Cooperative function of antioxidant and redox systems against oxidative stress in male reproductive tissues

Keywords: reactive oxygen species; superoxide dismutase; glutathione; thioredoxin; peroxiredoxin; aldo-keto reductase

Abstract

Reactive oxygen species (ROS) are produced under oxidative stress, such as high oxygen concentration and during the metabolic consumption of oxygen molecules. Male reproductive tissues appear to be continuously exposed to ROS produced by active metabolism. In addition, spermatozoa must pass through a high oxygen environment during the mating process. Thus, to maintain viable reproductive ability, a protective mechanism against oxidative stress is of importance. Here, we overview our current understanding of the cooperative function of antioxidative and redox systems that are involved in male fertility. Superoxide dismutase and glutathione peroxidase are major enzymes that scavenge harmful ROS in male reproductive organs. In turn, glutathione and thioredoxin systems constitute the main redox systems that repair oxidized and damaged molecules and also play a role in regulating a variety of cellular functions. While glutathione functions as an antioxidant by donating electrons to glutathione peroxidase and thioredoxin donates electrons to peroxiredoxin as a counterpart of glutathione peroxidase. In addition, aldo-keto reductases, which detoxify carbonyl compounds produced by oxidative stress, are present at high levels in the epithelia of the genital tract and Sertoli cells of the testis. Since these systems are involved in cross-talk, a comprehensive understanding will be required to maintain the physiological functions of male reproductive system.

1 Introduction

The lives of most organisms are dependent on ATP, produced by oxidative phosphorylation. However, reactive oxygen species (ROS), which are inevitably produced during the metabolic process, modify various cellular components by oxidation, thus impairing their physiologic function. Therefore, oxygen is generally considered to be a double-edged sword. ROS production is enhanced under pathological conditions, such as inflammation and ischemia/reperfusion and is involved in various diseases and aging [1].

Since more than 95 % respired oxygen is consumed by the mitochondrial electron transport system, ROS production in mitochondria is a continuous process and is enhanced under conditions of an abnormal oxygen supply [1]. In addition, the electron transport system exists on a limited scale in the endoplasmic reticulum. ROS are also produced by some enzymatic systems such as oxygenases and oxidases that utilize oxygen molecule as a substrate. Xanthine oxidase, an enzyme involved in purine metabolism, is often troublesome by generating ROS under ischemic conditions in the cardiovascular system [2]. Cyclooxygenase, which catalyzes the initial oxidation step of arachidonate to prostaglandins and is induced under inflammatory conditions, is also well known ROS-generating enzyme [3], in addition to the mitochondrial electron transport system.

Reproduction is a fundamental process that allows living organisms to preserve their progeny and evolve by transmitting genes. However, metabolism, which is supported by sex steroid hormones, is highly activated during spermatogenesis, indicating that the reproductive system itself generates high levels of ROS [4]. On the course of transferring male DNA to the female partner, spermatozoa are exposed to high concentrations of oxygen. Thus, reproduction as a whole is a process that is exposed to severe oxidative stress at various stages.

Mammals have evolved several mechanisms to suppress oxidative stress and minimize damage by ROS (Figure 1) [1]. One is an antioxidative system comprised of enzymes and low molecular weight compounds such as vitamins. This system scavenges harmful ROS before they have an opportunity to react with other important molecules and terminates the subsequent chain reaction. The other is a reduction-oxidation (redox) system that not only functions to detoxify harmful oxidants but reductively repairs oxidized molecules as well. Moreover, the redox system is positively involved in the regulation of a variety of physiological functions of cells, such as signal transduction for cell growth and death [5]. In fact both systems are involved in cross-talk and are not clearly separable [6]. In turn, carbonyl com-pounds, which are produced as a consequence of oxidative modification, are reductively detoxified by aldo-keto reductases (AKR) [7].

Figure 1. Antioxidative system and redox system in protection against oxidative stress and nitrosative stress. The antioxidative system primarily scavenges ROS. The redox system, in conjunction with antioxidative system, reductively supports cellular function by reducing oxidized molecules, thus minimizing dysfunction.

