Distribution of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in rat testis mitochondria

A white paper on Phospholipid Hydroperoxide Glutathione Peroxidase (GPx4) forty years later

The purification of a protein inhibiting lipid peroxidation led to the discovery of the selenoperoxidase GPx4 forty years ago. Thus, the evidence of the enzymatic activity was reached after identifying the biological effect and unambiguously defined the relationship between the biological function and the enzymatic activity. In the syllogism where GPx4 inhibits lipid peroxidation and its inhibition is lethal, cell death is operated by lipid peroxidation. Based on this rationale, this form of cell death emerged as regulated iron-enforced oxygen toxicity and was named ferroptosis in 2012. In the last decades, we learned that reduction of lipid hydroperoxides is indispensable and, in cooperation with prooxidant systems, controls the critical steady state of lipid peroxidation. This concept defined the GPx4 reaction as both the target for possible anti-cancer therapy and if insufficient, as cause of degenerative diseases. We know the reaction mechanism, but the details of the interaction at the membrane cytosol interface are still poorly defined. We know the gene structure, but the knowledge about expression control is still limited. The same holds true for post-transcriptional modifications. Reverse genetics indicate that GPx4 has a role in inflammation, immunity, and differentiation, but the observations emerging from these studies need a more specifically addressed biochemical evidence. Finally, the role of GPx4 in spermatogenesis disclosed an area unconnected to lipid peroxidation. In its mitochondrial and nuclear form, the peroxidase catalyzes the oxidation of protein thiols in two specific aspects of sperm maturation: stabilization of the mid-piece and chromatin compaction. Thus, although available evidence converges to the notion that GPx4 activity is vital due to the inhibition of lipid peroxidation, it is reasonable to foresee other unknown aspects of the GPx4 reaction to be disclosed.

Lipid peroxidation and ferroptosis: The role of GSH and GPx4

Ferroptosis (FPT) is a form of cell death due to missed control of membrane lipid peroxidation (LPO). According to the axiomatic definition of non-accidental cell death, LPO takes place in a scenario of altered homeostasis. FPT, differently from apoptosis, occurs in the absence of any known specific genetically encoded death pathway or specific agonist, and thus must be rated as a regulated, although not "programmed", death pathway. It follows that LPO is under a homeostatic metabolic control and is only permitted when indispensable constraints are satisfied and the antiperoxidant machinery collapses. The activity of the selenoperoxidase Glutathione Peroxidase 4 (GPx4) is the cornerstone of the antiperoxidant defence. Converging evidence on both mechanism of LPO and GPx4 enzymology indicates that LPO is initiated by alkoxyl radicals produced by ferrous iron from the hydroperoxide derivatives of lipids (LOOH), traces of which are the unavoidable drawback of aerobic metabolism. FPT takes place when a threshold has been exceeded. This occurs when the major conditions are satisfied: i) oxygen metabolism leading to the continuous formation of traces of LOOH from phospholipid-containing polyunsaturated fatty acids; ii) missed enzymatic reduction of LOOH; iii) availability of ferrous iron from the labile iron pool. Although the effectors impacting on homeostasis and leading to FPT in physiological conditions are not known, from the available knowledge on LPO and GPx4 enzymology we propose that it is aerobic life itself that, while supporting bioenergetics, is also a critical requisite of FPT. Yet, when the homeostatic control of the steady state between LOOH formation and reduction is lost, LPO is activated and FPT is executed.

Reversal of mitochondrial proteomic loss in Type 1 diabetic heart with overexpression of phospholipid hydroperoxide glutathione peroxidase

Mitochondrial dysfunction is a contributor to diabetic cardiomyopathy. Previously, we observed proteomic decrements within the inner mitochondrial membrane (IMM) and matrix of diabetic cardiac interfibrillar mitochondria (IFM) correlating with dysfunctional mitochondrial protein import. The goal of this study was to determine whether overexpression of mitochondria phospholipid hydroperoxide glutathione peroxidase 4 (mPHGPx), an antioxidant enzyme capable of scavenging membrane-associated lipid peroxides in the IMM, could reverse proteomic alterations, dysfunctional protein import, and ultimately, mitochondrial dysfunction associated with the diabetic heart. MPHGPx transgenic mice and controls were made diabetic by multiple low-dose streptozotocin injections and examined after 5 wk of hyperglycemia. Five weeks after hyperglycemia onset, in vivo analysis of cardiac contractile function revealed decreased ejection fraction and fractional shortening in diabetic hearts that was reversed with mPHGPx overexpression. MPHGPx overexpression increased electron transport chain function while attenuating hydrogen peroxide production and lipid peroxidation in diabetic mPHGPx IFM. MPHGPx overexpression lessened proteomic loss observed in diabetic IFM. Posttranslational modifications, including oxidations and deamidations, were attenuated in diabetic IFM with mPHGPx overexpression. Mitochondrial protein import dysfunction in diabetic IFM was reversed with mPHGPx overexpression correlating with protein import constituent preservation. Ingenuity Pathway Analyses indicated that oxidative phosphorylation, tricarboxylic acid cycle, and fatty acid oxidation processes most influenced in diabetic IFM were preserved by mPHGPx overexpression. Specific mitochondrial networks preserved included complex I and II, mitochondrial ultrastructure, and mitochondrial protein import. These results indicate that mPHGPx overexpression can preserve the mitochondrial proteome and provide cardioprotective benefits to the diabetic heart.

