Reactive oxygen species: antioxidant defence system and related nutritional needs

The redox balance, or the anti/pro-oxidative balance in human cells, is of utmost importance to survival. Reactive oxygen species (ROS), typically oxygen and NO radicals, are consistently produced in and by cells in normal physiological processes, serving an important role in cellular and physiological functions, such as cellular signalling, regulation of cytokines, growth factors, transcription, immunomodulation and apoptosis, as well as in other processes^{(Reference Camini, da Silva Caetano and Almeida14)}. The cellular ROS levels are tightly maintained by complex intracellular regulatory systems. However, an unbalanced, uncontrolled pro-oxidative redox state results in damage to DNA, lipids and proteins, as well as loss of cellular integrity, and is linked to the initiation, development, progression and outcome of most human diseases, including infectious diseases. To keep cellular homeostasis and prevent a deleterious oxidative state, a sophisticated and synergistic antioxidant defence system, consisting of both enzymic and non-enzymic factors, is continuously activated. Non-enzymic antioxidants are mainly dietary components, classified generally as essential vitamins and minerals; and non-essential components, including phytochemicals – such as polyphenols, carotenoids and organosulfur compounds. The endogenous enzymic antioxidant system is comprised mainly of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) as well as other enzymes. The major antioxidant enzymes SOD1–3 catalyse the dismutation of superoxide (O₂ ⁻) oxidative radicals into O₂ and H₂O₂. Followed by SOD activity, two other antioxidant enzymes, CAT, a tetrameric haemoprotein, and GPx, convert H₂O₂ to water and O₂. The importance of these enzymes to human health is evident through numerous studies, demonstrating that abnormal SOD is linked to several diseases, including amyotrophic lateral sclerosis, Down’s syndrome and carcinogenesis^{(Reference Klassen, Biesalski and Mazariegos15,Reference Powers, Oberley, Domann, Valacchi and Davis16)} . Similarly, studies have shown the involvement of GPx in cancer, diabetes, angiogenesis, endothelial dysfunction, atherosclerosis, and cardiac dysfunction^{(Reference Malvy, Richard and Arnaud17,Reference Lubos, Loscalzo and Handy18)} . The majority of detoxification enzymes depend on dietary minerals as cofactors for optimal activity. For example, of the three human SOD isoforms, the cytosolic SOD1 uses Cu or Zn ions, the mitochondrial SOD2 uses Mn, the extracellular SOD3 also uses Cu/Zn, and GPx uses Se. Furthermore, as elimination of ROS is usually an orchestrated process, where the activity of one enzyme is followed by another, if the activity of one enzyme is not optimal and balanced by that of the following enzyme, the generation of ROS is accelerated.

Fruits and vegetables are a rich source of exogenic antioxidants, such as vitamins C and E and minerals (Mg, Zn, Mn and Se) and also of non-essential phytochemicals (polyphenols and carotenoids). Numerous studies have shown that a diet rich in fruits and vegetables is associated with reduced risk of chronic diseases^{(Reference Griffiths, Aggarwal and Singh19,Reference López-Jaramillo, Otero and Camacho20)} . In fact, it has been shown that during oxidative stress, dietary components can modify total antioxidant capacity, an analyte frequently used to assess the antioxidant status of biological samples, improving redox status and consequently delaying or preventing progression and onset of diseases. Dietary total antioxidant capacity has been shown to be associated with risks of several chronic diseases, such as diabetes, hypertension and CVD, etc.^{(Reference Ghiselli, Serafini and Natella21–Reference Pourvali, Abbasi and Mottaghi30)}.