The significance of antioxidant and redox systems in the physiology and pathophysiology of male reproductive tissue and spermatozoa is currently well understood [4, 8-11]. Here, we overview the major antioxidative and redox systems that are currently assumed to play roles in the male reproductive system. Although AKR is generally not classified as a redox molecule, it plays an essential role in detoxify carbonyl compounds in conjunction with the redox system. Thus, the participation of AKR in male reproductive system is also discussed. Due to limited number of pages, only important papers are cited.

2 Antioxidant enzymes in male genital tract

The pivotal roles of nutritional antioxidants are clearly understood, as evidenced by the antioxidative function of vitamin E [12, 13], which is frequently used as a food additive as well as supplement to protect against oxidation. Vitamins A and C also have antioxidative functions [14]. Glutathione is a major, low molecular antioxidant produced by cells. These compounds react directly with ROS and decrease their toxicity. However, the stoichiometry of the reaction is generally one antioxidant to one reactive oxygen, which is not very efficient. In this regard, an enzymatic system is much more efficient. Although many enzymes can function as antioxidants [15, 16], the following have been recognized as the most popular members of the antioxidative enzymes and have been investigated in the male reproductive system.

The superoxide anion is produced by a one-electron reduction of an oxygen molecule and initiates a radical chain reaction. Since its discovery, it is believed that SOD, which dismutates superoxide anion to hydrogen peroxide and oxygen molecule, plays a central part in antioxidative reactions. Three isozymes are present in mammals. SOD1 encodes CuZnSOD that contains Cu and Zn as metal cofactors and is mostly cytosolic, while SOD2-encoding MnSOD is a mitochondrial isoform containing Mn. SOD3, encoding the extracellular form, is referred to as ECSOD, is structurally similar to CuZnSOD, and also contains Cu and Zn as metal cofactors.

Although many studies have been reported on CuZnSOD since its discovery, there are few papers that suggest its involvement in the male reproductive system. While the SOD1-deficient female is infertile [17, 18], no phenotype is known in the male reproductive system. A pivotal role of SOD in protection of testicular cells against heat stress-induced apoptosis has been demonstrated in vivo and in vitro [19, 20]. Since allopurinol, an inhibitor for xanthine oxidase, effectively suppresses damage by cryptorchidism, the inappropriate activation of xanthine oxidase may be a source of harmful superoxide. We recently found that allopurinol also suppresses germ cell apoptosis induced by heat stress, when incubated at 43 for 15 min (Matsuki et al., manuscript in preparation). The target of ROS appears to be mitochondria. If xanthine oxidase is a source of superoxide, a cytosolic enzyme, CuZnSOD would function as a primary scavenging enzyme. We are currently investigating this possibility by using SOD1-knockout mice in a heat-stress model.

MnSOD is a mitochondrial isoform, but its gene, SOD2, is encoded by nuclear DNA. SOD2 is inducible under various oxidative stress and inflammatory conditions and, hence, the regulatory mechanism of the gene has been extensively examined. Nuclear factor kappa-B (NF-kB) appears to be the most important transactivating factor responsible for gene induction [21]. Homozygous SOD2-deficient mice suffer severe cardiovascular damage and die soon after birth [22, 23]. No abnormality in the genital tract has been reported for heterozygous mice although some organs of these mice are susceptible to oxidative stress. On the contrary, transgenic male mice that express higher levels of MnSOD are infertile, but the mechanism for this is unknown [24]. Since SOD only dismutates superoxide anion to hydrogen peroxide, the resulting hydrogen peroxide may also cause a toxic effect in testicular cells.