Mitochondrial thiols in antioxidant protection and redox signaling: distinct roles for glutathionylation and other thiol modifications

SIGNIFICANCE

The mitochondrial matrix contains much of the machinery at the heart of metabolism. This compartment is also exposed to a high and continual flux of superoxide, hydrogen peroxide, and related reactive species. To protect mitochondria from these sources of oxidative damage, there is an integrated set of thiol systems within the matrix comprising the thioredoxin/peroxiredoxin/methionine sulfoxide reductase pathways and the glutathione/glutathione peroxidase/glutathione-S-transferase/glutaredoxin pathways that in conjunction with protein thiols prevent much of this oxidative damage. In addition, the changes in the redox state of many components of these mitochondrial thiol systems may transduce and relay redox signals within and through the mitochondrial matrix to modulate the activity of biochemical processes.

RECENT ADVANCES

Here, mitochondrial thiol systems are reviewed, and areas of uncertainty are pointed out, focusing on recent developments in our understanding of their roles.

CRITICAL ISSUES

The areas of particular focus are on the multiple, overlapping roles of mitochondrial thiols and on understanding how these thiols contribute to both antioxidant defenses and redox signaling.

FUTURE DIRECTIONS

Recent technical progress in the identification and quantification of thiol modifications by redox proteomics means that many of the questions raised about the multiple roles of mitochondrial thiols can now be addressed.

Mitochondrial oxidant stress triggers cell death in simulated ischemia-reperfusion

To clarify the relationship between reactive oxygen species (ROS) and cell death during ischemia-reperfusion (I/R), we studied cell death mechanisms in a cellular model of I/R. Oxidant stress during simulated ischemia was detected in the mitochondrial matrix using mito-roGFP, a ratiometric redox sensor, and by Mito-Sox Red oxidation. Reperfusion-induced death was attenuated by over-expression of Mn-superoxide dismutase (Mn-SOD) or mitochondrial phospholipid hydroperoxide glutathione peroxidase (mito-PHGPx), but not by catalase, mitochondria-targeted catalase, or Cu,Zn-SOD. Protection was also conferred by chemically distinct antioxidant compounds, and mito-roGFP oxidation was attenuated by NAC, or by scavenging of residual O(2) during the ischemia (anoxic ischemia). Mitochondrial permeability transition pore (mPTP) oscillation/opening was monitored by real-time imaging of mitochondrial calcein fluorescence. Oxidant stress caused release of calcein to the cytosol during ischemia, a response that was inhibited by chemically diverse antioxidants, anoxia, or over-expression of Mn-SOD or mito-PHGPx. These findings suggest that mitochondrial oxidant stress causes oscillation of the mPTP prior to reperfusion. Cytochrome c release from mitochondria to the cytosol was not detected until after reperfusion, and was inhibited by anoxic ischemia or antioxidant administration during ischemia. Although DNA fragmentation was detected after I/R, no evidence of Bax activation was detected. Over-expression of the anti-apoptotic protein Bcl-X(L) in cardiomyocytes did not confer protection against I/R-induced cell death. Moreover, murine embryonic fibroblasts with genetic depletion of Bax and Bak, or over-expression of Bcl-X(L), failed to show protection against I/R. These findings indicate that mitochondrial ROS during ischemia triggers mPTP activation, mitochondrial depolarization, and cell death during reperfusion through a Bax/Bak-independent cell death pathway. Therefore, mitochondrial apoptosis appears to represent a redundant death pathway in this model of simulated I/R. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Cloning, characterization, and expression analysis of goat (Capra hircus) phospholipid hydroperoxide glutathione peroxidase (PHGPx)

Phospholipid hydroperoxide glutathione peroxidase (PHGPx), as a ubiquitous antioxidant enzyme in the glutathione peroxidases (GPx) family, plays multiple roles in organisms. However, there is very little information on PHGPx in goats (Capra hircus). In this study, a full-length cDNA was cloned and characterized from Taihang black goat testes. The 844 bp cDNA contains an open reading frame (ORF) of 597 bp. The goat PHGPx nucleotide sequence contains a selenocysteine (sec) codon TGA^244-246, two potential start codons ATG^20-22 and ATG^108-110, a polyadenylation signal AATAAA^813-818 and selenocysteine insertion sequence (SECIS) motif AUGA^688-691, UGA^729-731 and AAA^703-705. As a selenoprotein, the active-site motifs and GPx family signature motifs LAFPCNQF^101-108 and WNFEK^165-170 were also found. The order of PHGPx mRNA expression levels was: testes >> heart > brain > epididymis > kidney > liver > lung > spleen > muscle. Real-time PCR and immunohistochemistry results revealed similar expression differences in different age testes, with high expression levels during adolescence. Immunofluorescence results suggested that PHGPx mainly expressed in Leydig cells and spermatids in mature goat testes.

