4000 Current Medicinal Chemistry, 2012, 19, 4000-4005

1875-533X/12 $58.00+.00 © 2012 Bentham Science Publishers

Glutathione-Related Factors and Oxidative Stress in Autism, A Review

A. Ghanizadeh*,1,2

, S. Akhondzadeh3, M. Hormozi

4,1, A. Makarem

5,1, M. Abotorabi-Zarchi

6 and

A. Firoozabadi1,2

1,2Research Center for Psychiatry and Behavioral Sciences;

2Department of Psychiatry, Shiraz University of Medical Sciences, School

of Medicine, Shiraz, Iran; 3Psychiatric Research Center, Roozbeh Psychiatric Hospital, Tehran University of Medical Sciences, South

Kargar Street, Tehran, 13337, Iran; 4Student Research Committee, Fasa University of Medical Sciences, Fasa, Iran;

5Student

Research Committee, Jahrom University of Medical Sciences, Jahrom, Iran; 6Department of Neurology, School of Medicine, Kerman

University of Medical sciences, Iran

Abstract: Autism spectrum disorders are complex neuro-developmental disorders whose neurobiology is proposed to be associated

with oxidative stress which is induced by reactive oxygen species. The process of oxidative stress can be a target for therapeutic

interventions. In this study, we aimed to review the role of oxidative stress, plasma glutathione (GSH), and related factors as the potential

sources of damage to the brain as well as the possible related factors which reduce the oxidative stress. Methylation capacity, sulfates

level, and the total glutathione level are decreased in autism. On the other hand, both oxidized glutathione and the ratio of oxidized to

reduced glutathione are increased in autism. In addition, the activity of glutathione peroxidase, superoxide dismutase, and catalase, as a

part of the antioxidative stress system are decreased. The current literature suggests an imbalance of oxidative and anti-oxidative stress

systems in autism. Glutathione is involved in neuro-protection against oxidative stress and neuro-inflammation in autism by improving

the anti-oxidative stress system. Decreasing the oxidative stress might be a potential treatment for autism.

Keywords: Autism, oxidative stress, glutathione, treatment, inflammation, methylation, sulfate.

INTRODUCTION

Autism is a complex neuro-developmental disorder whose prevalence has increased in the recent years [1]. Impaired social interaction, impaired language and communication, limited interests, and repetitive behaviors are the key symptoms of autism spectrum disorder. However, the etiology and neurobiology are not well clear. Of course, genetic factors play a significant role in this disorder [2]. Nevertheless, mendelian inheritance is not proposed for autism and multiple genes are involved [3]. Moreover, there is not any curative therapeutic approach for autism. In addition, there are only a few FDA approved medications for the management of its symptoms. Therefore, studying the cellular and molecular mechanisms underlying the clinical symptoms of autism can introduce novel therapeutic targets and interventions.

A common emerging target in research is the imbalance between oxidative systems and anti-oxidative stress systems. Therefore, the present study aims to review and summarize the research to date which supports the role of glutathione-related factors against the oxidative stress in autism.

OXIDATIVE STRESS

In general, there must be a balance between pro-oxidant and anti-oxidant systems in cells. Pro-oxidant agents in both pre- and/or postnatal periods can activate the oxidative stress in autism. Moreover, increased exposure to oxidative stress contributes to the progress and clinical manifestations of autism (James SJ, et al, 2006)[4]. The pro-oxidants of reactive oxygen species (ROS) include free radicals and some other molecules derived from molecular oxygen. ROS can be classified in two groups: a) radicals of oxygen, such as superoxide anion, hydroxyl radical, and peroxy radicals. b) Reactive non-radical oxygen species, such as hydrogen peroxide (H2O2), singlet oxygen, and peroxynitrite [5]. There is a wide range of other pro-oxidant factors, such as valproic acid [6], mercury [7, 8], lead (Pb) [9], viruses [10], air pollutants and toxins [11], thalidomide [12], and retinoic acid [13]. Oxidative stress and free radicals induce brain inflammation and edema through both cytotoxic and vasogrnic edema [14]. They impair blood brain barrier function, as well [14].

*Address correspondence to this author at the Research Center for Psychiatry and

Behavioral Sciences, Department of Psychiatry, Hafez Hospital, Shiraz, Iran; Tel:/Fax:

+98-711-627 93 19; E-mail: ghanizad@sina.tums.ac.ir

One of the sources of anion radical (O2-) is the enzyme of xanthine oxidase. This enzyme, which catalyses the production of uric acid from hypoxanthine and xanthine, can be found in any nucleated cell [15]. Red blood cell xanthine oxidase activity is increased in autism [16]. Furthermore, the increased activity of xanthine oxidase leads to the higher production of anion radical.