ECSOD, which was originally reported as an extracellular isoform in the lung, is present at high levels in the epididymis [25]. ECSOD is also localized in the nuclei in the seminiferous tubules of testis [26]. In this case, the carboxyl terminal stretch of basic amino acids, which bind heparan sulfate to vascular endothelial cells, appears to function as a nuclear import signal. Erectile function is improved by transferring the SOD3 gene to the penis in aged rats [27]. Scavenging superoxide elongates the half-life of nitric oxide (NO), which results in an increase in cGMP levels. It is probable that the elevation of cGMP, which is caused by prolonged NO, relaxes vascular smooth muscle and improves erectile responses. However, no recognizable phenotype in the reproductive system has yet been reported in SOD3 knockout mice [28].

Glutathione peroxidases detoxify various peroxides using the reduced form of glutathione (GSH) as an electron donor and constitute a large family of groups [29]. These enzymes are classified into two groups in terms of the active site amino acid: one of which contains selenocysteine (Sec) at its active center, while the other does not. Here we only describe the former group, because the latter group exhibits a lower activity and GPX, in the narrow sense, indicates the former group.

Since selenium (Se) deficiency is related to male infertility, the relationship between GPX activity and male fertility has been debated [30]. At least four isozymes belong to selenium-containing GPX in mammals. The cytosolic form, GPX1, is widely distributed in tissues and has been most extensively investigated [31]. GPX1, like other antioxidative enzymes, prevents apoptosis induced by oxidative stress and other stimuli [32]. However, GPX1-knockout mice show no abnormality in phenotype including reproductive capability [33]. GPX2 and GPX3 are gastrointestinal and plasma forms, respectively, and a number of studies on them regarding the reproductive process have appeared.

GPX4 encodes an isoform that specifically detoxifies phospholipid hydroperoxide and is expressed at high levels in the testis. Thus, a defect in GPX4 had been suspected as the source of infertility caused by the Se deficiency, although direct evidence for its requirement had been missing for a long period. That GPX4 protein represents about 50 % of the capsule material, which embeds the helix of mitochondria in the midpiece of spermatozoa, has been demonstrated recently [34]. A correlation between male infertility and a GPX4 defect has actually been reported [35, 36]. GPX4-knockout mice show premature embryonal death in the uteri, but the direct cause of the death is not clear [37, 38]. A novel member, which has a high sequence identity to GPX4 except for the N-terminal region, is specifically present in sperm nuclei and is considered to act as a protamine thiol peroxidase [39]. However, the isoform is actually encoded by the same gene as GPX4 and is produced by an alternate promoter and exon usage [40].

GPX5 is a non-selenium enzyme and, hence, should be classified into the nonselenium-dependent GPX group. However, we describe it here because it is named so and is highly associated with the male reproductive system. GPX5 is expressed exclusively in the epididymis [41] and is secreted and present in the caput and cauda epididymides lumens [42]. It constitutes 6 % of the secretory epididymal proteins [43]. The binding of GPX5 to sperm membrane has also been reported. Thus, the protection of the sperm membrane against peroxidation is a possible function of this epididymis-specific isoform [44]. Since selenium deficiency causes male infertility and Sec-containing GPX is suspected to be a candidate for the defective molecule, non-selenium type GPX5 is speculated to serve as the back up enzyme for the Sec-containing GPX. The activity of nonselenium dependent GPX is low and, hence, its contribution as a GSH-dependent peroxide scavenger is ambiguous.

Catalase exclusively detoxifies hydrogen peroxide and has no electron donor requirement. Although CAT is a well-known antioxidative enzyme and has been implicated in protection against hydrogen peroxide, its localization is limited to the peroxisome. It plays a role in organs such as liver, but its specific function in male genital tract is unknown.

