biochemical explanation for multisystem issues, such as increased frequency of infections, gastrointestinal pathology, impaired detoxification and neurologic pathology, associated with both autism and gluta‐

The abnormal metabolite levels in pathways of methionine, folate, and glutathione metabolism observed in autism may reflect subtle changes in gene products that regulate activity in these pathways [37, 110]. Even small variations in the gene expression and enzyme activity, when expressed chronically, could have a significant impact on downstream metabolism. Many autistic children have been shown to exhibit a threefold reduction in the ratio of "active" GSH to "inactive" glutathione (GSSSSG). Cysteine, another substance needed for GSH synthesis, was also significantly reduced, suggesting that the building blocks for GSH synthesis are

Glutamic acid is the major excitatory neurotransmitter that increases the firing of neurons in the central nervous system. It is converted into glutamine and in GABAergic neurons to GABA, an inhibitory neurotransmitter. Glutamic acid and its neurologically inactive sibling, gluta‐ mine, are amino acids. Glutamic acid is found in most foods but it is particularly abundant in gluten grains (wheat, barley, rye), soy/legumes/peanuts, dairy products, nuts, seeds, meats and the gluten-grain substitutes (quinoa, amaranth, tapioca as well as the non-gluten grains millet, flax and sorghum). Cells lining the intestinal tract convert glutamate to glutamine, which in turn is used by the villi to maintain the health and integrity. The conversion of glutamate to glutamine happens also in liver and kidneys, which is fortuitous because many food substances that are rich in glutamic acid – namely gluten grains, casein (from dairy), and soy – are three of the four food substances that damage the villi and their ability to make the conversion. Glutamine is converted to glutamate by a mitochondrial enzyme, the phosphate-

GSH is synthesized sequentially by glutamate-cysteine ligase (GCL) and GSH synthase (GS) [106]. The cystine/glutamate antiporter controls the biosynthesis of GSH by transporting cystine, the rate-limiting precursor of GSH synthesis, into the cell in exchange for glutamate [107]. Methylmercury (MeHg) administration induces oxidative stress in cortex which could be antagonized by riluzole induced GSH synthesis through activation of glutamate transport‐ ers (GluTs) [108].

GSH is the major cellular antioxidant and plays an important role in the protection of cells against damage from free radicals and other electrophils and also influences cellular radio‐ sensitivity, cellular response to hyperthermia, and cytotoxicity to some kinds of chemothera‐ peutic agents. It protects the body against the damage caused by exposure to toxins and is a powerful detoxifier of heavy metals. Early studies showed the role of GSH in inflammation [18]. GSH delivery to the central nervous system (CNS) is limited due to its poor stability and low bioavailability.

#### *3.4.1. Application/role of reduced glutathione (GSH) in autism*

The impact of glutathione in autism has been described [109]. Children with autism have been shown to have low plasma levels of metabolites in the pathway of glutathione redox metab‐ olism [110] suggesting that children with autism have a more oxidized extracellular GSSG. If dietary GSH is insufficient, oxidative stress, toxicity and cell damage may occur to mucosal cells in the small intestine. The elimination of fat-soluble compounds, especially heavy metals like mercury and lead are dependent upon adequate levels of glutathione.

In autism the methylation cycle was found to be blocked at methionine synthase, which is the step of homocysteine methylation for formation of methionine [11, 111]. A significant decrease in the level of plasma methionine and lowering of the ratio of S-adenosylmethionine to Sadenosylhomocysteine are two effects of this blocking. The latter change results in a decreased capacity to promote methylation reactions. In addition, the flow through the transsulfuration pathway was also reduced leading to lower plasma levels of cysteine and glutathione and a lowered ratio of reduced to oxidized glutathione. The lowered ratio of reduced to oxidized glutathione reflects a state of oxidative stress [112]. The block in the methylation cycle and alterations of glutathione were found to be linked, since supplements used to restore the normal function of the methylation cycle (methylcobalamin, folinic acid and trimethylglycine) also restored the levels of reduced and oxidized glutathione [111].

### *3.4.2. The role of GSH-enzymes in the metabolism of arachidonic acid*

**3.4. Reduced glutathione**

ers (GluTs) [108].

low bioavailability.

glycine (gamma-glutamylcysteinylglycine, GSH).

106 Pharmacology and Nutritional Intervention in the Treatment of Disease

*3.4.1. Application/role of reduced glutathione (GSH) in autism*

Glutathione is present in two forms in the body; in a "reduced" or an "oxidized" form. The reduced form or "L" type glutathione is the most active form and is found in healthy cells. Reduced glutathione is responsible for all vital biological activity /function of glutathione in the body. In normal healthy cells the oxidized glutathione is quickly recycled back to its active reduced state and the majority of glutathione in the body is present in its reduced form. Reduced glutathione is a tripeptide composed of the amino acids glutamine, cysteine, and

GSH is synthesized sequentially by glutamate-cysteine ligase (GCL) and GSH synthase (GS) [106]. The cystine/glutamate antiporter controls the biosynthesis of GSH by transporting cystine, the rate-limiting precursor of GSH synthesis, into the cell in exchange for glutamate [107]. Methylmercury (MeHg) administration induces oxidative stress in cortex which could be antagonized by riluzole induced GSH synthesis through activation of glutamate transport‐

GSH is the major cellular antioxidant and plays an important role in the protection of cells against damage from free radicals and other electrophils and also influences cellular radio‐ sensitivity, cellular response to hyperthermia, and cytotoxicity to some kinds of chemothera‐ peutic agents. It protects the body against the damage caused by exposure to toxins and is a powerful detoxifier of heavy metals. Early studies showed the role of GSH in inflammation [18]. GSH delivery to the central nervous system (CNS) is limited due to its poor stability and