Reactive oxygen species, viral infection and antioxidant enzymes

In the context of viral infections, many human viruses, including HIV, herpes simplex virus type 1, hepatitis B virus, hepatitis C virus (HCV), respiratory syncytial virus, influenza viruses and SARS (severe acute respiratory syndrome) coronavirus, produce ROS by diverse mechanisms^{(Reference Molteni, Principi and Esposito31–Reference Li, Wang and Jou35)}. As SARS-CoV-2 virus has 80 % homology to SARS coronavirus, it probably uses similar mechanisms^{(Reference Koyama, Platt and Parida36)}. In recent years, several reviews summarised the involvement of ROS in the pathogenesis of viral infections in general and in RNA viruses in particular^{(Reference Reshi, Su and Hong37-Reference Ha, Shin and Feitelson39)}. Although not the focus of this review, in brief, during viral infections, one of the virus’s infection strategy to promote viral pathogenesis is to modulate the intracellular redox state as a byproduct of survival efforts and as part of their replication mechanism. Ultimately, virus-induced host cells, as a mechanism of pathogen elimination and viral spread limitation^{(Reference Valyi-Nagy and Dermody40)}, secrete cytokines, which trigger host ROS production^{(Reference Ha, Shin and Feitelson39)}. Host-triggered phagocytes activate the NADPH oxidase complex and NO synthase, resulting in simultaneous release of ROS and pro-oxidant cytokines, such as TNF and IL-1^{(Reference Camini, da Silva Caetano and Almeida14)}. In turn, TNF and IL-1 trigger a chain of cellular events, such as mitochondrial pro-oxidant activity and stimulation of neutrophils to release lysosomal proteins, including lactoferrin, which may result in an elevated production of ROS. As viral multiplication progresses, more ROS are formed, causing an imbalance in cellular redox homeostasis, thus contributing to the severity of the inflammatory responses, cell death, weight loss and other typical phenomena^{(Reference Reshi, Su and Hong37,Reference Schwarz38)} . The cellular redox homeostasis imbalance can in turn further contribute to viral survival by selecting for certain viral mutants and activating transcription factors, such as NF-κB, which increases viral replication^{(Reference Ha, Shin and Feitelson39)}.

In general, viruses, although varying in the production of ROS, share a common pathogenic pathway which results in host antioxidant system depletion^{(Reference Schwarz38)}. For example, patients infected by hepatitis B virus show a reduction in Cu/Zn-SOD and GPx enzymes^{(Reference Ha, Shin and Feitelson39)}; patients with dengue fever exhibit alteration in oxidative stress status as the disease progresses, with decrease in the glutathione and total antioxidant status following infection^{(Reference Klassen, Biesalski and Mazariegos15)}; and HIV patients show significantly reduced levels of GSH, cysteine, vitamin C, GPx and SOD in plasma and leucocytes, and an increase in levels of lipid peroxidation^{(Reference Malvy, Richard and Arnaud17,Reference Droge, Eck and Mihm41,Reference Fuchs, Emerit and Levy42)} .

The critical importance of both antioxidant enzymes and nutritional components to the course of viral infection is clearly demonstrated by numerous in vivo and in vitro studies (mostly in animals), highlighting the therapeutic potential but also the limitations of this therapy^{(Reference Saso and Firuzi43,Reference Sgarbanti, Amatore and Celestino44)} . Some examples in a nutshell: injecting SOD conjugated with a pyran copolymer protected mice against a potentially lethal influenza virus infection^{(Reference Oda, Akaike and Hamamoto45)}; the administration of recombinant human CAT to mice infected with H1N1 influenza A virus decreased inflammatory cell infiltration, inflammatory cytokine levels and the mRNA levels of the Toll-like receptors and NF-κB^{(Reference Shi, Shi and Huang46)}. Moreover, it was demonstrated that oxidative stress positively affects viral RNA replication and that antioxidant treatment can significantly impair viral RNA replication, altering the amount of capped viral RNA^{(Reference Gullberg, Jordan Steel and Moon47)}. Clinical studies have shown that the addition of antioxidants can decrease liver injury caused by oxidative stress, suggesting that this could be a potential treatment for HCV infection^{(Reference Lozano-Sepulveda, Bryan-Marrugo and Cordova-Fletes48,Reference Gabbay, Zigmond and Pappo49)} . Studies have also shown that the administration of exogenous GSH inhibits dengue virus-2 viral production by modulating NF-κB activity and reducing ROS production^{(Reference Tian, Gao and Zhang50,Reference Wang, Chen and Gao51)} . Vitamin and micronutrient supplementation has been shown to improve outcomes in HIV-infected patients either alone or with antiretroviral treatment^{(Reference Irlam, Visser and Rollins52,Reference Isanaka, Mugusi and Hawkins53)} . Vitamin E supplementation decreased lung virus titres in mice infected with influenza^{(Reference Hayek, Taylor and Bender54,Reference Han, Wu and Ha55)} . SOD, CAT and GPx were significantly increased in rats after oral dosage of astaxanthin^{(Reference Ambati, Phang and Ravi56)}. A vitamin C supplement was demonstrated to have a beneficial effect in influenza infections, mainly in experimental models; however, this effect has not been reported in patients^{(Reference Hemilä and Chalker57)}. Herpes zoster infection patients receiving intravenous vitamin C supplement had a significant reduction in pain scale scores^{(Reference Chen, Chang and Feng58)}. However, it should be noted that the role of antioxidants in viral infections is more complex than the mere antiviral host defence and viral survival strategies, as it includes many other effects related to metabolic regulation both of host and viral survival. Thus, viral infection simultaneously increases the demand for micronutrients and causes their loss, which leads to an antioxidant deficiency that should be monitored and addressed as an essential part of viral treatment. However, due to the complexity of the viral–host interaction, the complex effects of ROS–antioxidant interactions and the lack of clinical studies, well-designed clinical trials are required to study the use of antioxidants as a therapy in viral infections.