3 Roles of glutathione redox system

Glutathione is a tripeptidyl molecule and present in either the reduced (GSH) or the oxidized state (GSSG) by forming a disulfide bond between two molecules. Since it contains a reactive sulfhydryl group, GSH participates in preserving the intracellular milieu in a reduced state in addition to electron donation to GPX. GSH levels are maintained by two metabolic pathways: One is the de novo synthesis from its building blocks, Cys, Glu, and Gly and catalyzed by two enzymes, g-glutamylcy-steine synthetase (gGCS) and glutathione synthetase (GS). The other is recycling by glutathione reductase (GR) using NADPH as an electron donor. Glutathione is pumped out when it is oxidized or forms conjugates with cytotoxic compounds, including xenobiotic chemicals. Plasma glutathione is hydrolyzed to the individual amino acids by g-glutamyl transpeptidase (GGT), which is localized at the cellular surface as a membrane protein. The released amino acids are taken up by corresponding transporters and reused by cells [45]. Since glutathione administration improves male infertility, compounds that increase GSH levels may be used in the treatment of male infertility [46, 47].

The rate-limiting step of glutathione synthesis is the first reaction, which forms g-glutamylcysteine and is catalyzed by gGCS. GSH levels increase in response to various stimuli, which is mainly due to the responsiveness of gGCS gene expression to the stimuli [45]. gGCS is a heterodimer with a heavy catalytic subunit and a light regulatory subunit. The nuclear factor-kB (NF-kB) appears to be the most important transcriptional activator of the gGCS gene in response to oxidative stress [48]. gGCS catalyzes the formation of g-glutamylcysteine from Glu and Cys using ATP as an energy source. GS then completes the GSH synthesis by adding Gly in an ATP-dependent reaction. Thus, two ATP molecules are consumed in the synthesis of one GSH molecule. Buthionine sulfoximine (BSO), a specific inhibitor for gGCS, is commonly used to deplete intracellular GSH. Although BSO-treatment reduces GSH levels to less than 1 % in most cells, GSH-depleted cells generally appear to be healthy. This is because Cys levels increase in response to the low GSH as a compensatory reaction and maintain a reduced cellular state. Diethylstilbestrol (DES), which induces apoptosis in spermatogenic cells, decreases gGCS levels as well as GPX and glutathione S-transferase activity in the testis [49].

Once GSH is oxidized to GSSG, it is recycled by GR. When GR catalyzes the reduction of GSSG to GSH, NADPH is utilized as an electron donor. Since the recycling of GSSG is simple and beneficial from an energetic point of view, the reduction reaction is dominant in most tissues. GR is inhibited by nitrosative stress such as nitroso glutathione [50]. Nitrosourea (BCNU) is commonly used to inhibit GR in vivo as well as in vitro. Since GSH forms conjugates with xenobiotic chemicals and decreases their effect, the suppression of GR makes the drugs more effective. One of the anti-cancer functions of BCNU is, therefore, attributed to the inhibition of GR and the lowering of GSH levels.

We investigated the physiologic roles of GR in the male genital tract and the female reproductive system [51, 52]. GR was highly expressed in the epithelia of the epididymis, seminal vesicles, prostate gland and vas deferens. In case of testes, the GR activity in Sertoli cells was about 8-times that of spermatogenic cells. To investigate the contribution of de novo synthesis and the recycling of GSH to the intracellular glutathione pool, the levels of GSH and GSSG were measured in cultured Sertoli cells and spermatogenic cells after treatment with BSO and BCNU. GSH levels in spermatogenic cells that were about twice as high as those of Sertoli cells. After a 24-h incubation, the total glutathione levels in Sertoli cells was decreased to 8 % and 14 % of the control by BSO and BCNU, respectively. However, these agents were less potent in spermatogenic cells. The presence of BSO accelerated the spontaneous decline of GSH, but BCNU was not as effective as BSO. BCNU exerted a more toxic effect on Sertoli cells than on spermatogenic cells. The highly expressed GR in epithelial tissues of the male genital tract appears to supply GSH to sperma-tozoa.