Mitochondria-specific transgenic overexpression of phospholipid hydroperoxide glutathione peroxidase (GPx4) attenuates ischemia/reperfusion-associated cardiac dysfunction

Ischemia/reperfusion (I/R) injury elicits damage to mitochondria. Antioxidants provide protection from I/R-induced mitochondrial damage. The goal of this study was to determine the impact of mitochondria-specific overexpression of GPx4 (PHGPx) on cardiac function following I/R. Transgenic mice were created in which PHGPx was overexpressed solely in the mitochondrion (mPHGPx). MPHGPx and littermate control hearts were subjected to global no-flow ischemia (20 min) followed by reflow reperfusion (30, 60, and 90 min). Following I/R, mPHGPx hearts possessed significantly better rates of contraction, developed pressures, and peak-systolic pressures as compared to controls (P<0.05). No differences were observed in rates of relaxation or end-diastolic pressures. Lipid peroxidation was significantly lower in mitochondria from mPHGPx hearts as compared to controls, following I/R (P<0.05). Electron transport chain (ETC) complex I, III, and IV activities were significantly higher in mPHGPx hearts as compared to controls, following I/R (P<0.05). MPHGPx overexpression enhanced ETC complex I, III, and IV activities in subsarcolemmal mitochondria (SSM; P<0.05), and ETC complex I and III activities in interfibrillar mitochondria (IFM; P<0.05) following I/R. These results indicate that mitochondria-specific GPx4 overexpression protects cardiac contractile function and preserves ETC complex activities following I/R. These results provide further rationale for the use of mPHGPx as a therapeutic protectant.

Analysis of age-associated changes in mitochondrial free radical generation by rat testis

Throughout spermatogenesis, mitochondria undergo a morphological and functional differentiation. Mitochondria are involved in the production of reactive oxygen species (ROS), considered one of the mediators of ageing. Particularly, lipid peroxidation is regarded as a major phenomenon by which ROS can impair cellular function. In the present study, we examined the production of superoxide anion, superoxide dismutase activity and the effect of Fe(2+)/ascorbate induced-lipid peroxidation on the respiratory chain activities of testis mitochondria throughout the process of spermatogenesis and ageing. Mitochondria from rat testes generated superoxide anion, mainly using NADH as substrate, which increased according to age. The activity of SOD is age-dependent and greatly stimulated during the first wave of spermatogenesis, but decreases in adulthood and old age. TBARS concentration was also markedly increased by ageing. The activity of mitochondrial respiratory chain complexes is differentially affected by oxidative stress induced by iron/ascorbate, succinate-dehydrogenase activity being less vulnerable than that of NADH-dehydrogenase and cytochrome c oxidase. The data suggest that ageing is accompanied by reduced activity of SOD, leading to excessive oxidative stress and enhanced lipid peroxidation that compromises the functionality of the electron transport chain. The data support the concept that mitochondrial function is an important determinant in ageing.

Elucidation of the Mechanism of Selenoprotein Glutathione Peroxidase (GPx)-Catalyzed Hydrogen Peroxide Reduction by Two Glutathione Molecules: A Density Functional Study

The mechanism of the hydrogen peroxide reduction by two molecules of glutathione catalyzed by the selenoprotein glutatione peroxidase (GPx) has been computationally studied. It has been shown that the first elementary reaction of this process, (E-SeH) + H(2)O(2) --> (E-SeOH) + H(2)O (1), proceeds via a stepwise pathway with the overall barrier of 17.1 kcal/mol, which is in good agreement with the experimental barrier of 14.9 kcal/mol. During reaction 1, the Gln83 residue has been found to play a key role as a proton acceptor, which is consistent with experiments. The second elementary reaction, (E-SeOH) + GSH --> (E-Se-SG) + HOH (2), proceeds with the barrier of 17.9 kcal/mol. The last elementary reaction, (E-Se-SG) + GSH --> (E-SeH) + GS-SG (3), is initiated with the coordination of the second glutathione molecule. The calculations clearly suggest that the amide backbone of the Gly50 residue directly participates in this reaction and the presence of two water molecules is absolutely vital for the reaction to occur. This reaction proceeds with the barrier of 21.5 kcal/mol and is suggested to be a rate-determining step of the entire GPx-catalyzed reaction H(2)O(2) + 2GSH --> GS-SG + 2H(2)O. The results discussed in the present study provide intricate details of every step of the catalytic mechanism of the GPx enzyme and are in good general agreement with experimental findings and suggestions.

Glutathionylation of mitochondrial proteins

Many proteins contain free thiols that can be modified by the reversible formation of mixed disulfides with low-molecular-weight thiols through a process called S-thiolation. As the majority of these modifications result from the interaction of protein thiols with the endogenous glutathione pool, protein glutathionylation is the predominant alteration. Protein glutathionylation is of significance both for defense against oxidative damage and in redox signaling. As mitochondria are at the heart of both oxidative damage and redox signaling within the cell, the glutathionylation of mitochondrial proteins is of particular importance. Here we review the mechanisms and physiological significance of the glutathionylation of mitochondrial thiol proteins.