In the context of the present review, one of the overlooked areas which requires scientific and clinical attention is the existence of functional polymorphisms in genes encoding antioxidant enzymes, which alter the function of the enzymes, with possible implications on antioxidant nutritional requirements and treatment (Table 1).

Table 1. Summary of oxidative enzyme polymorphism and nutrition interactions

SOD2, superoxide dismutase 2; CAT, catalase; GPx, glutathione peroxidase; HUVEC, human umbilical vein endothelial cells.

Superoxide dismutase polymorphisms and diseases

The human SOD1 gene is located on chromosome 21q22, SOD2 on chromosome 6q25^{(Reference Pourvali, Abbasi and Mottaghi30,Reference Bresciani, Cruz and de Paz59)} and SOD3 on chromosome 4q21^{(Reference Levanon, Lieman-Hurwitz and Dafni60,Reference Folz and Crapo61)} . Due to their essential role in conserving cellular integrity and redox balance, functional alterations in SOD1, SOD2 and SOD3 have been linked to common diseases, including inflammatory bowel disease^{(Reference Lih-Brody, Powell and Collier62,Reference Mulder, Verspaget and Janssens63)} , obesity^{(Reference Bełtowski, Wójcicka and Górny64,Reference Olusi65)} , diabetes and hypertension^{(Reference Pourvali, Abbasi and Mottaghi30,Reference Sundaram, Bhaskar and Vijayalingam66-Reference Olofsson, Marklund and Behndig70)} , chronic obstructive pulmonary disease^{(Reference Yigla, Berkovich and Nagler71–Reference Lewandowski, Kepinska and Milnerowicz75)} and CVD^{(Reference Bresciani, Cruz and de Paz59)}, etc. Surprisingly, although polymorphisms in the antioxidant genes may determine cellular oxidative stress levels, with significant implications for the pathogenesis of viral infections and their complications, scarce research exists on this issue. Farawela et al.^{(Reference Farawela, Khorshied and Shaheen76)} studied 100 Egyptian patients with B cell-non-Hodgkin lymphoma and 100 controls to test the association between HCV infection, oxidative stress gene polymorphisms and B cell-non-Hodgkin lymphoma risk. Concomitant HCV infection and GPx1 gene polymorphism (Pro¹⁹⁷Leu) had a synergetic effect on non-Hodgkin lymphoma risk with an OR of 15. SOD2 (Val16Ala) and CAT (C-262T) genetic polymorphisms were not found to confer increased non-Hodgkin lymphoma risk. Similarly, Ezzikouri et al.^{(Reference Ezzikouri, El Feydi and Chafik77)} found a significant association between homozygosity of the SOD2 (Val16Ala) variant polymorphism and hepatocellular carcinoma occurrence in HCV-infected Moroccan patients.