In spermatogenic cells, Cys, which is used to synthesize protamine rather than GSH itself, is directly required for spermiogenesis and, hence, the participation of GR would be small. Since the glutathione content of spermatozoa is very low [53], supplementation of GSH from Sertoli cells is required for spermatogenic cells both as protection from ROS and as an amino acid source for spermatogenesis (Figure 2). Sertoli cells appear to provide GSH by a direct interaction [54]. The significant role of GSH from Sertoli cells in the supply of cysteine is supported by data on GGT-deficient mice [55]. The GGT-knockout mouse has reduced testis and seminal vesicle sizes and is severely oligozoospermic and infertile. The administration GSH or N-acetylcysteine, a membrane permeable precursor of cysteine, totally restores the testis and seminal vesicle to normal sizes and renders the mutant mice fertile. This indicates that GGT in the cell surface metabolizes glutathione to individual amino acids. The released cystine is then taken up by the spermatogenic cells and can be used for protamine biosynthesis.

Figure 2. Sertoli cells have both de novo synthesis and recycling systems of glutathione and supply GSH to spermatogenic cells. While Sertoli cells have both a de novo pathway for GSH synthesis and a recycling system of GSSG, spermatogenic cells have much less potency. A helper function of Sertoli cells is to supply GSH to spermatogenic cells.

Thioredoxin (Trx) is a small redox protein and is distributed from bacteria to mammals. It was originally identified as an electron donor for ribonucleotide reductase. Trx also functions to regulate various enzymes and transactivating factors of genes and is intimately involved in cell growth, differentiation, and death [5]. Moreover, Trx directly donates electrons to a new family of antioxidative proteins as described below and is involved in the detoxification of peroxides. After the donation of electrons, an intramolecular disulfide bond is formed in Trx. Trx reductase (TR) is involved in recycling the oxidized molecule using electrons from NADPH.

Ribonucleotide reductase activity is high in the testes because deoxyribonucleotides are a source for DNA replication accompanying spermatogenesis. Thus, one essential role of Trx in the testis is to provide reducing equivalents to ribonucleotide reductase as its originally reported function. Since Trx-knockout mice are embryonically lethal [56], other functions performed by Trx in male reproductive systems are not clear. Recently, members of the Trx family have increased. They are classified into two groups: one with a Trx domain exclusively and the other with a second domain in addition to the Trx domain. The former is composed of Trx-1 and Trx-2. The latter includes Trx-like proteins, Txr-1 and Txr-2 and spermatid-specific thioredoxins, Sptrx-1 and Sptrx-2 [57-60]. The presence of testis-specific Trx members implies a significant function for these proteins in spermatogenesis. Further studies will be required to clarify their actual roles.

The molecular structure and reaction mechanism of TR are similar to GR, but their substrate specificity is strict (Figure 3). Cytosolic TR (TR1) [61] and mitochondrial TR (TR3) [62] that have been cloned from mammals are structurally similar to each other, except for the signal peptide, which is specific to TR3 and required for mitochondrial import. The other TR member, TR2, has reducing activity for GSSG as well as oxidized Trx [63]. Evolutional fusion of the TR and glutaredoxin domains appears to enable this unusual characteristic of the enzyme.

Figure 3. Comparison of reaction mechanisms between glutathione reductase and thioredoxin reductase. Both GR and TR function in dimeric forms. The same subunit interacts in a head to tail conformation in these enzymes. TR, however, has a Sec residue that forms secondary redox center at the penultimate position.

Although bacteria have a Trx/TR system, their TR molecules are quite different in size and structure from that of mammals. The specificity of mammalian TR is supported by the C-terminal redox center. Although active center of GR is located in its N-terminal domain, mammalian TR has secondary redox center in addition to N-terminal active center at its C-terminal domain, which contains a Sec residue at the penultimate position. Mutation of the Sec or nearby Cys diminishes TR activity, indicating that these Sec and Cys residues coordinately form the second redox center required for the reduction of oxidized Trx [64]. The selenocysteine insertion sequence (SECIS), which is required for Sec insertion into the TGA codon, is present at its 3'-noncording region of mRNA [65] as is known for GPX and other selenoprotein mRNAs. Since the redox potential is high in reactive thiols of Trx, selenol (SeH), which is more reactive than thiol (SH), would be required for the reduction of oxidized Trx. Since TR also requires selenium for activity, a defect in TR activity, which is caused by a selenium deficiency, may also be related to male infertility.