MAJOR ANTIOXIDANT PROTEINS

There are several factors in the brain making it highly susceptible to oxidative stress. In addition, the brain has a limited antioxidant capacity and the required energy to tackle the oxidative stress is also low. Moreover, the deficit in the antioxidant capacity causes cellular damage and changes the epigenetic gene expression [17].

Transferrin is an iron-binding protein and ceruloplasmin is a copper-binding one. The serum level of these proteins are decreased in autism compared to normal controls [18]. Moreover, there is a correlation between the lower level of these proteins and the higher degrees of loss of acquired language skills [18].

INSIGHTS REGARDING OXIDATIVE STRESS IN AUTISM AND ITS MANAGEMENT

A clinical manifestation of mitochondrial problems, such as mitochondrial DNA depletion [19], can be represented as autistic features [20, 21]. Moreover, the rate of autism in individuals with mitochondrial dysfunction is higher than the controls [22]. However, a very low percent of the children with autism have mitochondrial dysfunction [23]. Oxidative stress markers tend to increase in autism [24]. In addition, oxidative stress has a potential role in pathogenesis of Rett syndrome [25]. However, it cannot be guaranteed that there is a cause and effect association between autism, oxidative stress, and mitochondrial problems [26]. Oxidative stress targets mitochondrion and impairs the function of respiratory chain [27]. In addition, S-Adenosylhomocysteine induces phosphotidylserine exposure as well as apoptosis [28].

Neuro-inflammation may play a role in the etiology of autism [29, 30]. Meanwhile, the neuro-protective role of some factors, such as sonic hedgehog signaling and brain-derived neurotrophic factor against autism, is mediated through oxidative stress [31-33]. Sonic hedgehog improves the activities of superoxide dismutase and glutathione peroxidase [31]. Furthermore, nuclear factor kappa B is expected to play a role in the association between oxidative stress and neuro-inflammation in autism [34]. Moreover, the association

Oxidative Stress and Autism Current Medicinal Chemistry, 2012 Vol. 19, No. 23 4001

of neuro-inflammation and stereotypies in autism may be mediated by oxidative stress [35]. Lovastatin targets this neuro-inflammation in autism and it may improve autism co-morbid with epilepsy [36].

After the widespread use of acetaminophen for the management of fever in children, oxidative stress is proposed to play a major role in increasing the rate of autism. It is proposed that acetaminophen may mediate the oxidative stress in autism [37].

Metallothionein I/II may decrease oxidative stress in autism through the scavenging of reactive oxygen species [38]. In addition, medications, such as Zonisamide, which increase the glutathione levels in astroglial cells [39] may decrease oxidative stress in autism [40].

The hyperglutaminergic state in autism leads to excitotoxicity [41] and increased oxidative stress. Some interventions, such as c-Kit+ cells transplantation [42], ziconotide, as an N-type voltage-sensitive calcium channel blocker [43], targeting neurotensin [44], and targeting the glycine site on the NMDA receptor [45], are suggested to reduce this condition in autism leading to a possible management of some cases with the increased oxidative stress.

Methionine sulfoximine decreases neuroinflammation [46] and can also chelat lead whose level is high in some children with autism [47]. Therefore, methionine may decrease the oxidative stress in autism. Moreover, the protection of mitochondria from oxidative stress and neuro-inflammation by factors, such as Olesoxime [27], gold nanoparticle, and lipoic acid [48], are proposed to be beneficial in autism.

PLASMA GLUTATHIONE (GSH)

Glutathione is the most important intracellular defense in redox (reduction/oxidation) process. The glutamate, glycine, and cysteine make glutathione. It is produced through the following pathways: a) methionine cycle, b) transsulfuration pathway, and c) GSH-synthesis pathway [49] Fig. (1). Cysteine has a rate limiting effect for the production of glutathione. Besides, glutathione supplementation increases sulfate, cysteine, and taurine in autism spectrum disorders [50].

Fig. (1). The pathways of transsulfation and glutathione production.

Glutathione is also involved in pathophysiology of many psychiatric disorders. Moreover, in comparison to the control group, the levels of reduced, oxidized, and total GSH are significantly decreased in bipolar disorder, major depressive disorder, and schizophrenia [51]. There is a negative association between age and GSH level in all the brain areas in animals [52]; however, it is not gender related [52].

In addition to combat with oxidative stress, this endogenous antioxidant is also important for the excretion of toxic metals. It is also considered as a therapeutic strategy for the treatment of some neurodegenarative disorders, such as Alzheimer [53]. Both total glutathione level [54] and reduced plasma glutathione (GSH) are quite low in the children with autism spectrum disorders (ASD) [54, 55]. However, glutathione reductase activity is not different from the controls [54] (Table 1).