In humans, at least 111 SNP have been identified for SOD1 and 100 for SOD3; however, information regarding these polymorphisms in the context of chronic diseases is lacking^{(Reference Lewandowski, Kepinska and Milnerowicz75-Reference Crawford, Fassett and Geraghty78)}. Most of the SNP, summarised in a recent review, are not known to be functional, yet are located in non-coding intronic genetic regions with possible regulatory implications^{(Reference Lewandowski, Kepinska and Milnerowicz75)}. Of clinical significance is the functional SNP rs1799895 (worldwide MAF 0·5–10 %), which changes arginine to glycine at position 213 (R213G) at the SOD3 carboxy-terminus, resulting in alteration of the positive charge of the terminus, and consequent release of SOD3 from the extracellular matrix to the extracellular fluids such as plasma and epithelial lining fluids^{(Reference Gaurav, Varasteh and Weaver79)}. SOD3 is highly expressed in arteries^{(Reference Crapo and Day80)}, lungs, airways^{(Reference Oury, Chang and Marklund81)} and alveolar macrophages^{(Reference Kim, Morimoto and Ogami82)}. Studies in both human subjects and mice have shown that SOD3 plays a key role in decreasing lung injury by reducing oxidative stress^{(Reference Yao, Arunachalam and Hwang83)}. In fact, SOD3 R213G carriers have reduced risk of exacerbations of chronic obstructive pulmonary disease^{(Reference Li, Wang and Jou35)}. In a recent elegant study, Gaurav et al.^{(Reference Gaurav, Varasteh and Weaver79)} showed that knock-in mice analogous to the human SOD3 R213G SNP had lower airway hyper-responsiveness, inflammation and mucus hypersecretion with decreased IL-33 in bronchoalveolar lavage fluid and reduced type II innate lymphoid cells in the lungs. This study suggests the potential benefit of SOD3 R213G SNP carriers, as they highly express SOD3 in the airway-lining fluid, thus ameliorating allergic airway inflammation by diminishing the innate immune response, including IL-33-mediated changes in innate lymphoid cells^{(Reference Gaurav, Varasteh and Weaver79)}. Of note, the SOD3 R213G SNP was also reported to increase the risk of IHD in The Copenhagen City Heart Study^{(Reference Juul, Tybjaerg-Hansen and Marklund84)}.

The nuclear SOD2 gene is translated in the cytoplasm with a mitochondrial targeting sequence; the enzyme is then transported into the mitochondria, processed, and assembled into an active homo-tetramer^{(Reference Bresciani, Cruz and de Paz59)}. SOD2 is present in the mitochondria, the major ROS production organelle in aerobes, thus playing a pivotal role in health and disease^{(Reference Crawford, Fassett and Geraghty78)}. In humans, at least 190 SNP have been identified for SOD2 ^{(Reference Pourvali, Abbasi and Mottaghi30,Reference Bresciani, Cruz and de Paz59)} . The most studied SOD2 functional SNP is Ala16Val (rs4880) in exon 2, which causes a conformational change in the mitochondrial targeting domain, from α-helix to β-sheet secondary structure, consequently affecting SOD2 activity in the mitochondria^{(Reference Pourvali, Abbasi and Mottaghi30)}. Numerous studies have shown that the SOD2 Ala16Val SNP is significantly associated with altered progression and risk of different diseases, such as diabetes and diabetes co-morbidities^{(Reference Bresciani, Cruz and de Paz59,Reference Banerjee and Vats85–Reference Ascencio-Montiel Ide, Parra and Valladares-Salgado87)} , epilepsy^{(Reference Kegler, Cardoso and Caprara88)}, cancer^{(Reference Wang, Zhu and Xi89)}, pre-eclampsia^{(Reference Luo, Julien and Wei90)} and CVD^{(Reference Bresciani, Cruz and de Paz59,Reference Crawford, Fassett and Geraghty78,Reference Santl Letonja, Letonja and Ikolajević-Starcević91)} , etc. The SOD2 Ala16Val (rs4880) mean allelic frequency is highly variable in different populations, ranging from 11·7 % in the East Asian population, to 50 % in European cohorts, to 62 % in Latin American population (BLAST⁽⁹²⁾).

Superoxide dismutase 2 Ala16Val SNP and dietary factors

Several known disease risk factors have been shown to interact with the SOD2 Ala16Val polymorphism, including smoking and alcohol consumption. Alcohol promotes the generation of ROS through numerous processes, particularly in the liver, the main organ that metabolises and detoxifies alcohol. Nahon et al. ^{(Reference Nahon, Sutton and Rufat93)} reported that alcoholic cirrhosis patients who have at least one Ala16Val allele are at increased risk for hepatocellular carcinoma occurrence and death.