Role of mitochondrial cardiolipin peroxidation in apoptotic photokilling of 5-aminolevulinate-treated tumor cells

In 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), ALA taken up by tumor cells is metabolized to protoporphyrin IX (PpIX), which sensitizes photodamage leading to apoptotic or necrotic cell death. Since lipophilic PpIX originates in mitochondria, we postulated that photoperoxidation of highly unsaturated cardiolipin (CL), which anchors cytochrome c (cyt c) to the inner membrane, is an early proapoptotic event. As initial evidence, PpIX-sensitized photooxidation of liposomal CL to hydroperoxide (CLOOH) species precluded cyt c binding, but this could be reinstated by GSH/selenoperoxidase (GPX4) treatment. Further support derived from site-specific effects observed using (i) a mitochondrial GPX4-overexpressing clone (7G4) of COH-BR1 tumor cells, and (ii) an ALA treatment protocol in which most cellular PpIX is either inside (Pr-1) or outside (Pr-2) mitochondria. Sensitized cells were exposed to a lethal light dose, and then analyzed for death mechanism and lipid hydroperoxide (LOOH) levels. Irradiated Pr-1 vector control (VC) cells died apoptotically following cyt c release and caspase-3 activation, whereas 7G4 cells were highly resistant. Irradiated Pr-2 VC and 7G4 cells showed negligible cyt c release or caspase-3 activation, and both types died via necrosis. CLOOH (detected long before cyt c release) accumulated approximately 70% slower in Pr-1 7G4 cells than in Pr-1 VC, and this slowdown exceeded that of all other LOOHs. These and related findings support the hypothesis that CL is a key upstream target in mitochondria-dependent ALA-PDT-induced apoptosis.

Initiation of apoptotic signal by the peroxidation of cardiolipin of mitochondria

Overexpression of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in mitochondria of RBL2H3 cells (M15 cells) prevented the release of cytochrome c (cyt.c), the activation of caspase-3, and apoptosis caused by 2-deoxyglucose (2DG), whereas cells overexpressing nonmitochondrial PHGPx(L9) and control (S1) cells were induced to apoptosis. Hydro-peroxide levels in mitochondria of L9 and S1 cells were significantly enhanced by 2DG-induced apoptosis. In contrast, generation of hydroperoxide in mitochondria was protected in M15 cells, which also showed resistance to apoptosis by etoposide, staurosporine, UV irradiation, cycloheximide, and actinomycin D, stimuli that induce apoptosis by the liberation of cyt.c from mitochondria. Cyt.c preferentially binds to the monolayer of cardiolipin (CL), the specific phospholipid of the inner membrane of mitochondria. The amount of cyt.c bound to the monolayer of cardiolipin hydroperoxide (CL-OOH) was much lower than that bound to CL. Cyt.c bound to liposome containing CL was released by peroxidation with a radical initiator. Adenine nucleotide translocator (ANT), which regulates the opening and closing the permeability transition (PT) pore, potentially was inactivated in apoptosis-induced S1 cells 4 h after the addition of 2DG, coincidentally with cyt.c release from mitochondria. ANT activity was suppressed by the fusion of isolated mitochondria with liposomes containing CL-OOH. ANT activity was expressed in proteoliposomes containing 10% CL, but it was competitively inhibited by the addition of CL-OOH. This study suggests that CL peroxidation might have an initiating role in the liberation of cyt.c from the inner membrane, and in the opening of the PT pore via inactivation of ANT. Mitochondrial PHGPx might play a role as an anti-apoptotic factor by protecting CL and reducing CL-OOH.

Effects of 17beta-estradiol and tamoxifen on the selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx) mRNA expression in male reproductive organs of rats

The selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx) is highly expressed in testes under gonadotropin control. The expression patterns of PHGPx mRNA by 17beta-estradiol (E2) as an estrogen and tamoxifen (Tam) as an estrogen antagonist were investigated in the reproductive organs of male rats. Twelve-week-old male Sprague-Dawley rats were subcutaneously injected with E2 (7.5 microg/kg/day) or Tam (5 mg/kg/day) for 1 week. The E2 treatment significantly increased the levels of PHGPx mRNA in both testes and prostates, whereas the Tam treatment significantly decreased the levels of PHGPx mRNA, compared to the vehicle control (p<0.01). The treatment with E2 or Tam slightly decreased the levels of PHGPx mRNA in epididymides. In histopathological examination, severe vacuolization and depletion of germ cells in the seminiferous tubules, cell debris in the tubular lumen, and mild proliferative changes in interstitial tissues were observed in the testes of Tam-treated rats, whereas only mild spermatogonial proliferation was observed in the seminiferous tubules of E2-treated rats. There were no typical histopathological changes in the epididymides of any of the laboratory rats but mild epithelial proliferation in the prostates of E2- and Tam-treated rats. These results suggest that PHGPx mRNA expression may be influenced by estrogen in the male reproductive organs.

Spatiotemporal changes of levels of a moonlighting protein, phospholipid hydroperoxide glutathione peroxidase, in subcellular compartments during spermatogenesis in the rat testis

We studied temporal changes in the subcellular localization and levels of a moonlighting protein, phospholipid hydroperoxide glutathione peroxidase (PHGPx), in spermatogenic cells and mature sperm of the rat by immunofluorescence and immunoelectron microscopy. The PHGPx signals were detected in chromatoid bodies, clear nucleoplasm, mitochondria-associated material, mitochondrial aggregates, granulated bodies, and vesicles in residual bodies in addition to mitochondria, nuclei, and acrosomes as previously reported. Within mitochondria, PHGPx moved from the matrix to the outermost membrane region in step 19 spermatid, suggesting that this spatiotemporal change is synchronized with the functional change of PHGPx in mitochondria. In the nucleus, PHGPx was associated with electron-lucent spots and with the nuclear envelope, and PHGPx in the latter region increased after step 16. In early pachytene spermatids, PHGPx signals were noted in the nuclear material exhibiting a very similar density to chromatoid bodies and in the intermitochondrial cement, supporting the previous proposal that chromatoid bodies originate from the nucleus and intermitochondrial cement. The presence of PHGPx in such various compartments suggested versatile roles for this protein in spermatogenesis. Quantitative immunoelectron microscopic analysis also revealed dynamic changes in the labeling density of PHGPx in different subcellular compartments as follows: 1). Total cellular PHGPx rapidly increased after step 5 and reached a maximum at step 18; 2). mitochondrial labeling density increased after step 1 and achieved a maximum in steps 15-17; 3). nuclear labeling density suddenly increased in steps 12-14 to a maximum; 4). in cytoplasmic matrix, the density remained low in all steps; and 5). the labeling density in chromatoid bodies gradually decreased from pachytene spermatocytes to spermatids at step 18. These spatiotemporal changes in the level of PHGPx during the differentiation of spermatogenic cells to sperm infer that PHGPx plays a diverse and important biological role in spermatogenesis.