METHYLATION CAPACITY

S-adenosylmethionine (SAM) level which is produced from methionine is quite low in the children with autism compared to the neuro-typical children [55]. The ratio of SAM/ S- adenosyl homocysteine (SAH) level is also less in the children with autism [55, 56]. SAM is the primary donor of methyl. Methionine adenyosyl transferase forms SAM from methionine by using ATP Fig. (2). Therefore, the decreased level of ATP can lead to the lower level of SAM. S-adenosylhomocysteine (SAH) is formed from SAM after the transfer of a methyl group. Therefore, the ratio of SAM/SAH indicates the capacity of methylation. The decreased capacity is specific to autism [17]. Moreover, compared to the controls, the plasma level of ATP is lower in autism [55].

Methylcobalamin provides methyl groups for producing methionine and SAM. Furthermore, methylcobalamin and folinic acid improve glutathione Redox and, at the same times, increase the capacity for management of autism [57].

OXIDIZED GLUTATHIONE (GSSG)

The level of oxidized glutathione (GSSG) is higher in autism [55, 56]. Glutathione, sulfate, and taurine can be produced from cysteine [58] Fig. (1). Moreover, in comparison to the control group, the level of reduced GSH, cysteine, taurine, sulfate, and specially free sulfate are lower in autism [59]. In addition, red blood cell glutathione level is associated with the severity of autism [60]. Besides, low glutathione level is proposed to be a reason for low natural killer cell activity in autism [61].

GSH/GSSG RATIO

Glutathione reductase reduces GSSG to GSH. The ratio of oxidized to reduced glutathione (GSSG:GSH) is higher in the children with autism [55]. This ratio represents the intracellular antioxidant ability of the cells [62]. Oxidative stress decreases the GSH/GSSG ratio [54, 63] and increases free radical generation in autism [63]. The decline of the GSH/GSSG ratio is associated with the increase of age [52].

GLUTATHIONE PEROXIDASE (GSH-PX)

This is an antioxidant enzyme. The level of erythrocyte and plasma GSH-Px in autistic children are less than the normal children [64]. Of course, its level is age related in autism [65]. Another study reported the increased activity level of GSH-Px in autism [66].

GLUTATHIONE- S-TRANSFERASE (GST)

Homozygous glutathione S-transferase M1 is associated with autism [67, 68]. Moreover, compared to the controls, the activity level of GST is lower in autism [54]. This family of enzymes plays a major role in detoxification of xenobiotics [69].

NUTRITIONAL AND METABOLIC STATUS

Nutritional and metabolic status of the children with autism indicated vitamin insufficiency as well as the reduced capacity for energy transport, sulfation, and detoxification [55].

SULFATE LEVELS

About 80% of sulfate in the body is produced from the oxidation of dietary amino acids of methionine or cysteine. Sulfation and detoxification capacity in autism is decreased in comparison to the control group [55, 70]. Of course, they lose a higher level of sulfate in urine.

Taurine Glutathione

sulfate

Methionine

Methionine cycle

Cysteine

Transsulfation pathway

Homocysteine Cystathionine

4002 Current Medicinal Chemistry, 2012 Vol. 19, No. 23 Ghanizadeh et al.

Table 1. The Status of Glutathione-Related Factors in Autism

Pro-Oxidant Factors Status Ant-Oxidant Factors Status

Reactive oxygen species (ROS) Increased Transferrin Decreased

Oxanthine oxidase activity Increased Ceruloplasmin Decreased

Total Level of Glutathione Decreased

Oxidized glutathione level Increased The Level of Reduced Glutathione Decreased

Ratio of oxidized to reduced glutathione (GSSG:GSH)

Increased

Glutathione Reductase Activity No Changed

Uridine level Increased Free Sulfate Level Decreased

Adenosine level Increased Total Sulfate Level Decreased

Adenosine deaminase activity Decreased/No Changed S-Adenosylmethionine (SAM) Decreased

S-Adenosylmethionine (SAM)/ S-Adenyosylhomocysteine (SAH) Ratio Decreased

ATP Decreased

NADH Decreased

NADPH Decreased

G6PDH Decreased

Glutathione Peroxidase Decreased/Increased

Glutathione- s-transferase (GST) Decreased

Superoxide Dismutase (SOD) Decreased/Increased/

No change

Catalase Decreased

Sulfation and Detoxification Capacity Decreased

Fig. (2). The Methionine cycle.