Conflicting evidence exists regarding the interaction between the SOD2 Ala16Val polymorphism and dietary components. Women homozygous for the SOD2 Ala16Val variant allele have a 4-fold increased risk for breast cancer compared with women who are homozygous or heterozygous for the common allele (OR 4·3; 95 % CI 1·7, 10·8); this effect is particularly evident in premenopausal women. This association was found to be most evident among women whose intake of fruits and vegetables and dietary ascorbic acid and α-tocopherol is below the median^{(Reference Ambrosone, Freudenheim and Thompson94–Reference Nowell, Ahn and Ambrosone97)}. The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study found that men homozygous for the variant allele had a 70 % increased risk for prostate cancer compared with men homozygous for the wild-type allele (OR 1·72; 95 % CI 0·96, 3·08)^{(Reference Woodson, Tangrea and Lehman98)}. However, supplementation with α-tocopherol had no impact on the SOD2–prostate cancer association. The Physicians’ Health Study found significant interaction between prostate cancer risk, SOD2 homozygous variant genotype and low baseline plasma antioxidant levels, where homozygote genotype and low antioxidant levels incurred almost 4-fold increased risk of prostate cancer. Men homozygous for the variant allele genotype had a 10-fold increased risk for aggressive prostate cancer across quartiles of antioxidant status^{(Reference Li, Kantoff and Giovannnucci99)}.

Tong et al.^{(Reference Tong, Lee and Song100)} studied the interaction between SOD2 genotypes and cervical carcinogenesis risk and the modulating effects of serum antioxidant nutrient status (β-carotene, lycopene, zeaxanthin/lutein, retinol, α-tocopherol and γ-tocopherol). They found that the reduced risk of cervical carcinogenesis subtype was associated with the variant allele only among those with above median levels of serum β-carotene and γ-tocopherol. Two meta-analyses have examined the association between the SOD2 Val16Ala polymorphism, breast cancer risk and vitamin C, vitamin E and carotenoid^{(Reference Kim, Morimoto and Ogami82)} and fruit and vegetable consumption^{(Reference Wang, Wang and Shi101)}. Their results suggest that the SOD2 Val16Ala polymorphism may contribute to cancer development through a disturbed antioxidant balance; while both meta-analyses showed no independent effect of genotype on breast cancer risk, intakes of antioxidants were shown to modify risk in premenopausal women^{(Reference Wang, Wang and Shi101)}, while fruit and vegetable consumption did not^{(Reference Chen and Pei102)}. Similarly, Kakkoura et al. ^{(Reference Kakkoura, Demetriou and Loizidou103)} showed that in Greek-Cypriot women who were carriers of at least one SOD2 variant allele, high vegetable intake lowered breast cancer risk by almost half compared with low vegetable intake. Taken together, despite inconsistencies, the overall results suggest that the SOD2 Ala16Val SNP can be modulated by dietary factors. However, further studies are needed to establish the nature of this association^{(Reference Pourvali, Abbasi and Mottaghi30,Reference Bresciani, Cruz and de Paz59)} .

Catalase polymorphisms and dietary factors

The CAT gene, mapped to chromosome 11p13, encodes a tetrameric haemoprotein expressed in all aerobes; the highest levels of the enzyme are found in the liver, kidney and erythrocytes. Numerous CAT polymorphisms have been described in the promoter, 5´ and 3´- untranslated regions, exons and introns^{(Reference Da Costa, Badawi and El-Sohemy104,Reference Kodydková, Vávrová and Kocík105)} . Significant associations were found between CAT polymorphisms and the risk of various diseases, including diabetes^{(Reference Chistiakov, Savosťanov and Turakulov106,Reference Tarnai, Csordás and Sükei107)} , hypertension^{(Reference Watanabe, Metoki and Ohkubo108)}, asthma^{(Reference Polonikov, Ivanov and Solodilova109,Reference Wang and Karmaus110)} , breast cancer^{(Reference Ahn, Nowell and McCann111)} and others. However, results of the various studies are inconsistent, explained at least in part by different populations, methodologies and disease course.