Phospholipid hydroperoxide glutathione peroxidase induces a delay in G1 of the cell cycle

Phospholipid hydroperoxide glutathione peroxidase (PhGPx) is an antioxidant enzyme that reduces cellular phospholipid hydroperoxides (PLOOHs) to alcohols. Cellular peroxide tone has been implicated in cell growth and differentiation. By reducing the PLOOH level in the cell membrane, PhGPx regulates the peroxide tone and thereby might be involved in cell growth. We hypothesized that overexpression of PhGPx in human breast cancer cells would decrease their growth rate. We stably transfected MCF-7 cells (Wt) with L-PhGPx and measured cell doubling time, plating efficiency, and cell cycle phase transit times. P-4 cells (8-fold increase in PhGPx activity) showed a 2-fold increase in doubling time; doubling time increased directly with PhGPx activity (r = 0.95). The higher the PhGPx activity, the lower the plating efficiency (r = -0.86). The profile of other antioxidant enzymes was unchanged. Overexpression of PhGPx lowered the steady-state level of PLOOH (by > 60%). Results from bromodeoxyuridine pulse-chase experiments and flow cytometry indicate that PhGPx induced a delay in MCF-7 proliferation that was primarily due to a slower progression from G1 to S. These results support the hypothesis that PhGPx plays a regulatory role in the progression of MCF-7 cells from G1 to S possibly by regulating the steady-state levels of PLOOH. These data suggest that PhGPx can lower the peroxide tone, which might change the cellular redox environment resulting in a delay in G1 transit. Thus, PhGPx could be an important factor in cell growth.

Interactions of mitochondrial thiols with nitric oxide

The interaction of nitric oxide (NO) with mitochondria is of pathological significance and is also a potential mechanism for the regulation of mitochondrial function. Some of the ways in which NO may affect mitochondria are by reacting with low-molecular-weight thiols such as glutathione and with protein thiols. However, the detailed mechanisms and the consequences of these interactions for mitochondria are uncertain. Here we review mitochondrial thiol metabolism, outline how NO and its metabolites interact with thiols, and discuss the implications of these reactions for mitochondrial and cell function.

Protection from inactivation of the adenine nucleotide translocator during hypoglycaemia-induced apoptosis by mitochondrial phospholipid hydroperoxide glutathione peroxidase

We demonstrated that mitochondrial phospholipid hydroperoxide glutathione peroxidase (PHGPx) first suppressed the dissociation of cytochrome c (cyt c) from cardiolipin (CL) in mitochondrial inner membranes and then apoptosis caused by the hypoglycaemia by the prevention of peroxidation of CL [Nomura, Imai, Koumura, Arai and Nakagawa (1999) J. Biol. Chem. 274, 29294-29302; Nomura, Imai, Koumura, Kobayashi and Nakagawa (2000) Biochem. J. 351, 183-193]. The present study shows the involvement of peroxidation of CL in the inactivation of adenine nucleotide translocator (ANT) and the opening of permeability transition pores by using the system of ANT-reconstituted liposome and isolated mitochondria. ANT activity appeared in dioleoyl phosphatidylcholine proteoliposome containing 10% (mol/mol) CL or phosphatidylglycerol (PG), but not other classes of phospholipids. ANT activity was competitively inhibited by the addition of cardiolipin hydroperoxide (CLOOH) in reconstituted liposomes containing CL. However, phosphatidylcholine hydroperoxide failed to inactivate the activity of ANT. The activity of ANT in reconstituted liposomes, including CLOOH, recovered when CLOOH in reconstituted liposome was reduced to hydroxycardiolipin by incubation with PHGPx. The activity of ANT was determined in rat basophil leukaemia RBL2H3 cells after their exposure to 2-deoxyglucose. ANT activity decreased to 50% of the control level by 4 h in response to apoptosis. In parallel, cyt c and apoptosis-inducing factor (AIF) were released from mitochondria. Suppression of the accumulation of CLOOH by overexpression of PHGPx in mitochondria effectively prevented the inactivation of ANT, the opening of permeability transition pores and the release of cyt c and AIF from mitochondria in hypoglycaemia-induced apoptotic cells. These findings suggest that mitochondrial PHGPx might be involved in the modulation of the activity of ANT and the opening of pores for the release of cyt c via the modulation of levels of CLOOH in the mitochondria.