Although Trx is involved in the reduction of oxidized proteins and other molecules, the direct reactivity of Trx to ROS is not high. It is well known that GSH donates electrons to peroxides via the catalytic activity of GPX as mentioned above. However, the presence of Trx-dependent peroxidase was not known. The enzyme family that exhibits Trx-dependent peroxidase activity has now been found and is formally referred to as peroxire-doxin (Prx). In addition to its enzymatic activity, various other functions have been attributed to Prx members.

Since details of this newly identified family were reviewed recently [66], we briefly mention the functions of its members that are involved in male reproduction. The up-regulation of Prx1 and Prx2 by radiation has been reported in mouse testis [67]. However, no specific function was implied for these family members in reproductive system to date. We examined Prx4 and Prx6 in male reproductive systems and found Prx4 especially unique [68-70]. Prx4 is the only member that has a signal peptide required for its secretion from cells [71]. In fact, it has been identified as a plasma form in most tissues. It binds heparan sulfate and, hence, would be expected to function to protect the vascular system from oxidative stress [72]. In immature testis, most of the Prx4 is present as the secreted form in which the signal peptide has been cleaved. However, in mature testis where spermatogenesis proceeds, Prx4 is present as a large precursor form with the intact signal peptide. Since immunoelectron microscopy identified a Prx4-bound, multilamellar structure in the cytosol, we hypothesize that this precursor form is actually involved in membrane remodeling during the spermiogenic process [66, 69]. Further study, such as gene disruption, will clarify the actual function of this unprocessed form of Prx4.

Carbonyl compounds, such as 4-hydroxynonenal and acrolein (Figure 4), that are produced by the oxidation of organic compounds, such as unsaturated fatty acids, are highly reactive. They modify the reactive sulfhydryl groups commonly present in redox molecules, such as GR [73], and result in the impairment of the systems. Mammals have several enzymatic systems that function to detoxify carbonyl compounds. The AKR family includes enzymes that reduce carbonyl groups to alcohol using NAD(P)H as an electron donor. Among the members of the AKR family, aldose reductase (AR), the AKR1B gene product, has been most extensively studied, because it is deeply involved in diabetic complications [74]. Aldehyde reductase (ALR), the AKR1A gene product, exhibits the highest similarity to AR among the family members [75, 76]. Since the substrate specificity of the family enzymes is similar to each other, an immunological method is convenient to specify and localize individual members of the family. While ALR is constitutively expressed, AR is induced under pathological conditions including diabetes and cancer, as well as oxidative stress [77, 78]. Since the metabolic rates of malignant cells are fast due to their rapid proliferation, AR induced in these cells would be required for protection against carbonyl compounds produced within these cells.

Figure 4. Structure of 4-hydroxynonenal and acrolein and their reduction by AKR. Carbonyl compounds such as 4-hydroxynonenal and acrolein can be reduced to corresponding alcohols by AKR in an NADPH-dependent manner.

The male genital tract is an organ that is under various stresses caused by activated cellular functions, such as steroidogenesis and continuous cell growth. ROS and, in consequence, carbonyl compounds can be produced by activated metabolism. Thus, the detoxification by AKR appears to contribute to maintain the potency of the genital tract. In fact, Sertoli cells and epithelia of the genital tract have high levels of AR and ALR [79]. In addition, an aldose reductase-like protein (AKR1B7), which is highly expressed in the vas deferens epithelium and zona fasciculata of the adrenal cortex, has been reported [80, 81]. The separate role of these enzymes in maintaining reproductive function is a matter of concern. Since steroid hormones and their derivatives have carbonyl groups and can serve as substrates for AKR [82], the enzymes that are highly expressed in Sertoli cells participate in their detoxification as well as other carbonyl compounds.