Of the CAT SNP presented in the National Center for Biotechnology Information (NCBI) database, the most studied in relation to human diseases is CAT C-262T (rs1001179), with mean world population frequency ranging from 8 to 26 %. The common CAT C262T polymorphism is located in the gene promoter region and its correlation with CAT activity is controversial^{(Reference Ahn, Nowell and McCann111–Reference Mak, Ho and Yu113)}. However, the CAT C262T polymorphism was linked to host response to oxidative stress^{(Reference Ahn, Nowell and McCann111)}. Indeed, variant CAT T alleles have been associated with increased risk for conditions related to oxidative stress, such as hypertension^{(Reference Zhou, Cui and DeStefano114)} and vitiligo^{(Reference Casp, She and McCormack115)}. Ahn et al.^{(Reference Ahn, Gammon and Santella116)} (Long Island Breast Cancer Study Project) have shown that women homozygous for the common C allele had a 17 % reduction in risk of breast cancer compared with those with at least one variant T allele. Women homozygous for the common C allele who consumed a high-fruit diet showed a significantly lower risk for breast cancer (OR 0·59; 95 % CI 0·38, 0·89). In a follow-up study, Ahn et al. ^{(Reference Ahn, Gammon and Santella116)} found that differences in CAT activity by genotype were most pronounced among those in the highest tertiles of fruit and vegetable consumption. Similarly, Kakkoura et al.^{(Reference Kakkoura, Demetriou and Loizidou103)} found that high vegetable intake lowered breast cancer risk in Greek-Cypriot women with at least one CAT -262C allele (OR high v. low for -262CC = 0·66, 95 % CI 0·47, 0·92; for -262CT = 0·53, 95 % CI 0·35, 0·81). Analysing the Cancer Prevention Study-II Nutrition Cohort, Li et al.^{(Reference Li, Ambrosone and McCullough117)} studied the interaction of a combined haplotype of SNP of common antioxidant enzymes with the level of vegetable and fruit intake on breast cancer risk in postmenopausal women. They found joint effects of endogenous and exogenous antioxidants, where among women with low vegetable and fruit intake (< median), the low-risk CAT CC (OR 1·33; 95 % CI 0·89, 1·99) genotype appeared to be associated with higher breast cancer risk, with significantly increased risks observed in those with ≥ 4 low-risk alleles compared with participants with ≤ 2 low-risk alleles (OR 1·77; 95 % CI 1·05, 2·99; P _interaction = 0·006)^{(Reference Ahn, Gammon and Santella116)}. Similarly, Jansen et al.^{(Reference Jansen, Robinson and Stolzenberg-Solomon118)} found that inter-individual variation in antioxidant genes, including the CAT rs12807961 SNP, could interact with dietary intake to influence pancreatic cancer risk.

Another CAT SNP in the gene promoter region (rs7943316, worldwide MAF 25–48 %), an A>T substitution at position −21 (A-21T), has been studied in relation to interaction with nutritional factors. Polonikov et al.^{(Reference Polonikov, Ivanov and Solodilova109)} found that the frequencies of both allele -21A and -21AA CAT genotypes were higher among asthmatics than among healthy controls. Notably, no association of CAT genotype -21AA with asthma was found in high fruit and vegetable consumers, whereas low fruit and vegetable consumers (once per d or less) possessing this genotype were at increased risk of both allergic and non-allergic asthma.

Taken together, there is growing evidence pointing to a possible interaction between CAT polymorphisms and relevant nutrients, indicating that an individual’s genome should be taken into consideration when planning nutrient intake and that interaction between dietary components and the personal genome is a significant factor in health and disease.

Catalase polymorphisms and viral infection

Most of the quite limited publications regarding CAT polymorphisms and viral infection are related to either HCV and hepatic carcinoma or to HIV, with only initial and conflicting findings, warranting much needed future studies^{(Reference Farawela, Khorshied and Shaheen76,Reference Sousa, Carmo and Vasconcelos119,Reference Liu, Xie and Zhao120)} . SNP have been associated with airway diseases, including asthma and chronic obstructive pulmonary disease^{(Reference Wu, Yuan and López121)}; however, little is known in this context regarding virus-induced lung disorders. Chambliss et al.^{(Reference Chambliss, Ansar and Kelley122)} have shown that respiratory syncytial virus infection is associated with oxidative lung injury, decreased levels of antioxidant enzymes and degradation of the transcription factor NF-E2-related factor 2, a master regulator of antioxidant enzyme expression. Additionally, Chambliss et al.^{(Reference Chambliss, Ansar and Kelley122)} demonstrated that the CAT rs1001179 (-262C/T) polymorphism in the lung may play an important and protective role in respiratory syncytial virus-associated lower respiratory tract infections in children heterozygous or homozygous for the variant allele. Similarly, the presence of the CAT rs1001179 (-262C/T) T allele has been previously associated with a decreased risk of asthma in non-smokers in the Hong Kong Chinese population^{(Reference Mak, Leung and Ho123)}, yet with an increased risk of new-onset of asthma among Hispanic and Caucasian children^{(Reference Islam, McConnell and Gauderman124)}. To the best of our knowledge, no research has been conducted on the interaction between CAT polymorphism, viral infection and nutrition. Due to the important and proven interaction between CAT and nutritional factors, future such studies are needed.