Cloning and expression analysis of chicken phospholipid-hydroperoxide glutathione peroxidase

Phospholipid-hydroperoxide glutathione peroxidase (GPX4 or PHGPX) is a unique selenium dependent glutathione peroxidase that reduces phospholipid, cholesterol, and cholesteryl ester hydroperoxides. Phospholipid-hydroperoxide glutathione peroxidase has been shown to exist as both a 197 amino acid mitochondrial targeting protein and as a 170 amino acid non-mitochondrial protein. The cDNA encoding the non-mitochondrial chicken GPX4 (cGPX4) has been isolated from an immortalized DF-1 chicken embryonic fibroblast (CEF) cell line cDNA library. The nucleotide sequence of cGPX4 was 802 bp in length with an open reading frame (ORF) that encoded 170 amino acids but lacked the N-terminal domain that encoded the mitochondrial leader sequence (MLS). Chicken non-mitochondrial GPX4 was highly expressed in brain and stromal tissues. Surprisingly, it was found that ovarian stromal tissue cGPX4 expression is regulated quite differently according to the reproductive status of the bird, suggesting that GPX4 may play an important role in reproduction in response to steroid hormones, in addition to its general antioxidant functions.

Differential splicing of the phospholipid hydroperoxide glutathione peroxidase gene in diploid and haploid male germ cells in the rat

Phospholipid hydroperoxide glutathione peroxidase (PHGPx, 20 kDa) and sperm nuclei glutathione peroxidase (snGPx, 34 kDa) are two selenoproteins present in mammalian testis and epididymal spermatozoa. They originate from the differential splicing of the PHGPx gene and appear to play important roles in sperm physiology. To determine the stages of spermatogenesis in which they are present, we compared the expression pattern of these two enzymes in highly purified populations of germ cells during specific phases of differentiation. In Northern and Western blotting experiments, both PHGPx transcript and protein were markedly expressed in pachytene spermatocytes and round spermatids. In contrast, the testis-specific snGPx was detected at both the mRNA and protein level only in haploid round spermatids. Accordingly, the developmental analysis of testicular RNAs from rats of different ages first revealed the appearance of PHGPx and snGPx transcripts at Day 20 and Day 30, respectively. Furthermore, both meiotic and postmeiotic cells contained catalytically active PHGPx/snGPx, with higher activity in the haploid cells. The intracellular distribution of PHGPx in mitochondria and nuclei of meiotic cells was demonstrated by immunocytochemical electron microscopy and Western blotting. These findings provide evidence that the PHGPx gene is differentially spliced during the meiotic prophase and haploid cell phases of spermatogenesis.

Trends in selenium biochemistry

The biochemistry of selenium-containing natural products, including selenoproteins, is reviewed up to May 2002. Particular emphasis is placed on the assimilation of selenium from inorganic and organic selenium sources for selenoprotein synthesis, the catalytic role of selenium in enzymes, and medical implications of an unbalanced selenium supply. The review contains 393 references on key discoveries and recent progress.

Enzymatic and immunochemical evaluation of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in testes and epididymal spermatozoa of rats of different ages

Selenium (Se) and selenoproteins such as glutathione peroxidases are necessary for the proper development and fertilizing capacity of sperm cells. Phospholipid hydroperoxide glutathione peroxidase (PHGPx, E.C. 1.11.1.12) is a monomeric seleno-enzyme present in different mammalian tissues in soluble and bound form. Its function, like the other glutathione peroxidases, was originally viewed as a protective role against hydroperoxides, but direct and indirect evidence indicates that it has additional regulatory roles. PHGPx is present in testis cells and sperm cells, and its appearance is hormone regulated. We present here biochemical data, which clearly indicate that the enzyme specific activity in rat is age-dependent during the life-span monitored (from 36 to 365 days), with a maximum at 3 months of age in the testis germ cells and at 6 months of age in the isolated epididymal sperm cells. Western blotting and immunocytochemical analysis by means of anti-PHGPx antibodies show the different distribution and the strong binding of PHGPx in the testes and sperm cell subcellular compartments (nucleus, acrosome, mitochondria and residual bodies) of rats of different age. The presence of the protein exhibits in the testis cells a pattern different from that of the catalytic activity, with a maximum at 6 months of age. The subcellular distribution of PHGPx is qualitatively, but not quantitatively, unchanged during ageing. These different behaviours are compared and discussed.

Polyunsaturated fatty acids of germ cell membranes, glutathione and blutathione-dependent enzyme-PHGPx: from basic to clinic

The lipid metabolism in sperm cells is important both for energy production and for cell structure. A special composition of membrane phospholipids, rich in polyunsaturated fatty acids (PUFA), and the different composition of sperm and immature germ cell membrane are described and discussed. Testis germ cells as well as epididymal maturing spermatozoa are endowed with enzymatic and non-enzymatic scavenger systems to prevent lipoperoxidative damage. Catalase, superoxide dismutase, and glutathione-dependent oxidoreductases are present in variable amounts in the different developmental stages. Phospholipid hydroperoxide glutathione peroxidase (PHGPx) activity and roles in caput and cauda epididymal sperm cells are discussed. Also seminal plasma has a highly specialized scavenger system that defends the sperm membrane against lipoperoxidation and the degree of PUFA insaturation acts to achieve the same goal. Systemic predisposition and a number of pathologies can lead to an anti-oxidant/pro-oxidant disequilibrium. Scavengers, such as glutathione can be used to treat these cases as they can restore the physiological constitution of PUFA in the cell membrane.