The impact of glutathione in autism has been described [109]. Children with autism have been shown to have low plasma levels of metabolites in the pathway of glutathione redox metab‐ olism [110] suggesting that children with autism have a more oxidized extracellular GSSG. If dietary GSH is insufficient, oxidative stress, toxicity and cell damage may occur to mucosal cells in the small intestine. The elimination of fat-soluble compounds, especially heavy metals

In autism the methylation cycle was found to be blocked at methionine synthase, which is the step of homocysteine methylation for formation of methionine [11, 111]. A significant decrease in the level of plasma methionine and lowering of the ratio of S-adenosylmethionine to Sadenosylhomocysteine are two effects of this blocking. The latter change results in a decreased capacity to promote methylation reactions. In addition, the flow through the transsulfuration pathway was also reduced leading to lower plasma levels of cysteine and glutathione and a lowered ratio of reduced to oxidized glutathione. The lowered ratio of reduced to oxidized glutathione reflects a state of oxidative stress [112]. The block in the methylation cycle and alterations of glutathione were found to be linked, since supplements used to restore the normal function of the methylation cycle (methylcobalamin, folinic acid and trimethylglycine)

like mercury and lead are dependent upon adequate levels of glutathione.

also restored the levels of reduced and oxidized glutathione [111].

The tissue content of GSH is normally very high, in some tissues concentration up to 5 mM is found. The function of GSH is often tissue protective, and GSH plays a central role as a cofactor in numerous enzyme reactions. GSH-peroxidase (GSH-Px) is one of the GSH-enzymes located in the circulation almost exclusively in the red cells, various GSH-transferases that have peroxidise-like activity and bind chemicals, and γ-glutamyl transferase that reflects the liver function and is involved in the transport of amino acids across the cell membrane. GSH is also consumed by some cytochromes, most notably cytochrome P-450. Several steps in the metabolism of arachidonic acid may be normally regulated by GSH-enzymes [113]. An early observation was that GSH may function as a chemical cofactor or coenzyme in the formation of some PGs, particularly PGEs [114].

The findings of several studies indicate that alterations of cellular methylation capacity, antioxidant defence, and oxidative stress contribute to the pathophysiology of autism. An imbalance in intracellular levels of GSH and GSSG could provide a biochemical explanation for multisystem issues, such as increased frequency of infections, gastrointestinal pathology, impaired detoxification and neurologic pathology, associated with both autism and gluta‐ thione depletion.

The abnormal metabolite levels in pathways of methionine, folate, and glutathione metabolism observed in autism may reflect subtle changes in gene products that regulate activity in these pathways [37, 110]. Even small variations in the gene expression and enzyme activity, when expressed chronically, could have a significant impact on downstream metabolism. Many autistic children have been shown to exhibit a threefold reduction in the ratio of "active" GSH to "inactive" glutathione (GSSSSG). Cysteine, another substance needed for GSH synthesis, was also significantly reduced, suggesting that the building blocks for GSH synthesis are insufficient in ASD [11].

#### **3.5. Gluten vs. Glutamate (glutamic acid) vs. Glutamine**

Glutamic acid is the major excitatory neurotransmitter that increases the firing of neurons in the central nervous system. It is converted into glutamine and in GABAergic neurons to GABA, an inhibitory neurotransmitter. Glutamic acid and its neurologically inactive sibling, gluta‐ mine, are amino acids. Glutamic acid is found in most foods but it is particularly abundant in gluten grains (wheat, barley, rye), soy/legumes/peanuts, dairy products, nuts, seeds, meats and the gluten-grain substitutes (quinoa, amaranth, tapioca as well as the non-gluten grains millet, flax and sorghum). Cells lining the intestinal tract convert glutamate to glutamine, which in turn is used by the villi to maintain the health and integrity. The conversion of glutamate to glutamine happens also in liver and kidneys, which is fortuitous because many food substances that are rich in glutamic acid – namely gluten grains, casein (from dairy), and soy – are three of the four food substances that damage the villi and their ability to make the conversion. Glutamine is converted to glutamate by a mitochondrial enzyme, the phosphateactivated glutaminase.

#### **3.6. N-Acetyl-L-Cysteine and glutathione**

N-Acetyl-L-Cysteine (NAC) is an antioxidant that helps increase glutathione synthesis which, in turn, helps the body defends against harmful toxins. NAC is the acetyl derivative of L-cysteine. While L-cysteine plays important metabolic roles as a key antioxidant, a gluta‐ thione precursor and a natural source of sulfur for metabolism, it is unstable and can be‐ come degraded during absorption. NAC on the other hand, is more stable than L-cysteine. Taken orally, NAC converts into L-cysteine after being absorbed, and raises blood and tis‐ sue cysteine levels.

**Author details**

**References**

Maija L. Castrén1,2, Tuomas Westermarck3

Central Hospital, Helsinki, Finland

3 Rinnekoti Foundation, Espoo, Finland

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and Faik Atroshi4

http://dx.doi.org/10.5772/57512

109

Oxidative Stress and Dietary Interventions in Autism: Exploring the Role of Zinc, Antioxidant Enzymes and Other…

2 Division of Child Neurology, Hospital for Children and Adolescents, Helsinki University

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1 Institute of Biomedicine/Physiology, University of Helsinki, Helsinki, Finland

4 Department of Pharmacology and Toxicology, University of Helsinki, Finland

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