Glutathione peroxidases

The glutathione peroxidases (GPx) are a family of Se-dependent enzymes encoded by discrete genes located on different chromosomes. The human genome harbours twenty-five selenoprotein genes, of which eight GPx paralogues have been identified, namely GPx1 (locus 3p21.3), GPx2 (locus 14q24.1), GPx3 (locus 5q23), GPx4 (locus 19p13.3), GPx5 (locus 6p22.1), GPx6 (locus 6p22.1), GPx7 (locus 1p32) and GPx8 (locus 5q11.2)^{(Reference Zmorzyński, Świderska-Kołacz and Koczkodaj125)}. Five of these eight GPx paralogues contain a selenocysteine residue in the catalytic site (GPx1–GPx4, GPx6) and three have a cysteine instead (GPx5, GPx7 and GPx8)^{(Reference Guillin, Vindry and Ohlmann9)}. GPx1 and GPx4 are ubiquitously expressed; GPx1 (mostly abundant in erythrocytes, kidney and liver) is cytoplasmic, while GPx4 is localised to the cytoplasmic, mitochondrial and nuclear cellular compartments. GPx2 is present in epithelial tissues including the gastrointestinal tract, lung, skin and liver. GPx3 is secreted to the plasma and excreted mostly by the kidney. GPx6 is only found in the olfactory epithelium and embryonic tissues. The enzymic activity of GPx is directly proportional to Se intake; therefore, there is a strong link between Se deficiency and oxidative stress. Consequently, several GPx SNP have been shown to have significant association with both Se status biomarkers and health outcome^{(Reference Hurst, Collings and Harvey126)}. For example, the T allele for GPx1 SNP rs1050450 has been shown to have a significant impact on high-grade prostate cancer risk, over a range of plasma/serum Se concentrations^{(Reference Penney, Schumacher and Li127,Reference Steinbrecher, Meplan and Hesketh128)} . Many SNP have been identified in human GPx isoforms: 46 in GPx1, 73 in GPx2, 120 in GPx3, 88 in GPx4 and 93 in GPx5 ^{(Reference Crawford, Fassett and Geraghty78)}. However, functional consequences have been demonstrated in in vitro and in vivo studies in only a small number of those SNP in genes encoding selenoproteins^{(Reference Méplan and Hesketh129)}. GPx1 has four functional SNP, of which Pro¹⁹⁷Leu (rs1050450; worldwide MAF 22–35 %) has been studied most extensively in association with many diseases, including cancer^{(Reference Crawford, Fassett and Geraghty78,Reference Ahn, Gammon and Santella116,Reference Hu and Diamond130)} , diabetes^{(Reference Nemoto, Nishimura and Sasaki131,Reference Zotova, Savost’ianov and Chistiakov132)} , kidney diseases and vascular diseases^{(Reference Crawford, Fassett and Geraghty78,Reference Méplan and Hesketh129)} . However, studies assessing the association between the GPx1 Pro¹⁹⁷Leu SNP genotypes and diabetes, stroke, brain tumours and prostate cancer are currently inconclusive.

Regarding functional consequences of SNP of the other GPx in relation to human diseases, there is at present very little conclusive data: GPx1 rs1800668 was studied in the context of cancer and was found to be associated with an increased risk of oesophageal cancer; three functional SNP of the GPx2 isoform, three of GPx3 and six of GPx3 coding regions have been scarcely studied in relation to disease^{(Reference Crawford, Fassett and Geraghty78)}.