Mitochondrial respiratory chain-dependent generation of superoxide anion and its release into the intermembrane space

It has been generally accepted that superoxide anion generated by the mitochondrial respiratory transport chain are vectorially released into the mitochondrial matrix, where they are converted to hydrogen peroxide through the catalytic action of Mn-superoxide dismutase. Release of superoxide anion into the intermembrane space is a controversial topic, partly unresolved by the reaction of superoxide anion with cytochrome c, which faces the intermembrane space and is present in this compartment at a high concentration. This study was aimed at assessing the topological site(s) of release of superoxide anion during respiratory chain activity. To address this issue, mitoplasts were prepared from isolated mitochondria by digitonin treatment to remove portions of the outer membrane along with portions of cytochrome c. EPR analysis in conjunction with spin traps of antimycin-supplemented mitoplasts revealed the formation of a spin adduct of superoxide anion. The EPR signal was (i) abrogated by superoxide dismutase, (ii) decreased competitively by exogenous ferricytochrome c and (iii) broadened by the membrane-impermeable spin-broadening agent chromium trioxalate. These results confirm the production and release of superoxide anion towards the cytosolic side of the inner mitochondrial membrane. In addition, co-treatment of mitoplasts with myxothiazol and antimycin A, resulting in an inhibition of the oxidation of ubiquinol to ubisemiquinone, abolished the EPR signal, thus suggesting that ubisemiquinone autoxidation at the outer site of the complex-III ubiquinone pool is a pathway for superoxide anion formation and subsequent release into the intermembrane space. The generation of superoxide anion towards the intermembrane space requires consideration of the mitochondrial steady-state values for superoxide anion and hydrogen peroxide, the decay pathways of these oxidants in this compartment and the implications of these processes for cytosolic events.

Mitochondrial phospholipid hydroperoxide glutathione peroxidase inhibits the release of cytochrome c from mitochondria by suppressing the peroxidation of cardiolipin in hypoglycaemia-induced apoptosis

Cytochrome c (cyt. c) is a proapoptotic factor that binds preferentially to cardiolipin (CL), a mitochondrial lipid, but not to cardiolipin hydroperoxide (CL-OOH). Cyt. c that had bound to CL liposomes was liberated on peroxidation of the liposomes by a radical. The generation of CL-OOH in mitochondria occurred before the release of cyt. c in rat basophile leukaemia (RBL)2H3 cells that had been induced to undergo apoptosis by exposure to hypoglycaemia with 2-deoxyglucose (2DG). The amount of cyt. c bound to CL prepared from the mitochondria of 2DG-treated cells was lower than that of untreated cells. The release of cyt. c was completely suppressed when the production of CL-OOH in mitochondria was inhibited by the overexpression of mitochondrial phospholipid hydroperoxide glutathione peroxidase (PHGPx). The fluorescence from CL-labelling dye (10-N-nonyl Acridine Orange) decreased on the induction of apoptosis by 2DG. However, no decrease in fluorescence was observed in PHGPx-overexpressing cells. Cyt. c was released from mitochondria that had been isolated from control cells on peroxidation by t-butylhydroperoxide, but no similar liberation of cyt. c from mitochondria isolated from mitochondrial PHGPx-overexpressing cells was observed. These findings suggest that the generation of CL-OOH in mitochondria might be a primary event that triggers the release of cyt. c from mitochondria in the apoptotic process in which mitochondrial PHGPx participates as an anti-apoptotic factor by preventing the formation of CL-OOH.

Testis glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase activities in aminoguanidine-treated diabetic rats

Severe steroidogenic and spermatogenic alterations are reported in association with diabetic manifestations in humans and experimental animals. This study was planned to determine whether oxidative stress is involved in diabetes-induced alterations in the testes. Diabetes was induced in male rats by injection of 50 mg/kg of streptozotocin (STZ). Ten weeks after injection of STZ, levels of selenium and activities of selenium dependent-glutathione peroxidase (GPx) and phospholipid hydroperoxide glutathione peroxidase (PHGPx) were measured in rat testis. Lipid and protein oxidations were evaluated as measurements of testis malondialdehyde (MDA) and protein carbonyl levels, respectively. Testis sulfydryl (SH) levels were also determined. The control levels of GPx and PHGPx activities were found to be 46.5 +/- 6.2 and 108.8 +/- 19.8 nmol GSH/mg protein/min, respectively. Diabetes caused an increase in testis GPx (65.0 +/- 21.1) and PHGPx (155.9 +/- 43.1) activities but did not affect the levels of selenium or SH. However, the testis MDA and protein carbonyl levels as markers of lipid and protein oxidation, respectively, did not increase in the diabetic group. Aminoguanidine (AG) treatment of diabetic rats returned the testis PHGPx activity (136.5 +/- 24.9) to the control level but did not change the value of GPx activity (69.2 +/- 17.4) compared with diabetic group. MDA and protein carbonyl levels in testis were not affected by AG treatment of diabetic rats, but interestingly AG caused SH levels to increase. The results indicate that reactive oxygen radicals were not involved in possible testicular complications of diabetes because diabetes-induced activations of GPx and PHGPx provided protection against oxidative stress, which was reported to be related to some diabetic complications.