In addition to detoxification of harmful carbonyls, AR, together with sorbitol dehydrogenase (SDH), forms a potentially important metabolic pathway, the so-called polyol pathway. A function of SDH retained in spermatozoa appears to convert sorbitol to fructose in the seminal plasma and uterine fluid. Since SDH in conjunction with the AR bypath one of the rate-determining step of glycolysis [83], it would provide advantages to spermatozoa in terms of energy supply. Blood sugar is typically glucose in mammals, because most ingested sugar molecules are converted to glucose in the liver. However, fetal blood in ungulates contains significant amounts of fructose. We recently demonstrated the colocalization of AR and SDH in male and female genital tract [79, 84]. In the testes, however, their localization does not match correspondingly. SDH is known to correlate with the maturation of testes and is present at high concentrations in spermatogenic cells. AR, on the other hand, is high in the Sertoli cells. The individual function for these enzymes is currently unclear. Since the detoxification of carbonyls is activated by the binding of GSH [85], there is also crosstalk between the GSH redox system and AKR.

Although histones, which contain few cysteine residues, stabilize DNA, they are substituted by prota-mines, which are rich in cysteine and arginine in sperma-tozoa. The proper oxidation of protamines is known to protect sperm DNA against oxidative damage during the mating process [86], although this appears to be para-doxical. In fact, protamines in the nuclei of spermatozoa are oxidatively cross-linked via disulfide bonds among nearby sulfhydryl groups. Flavin-dependent sulfhydryl oxidation in seminal vesicles may participate in disulfide bond formation [87]. This makes nuclei packed into about one-twentieth the size and enables quick movement for spermatozoa. After mating, the male pronucleus is reductively activated in the egg [88]. GSH may participate in pronucleus activation [49]. Thus, the mammalian reproduction process fully depends on a redox system.

About 15 % of couples worldwide remain childless because of infertility [22]. Developing IVF/ICSI techniques enable infertile couples to have a chance to have a baby. On the other hand, the mechanism of the spermatogenic process and infertility remain largely unknown and further study is clearly required. Spermatogenic cells are prone to undergo natural apoptosis, and apoptotic cell death is stimulated by various stress conditions, including heat exposure [89]. This is explained by means of the quality control of genetic material, DNA, from mutation caused by damage. However, excess apoptosis would cause infertility due to germ cell loss. Since the production of ROS is high in the reproductive tissue, the primary response of spermatogenic cells is scavenging by antioxidative systems, such as SOD and GPX. When ROS levels exceed the scavenging capacity of the system, they impair susceptible molecules such as polyunsaturated fatty acids. A redox system, under such situations, can repair oxidized and damaged molecules by the corresponding enzymes with NADPH as an original electron source. Since NADPH is shared in many systems other than glutathione or Trx redox systems (Figure 5), an abrupt consumption causes a redox imbalance and results in the impairment of cells, including death [90]. Thus, it is desirable to minimize harsh oxidative stress and to maintain a high redox potential to maintain the reproductive systems in a healthy state.

Figure 5. Crosstalk among antioxidative/redox systems via the NADPH pool. Although there are many redox systems that depend their redox potential on NADPH, only three systems are presented here. Since NADPH is origin of electrons, its levels determine the antioxidative/redox capacity of cells. A main source for NADPH is the pentose phosphate pathway. Severe oxidative stress depletes the NADPH pool and leads to dysfunction of entire system.

Supported in part by a Grant-in-Aid for Scientific Research (C) (No. 13670111), Young Scientists (B) (No. 14770797) and JSPS Fellows (No. 13007321) from the Japan Society for the Promotion of Science (JSPS).

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Correspondence to: Dr. Junichi Fujii, Department of Biochemistry, Yamagata University School of Medicine, 2-2-2 Iidanishi, Yamagata 990 9585, Japan.
Tel: +81-23-628 5227, Fax: +81-23-628 5230
E-mail: jfujii@med.id.yamagata-u.ac.jp
Received 2003-06-18  Accepted 2003-07-24

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