Glutathione peroxidase polymorphisms and dietary factors

Evidently, the most studied nutrient in regard to GPx in general and GPx polymorphisms in particular is Se. Several SNP in selenoprotein-coding genes have been shown to be functionally significant and to affect the response of biomarkers of Se status to Se supplementation^{(Reference Méplan, Crosley and Nicol133–Reference Jablonska, Gromadzinska and Reszka135)}. In particular, rs1050450 in GPx1, rs713041 in GPx4 and rs7579 in the selenoprotein-P gene are known to affect the expression of the respective selenoproteins. Of those, the GPx1 Pro¹⁹⁸Leu (rs1050450) SNP is the most studied. This polymorphism has been shown to affect GPx activity in some, although not all studies^{(Reference Da Costa, Badawi and El-Sohemy104)}. Carriers of the variant allele have been shown to have significantly higher levels of lipid pre-oxidation components^{(Reference Shuvalova, Kaminnyi and Meshkov136)}. This polymorphism has also been associated with several types of cancer, with conflicting results reported. A pilot study by Cardoso et al.^{(Reference Cardoso, Busse and Hare137)} examined the effects of GPx1 Pro¹⁹⁸Leu in response to Se supplementation via dietary Brazil nuts. GPx1 Pro¹⁹⁸Leu genotypes differentially affected the Se status and GPx activity. Similarly, a later study by Donadio et al.^{(Reference Donadio, Rogero and Cockell138)} showed that Brazil nut supplementation significantly increased GPx1 mRNA expression only in subjects with the CC genotype. Crosley et al.^{(Reference Crosley, Bashir and Nicol139)} have demonstrated elevated adhesion levels in human umbilical vein endothelial cells (HUVEC) and monocytes in individuals homozygous for the T-variant of functional GPx4 (c718t) as compared with carriers of the C-variant. This effect was modified by Se and PUFA. Méplan et al.^{(Reference Méplan, Crosley and Nicol140)} showed that following clinical intervention with Se supplementation for 6 weeks in non-smokers, both lymphocyte GPx1 protein concentrations and plasma GPx3 activity increased significantly in homozygote CC individuals in the GPx4 718 T/C (rs713041) SNP but not in homozygote TT participants. After Se withdrawal, there was a significant fall in both lymphocyte GPx4 protein concentration and activity in the homozygote TT, but not in homozygote CC participants^{(Reference Méplan, Crosley and Nicol140)}.

Several studies, although scarce, show suggestive interaction between GPx1 polymorphisms and other dietary components: Hu & Diamond^{(Reference Hu and Diamond130)} have shown that the GPx1 Pro¹⁹⁸Leu variant allele results in lesser response to the stimulation of GPx1 enzyme activity during Se supplementation compared with the common allele. Significant gene–diet interactions were found in the prospective Diet, Cancer and Health Study, where homozygotes for the variant allele had higher colorectal cancer risk with alcohol consumption and homozygotes for the common allele with higher dietary vitamin C intake had reduced risk of colorectal cancer^{(Reference Hansen, Krath and Frederiksen141)}.

In summary, most of the published data regarding GPx polymorphisms and dietary components are related to Se, both in cancer and in viral infection. Although scarce data exist regarding other nutritional factors, it is quite clear from the publications so far that GPx are affected by dietary components, especially by Se, and that GPx polymorphisms can alter the need for dietary components and vice versa. This is particularly relevant to cancer and to viral infections.

Glutathione peroxidase 1 polymorphisms and viral infection

Se deficiency, which is a major regulator of selenoprotein expression, has been associated with the pathogenicity of several viruses. Moreover, several selenoprotein family members, including GPx, suggestively have an important role in different models of viral replication^{(Reference Guillin, Vindry and Ohlmann9)}. For instance, in Epstein–Barr virus infection, GPx activity reduction is associated with elevation in viral load^{(Reference Sumba, Kabiru and Namuyenga142)}. Supplementing herpes simplex virus-2 patients with selenium aspartate and multi-supplementation results in faster recovery, reduction in viral load and elevation in antiviral cytokines^{(Reference De Luca, Kharaeva and Raskovic143)}. The effect of GPx on inhibiting HIV activation is well documented. Correspondingly, Se can alter mutagenesis rates, both in viral genomes and in the DNA of mammalian cells exposed to carcinogens^{(Reference Guillin, Vindry and Ohlmann9)}. Similar to CAT, most published literature regarding GPx1 polymorphisms and viral infection is regarding chronic hepatitis C. Sousa et al.^{(Reference Sousa, Carmo and Vasconcelos119)} found that homozygosity to the common GPx1 Pro¹⁹⁸Leu (rs1050450) allele was significantly associated with severity of liver fibrosis and chronic hepatitis C. Thus, they concluded that GPx1 polymorphisms may be implicated in the severity of liver fibrosis and HCC caused by HCV^{(Reference Jansen, Robinson and Stolzenberg-Solomon118)}. Farawela et al. ^{(Reference Farawela, Khorshied and Shaheen76)} found that HCV infection and GPx1 gene polymorphisms had a synergetic effect on non-Hodgkin lymphoma risk (OR 15; 95 % CI 2·2, 69·6; P<0·0001) in Egyptians.