The assessment of transsulfuration metabolites is recommended in autism [59, 71]. Moreover, the lower sulfation ability in autism decreases the detoxification of xenobiotics and heavy metals [71]. Heavy metals and xenobiotics are proposed to have a significant role in the neurobiology of autism [72, 73].

PLASMA URIDINE LEVEL

Methylation converts uridine to thymidine and the capacity for methylation is decreased in autism [56, 68] Fig. (2). On the other hand, plasma uridine level which is a marker of methylation status is higher in the children with autism [55].

There is a negative association between plasma uridine and SAM [55].

ADENOSINE

In more than one third of the children with autism, adenosine level is higher than the control group [55]. This increase may show the impairment of adenosine deaminase (ADA). This increment inhibits SAH hydrolase leading to the inhibition of the conversion of SAH to homocysteine Fig. (2), which can be an explanation for the higher level of SAH in autism, as well. The association of the low-activity ADA2 allele in autistic patients is reported by several genetic studies [74,

Uridine

Thymidine

Methionine adenyosyl transferase + ATP

Donation of Methly

Diet Methionine S-adenosylmethionine (SAM)

S-adenyosylhomocysteine (SAH)

Adenosine deaminase

Methionine synthase or methyl transferase+ methyl-B12 and 5-methyl-tetrahydrofolate

Homocysteine

Adenosine Inosine

Oxidative Stress and Autism Current Medicinal Chemistry, 2012 Vol. 19, No. 23 4003

75]. However, another study reported that ADA activity of the red blood cells is not changed in autism [16].

NADH, NADPH

The activity of enzymes involved in glutathione re-production and utilization is gender and age related. The activity of NADP-linked isocitrate dehydrogenase (NADP-ICDH), glucose-6-phosphate dehydrogenase (G6PDH), and glutathione reductase (GR) in the cortex of female animals is more than the male ones [76]. Nevertheless, as age increases,, this activity is decreased [76]. NADPH is needed for the reduction of GSSG to GSH Fig. (3). The plasma level of NADH and NADPH are decreased in autism and these decrement are associated with the oxidative stress [55]. NAD(P)H directly and indirectly functions as an antioxidant factor [77] and its indirect effect is through the reduction of the oxidized glutathione Fig. (3). NAD(P)H directly scavenges toxic free radicals [77]. NADH is involved in mitochondrial function, while NADPH plays a role in anabolic reactions (antioxidation and reductive biosynthesis) [78].

Fig. (3). The cycle of oxidation and reduction of glutathione.

GLUCOSE-6-PHOSPHATE DEHYDROGENASE (G6PDH)

G6PDH plays a direct role in the reproduction of GSH through providing NADPH [76]. The revival of erythrocyte GSH in the patients with declined G6PD level is declined [79]. Moreover, G6PDH is required for regeneration of NADPH [77].

ATP

Energy metabolism is impaired in autism [80]. In addition, the plasma level of ATP is decreased in autism [55]. In fact, decrease of ATP declines the activity of the anti-oxidative system Fig. (2).

MALONDIALDEHYDE (MDA)

The high level of malondialdehyde is proposed to be a biomarker for oxidative stress in the children older than 6 who suffer from autism [65].

SUPEROXIDE DISMUTASE

Superoxide dismutase (SOD) is one of the enzymes for the protecting the cells against oxidative stress. SOD and glutathione peroxidase are antioxidant enzymes. SOD selectively scavenges radical O2

–. This enzyme catalysis radical O2– to hydrogen peroxide.

Then, H2O2 is catalyzed to water and molecular oxygen by catalase [81]. The activity of SOD in red blood cells is increased in autism [16, 66]. Nevertheless, another study reported that the activities of erythrocyte SOD are decreased in autism [64]. However, another study reported that the activity level of SOD in the patients younger than 6 suffering from autism is not different from the control group [65].

CATALASE ACTIVITY

The enzyme of Catalase is directly involved in the elimination of reactive oxygen species. Red blood cell catalase activity is decreased in autism [16].

CONCLUSION

While the oxidative stress is increased in autism, the function of anti-oxidative stress is decreased. Furthermore, methylation processes is impaired and the level of glutathione is decreased. Glutathione plays a key role in protection against the oxidative stress. Therefore, it is expected that the strategies which increase the glutathione precursors as well as its reduced form improve some cases with autism through enhancement of the anti-oxidative stress system.

CONFLICT OF INTEREST

The author(s) confirm that this article content has no conflicts of interest.

ACKNOWLEDGEMENT

Research improvement center of Shiraz University of Medical Sciences and Ms. A. Keivanshekouh are appreciated for improving the use of English in the manuscript.

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Received: Janaury 31, 2012 Revised: May 22, 2012 Accepted: May 22, 2012