Mitochondrial oxidative stress in mice lacking the glutathione peroxidase-1 gene

Oxidative stress resulting from mitochondrially derived reactive oxygen species (ROS) has been hypothesized to damage mitochondrial oxidative phosphorylation (OXPHOS) and to be a factor in aging and degenerative disease. If this hypothesis is correct, then genetically inactivating potential mitochondrial antioxidant enzymes such as glutathione peroxidase-1 (Gpx1; EC 1.11.1.9) should increase mitochondrial ROS production and decrease OXPHOS function. To determine the expression pattern of Gpx1, isoform-specific antibodies were generated and mutant mice were prepared in which the Gpx1 protein was substituted for by beta-galactosidase, driven by the Gpx1 promoter. These experiments revealed that Gpx1 is highly expressed in both the mitochondria and the cytosol of the liver and kidney, but poorly expressed in heart and muscle. To determine the physiological importance of Gpx1, mice lacking Gpx1 were generated by targeted mutagenesis in mouse ES cells. Homozygous mutant Gpx1(tm1Mgr) mice have 20% less body weight than normal animals and increased levels of lipid peroxides in the liver. Moreover, the liver mitochondria were found to release markedly increased hydrogen peroxide, a Gpx1 substrate, and have decreased mitochondrial respiratory control ratio and power output index. Hence, genetic inactivation of Gpx1 resulted in growth retardation, presumably due in part to reduced mitochondrial energy production as a product of increased oxidative stress.

Lipid peroxidation and antioxidant activities in rat testis after chronic cocaine administration

OBJECTIVES

Our recent study has shown that cocaine has adverse action on spermatogenesis and fertility in male rats. Adverse effects of cocaine on the testes may be mediated by oxidative damage and subsequent lipid peroxidation. Glutathione is a cellular antioxidant and is found in high concentrations in the rat testes. In this study, the effects of chronic cocaine administration on the activities of glutathione peroxidase, the level of testicular reduced glutathione, and lipid peroxidation were investigated.

METHODS

Thirty-day-old male Sprague-Dawley rats were given cocaine hydrochloride (15 mg/kg body weight) subcutaneously daily for 90 days. Control animals received equal volumes of normal saline daily for 90 days. Testes were removed at 15, 30, 60, and 90 days after cocaine injection. Tissues were washed and homogenized in ice-cold metaphosphoric acid solution or Tris-HCI buffer. Reduced glutathione, glutathione peroxidase, and malonaldehyde levels were determined by colorimetric assay. Statistical analysis was performed using analysis of variance.

RESULTS

Testicular reduced glutathione and glutathione peroxidase were significantly decreased in the treatment testes 15, 30, 60, and 90 days after chronic cocaine injection compared with controls (P <0.05). The testicular malonaldehyde level was 20.8% (P <0.05), 22.1% (P <0.05), 31.2% (P <0.05), and 24.7% (P <0.05) above the control value on days 15, 30, 60, and 90, respectively.

CONCLUSIONS

Our findings demonstrate that chronic administration of cocaine to male rats induces a depletion of reduced glutathione and glutathione peroxidase. Adverse effects of cocaine on the testes are at least in part due to impairment of the function of the antioxidant defense and further enhancement of lipid peroxidation.

Tissue-specific functions of individual glutathione peroxidases

The family of glutathione peroxidases comprises four distinct mammalian selenoproteins. The classical enzyme (cGPx) is ubiquitously distributed. According to animal, cell culture and inverse genetic studies, its primary function is to counteract oxidative attack. It is dispensible in unstressed animals, and accordingly ranks low in the hierarchy of glutathione peroxidases. The gastrointestinal isoenzyme (GI-GPx) is most related to cGPx and is exclusively expressed in the gastrointestinal tract. It might provide a barrier against hydroperoxides derived from the diet or from metabolism of ingested xenobiotics. The extreme stability in selenium deficiency ranks this glutathione peroxidase highest in the hierarchy of selenoproteins and points to a more vital function than that of cGPx. Plasma GPx (pGPx) behaves similar to cGPx in selenium deficiency. It is directed to extracellular compartments and is expressed in various tissues in contact with body fluids, e.g., kidney, ciliary body, and maternal/fetal interfaces. It has to be rated as an efficient extracellular antioxidant device, though with low capacity because of the limited extracellular content of potential thiol substrates. Phospholipid hydroperoxide glutathione peroxidase (PHGPx), originally presumed to be a universal antioxidant enzyme protecting membrane lipids, appears to have adopted a variety of specific roles like silencing lipoxygenases and becoming an enzymatically inactive structural component of the mitochondrial capsule during sperm maturation. Thus, all individual isoenzymes are efficient peroxidases in principle, but beyond their mere antioxidant potential may exert cell- and tissue-specific roles in metabolic regulation, as is evident for PHGPx and may be expected for others.

Purification, redox sensitivity, and RNA binding properties of SECIS-binding protein 2, a protein involved in selenoprotein biosynthesis

In mammalian selenoprotein mRNAs, the highly structured 3' UTR contains selenocysteine insertion sequence (SECIS) elements that are required for the recognition of UGA as the selenocysteine codon. Our previous work demonstrated a tight correlation between codon-specific translational read-through and the activity of a 120-kDa RNA-binding protein that interacted specifically with the SECIS element in the phospholipid hydroperoxide glutathione peroxidase mRNA. This study reports the RNA binding and biochemical properties of this protein, SECIS-binding protein 2 (SBP2). We detected SBP2 binding activity in liver, hepatoma cell, and testis extracts from which SBP2 has been purified by anion exchange and RNA affinity chromatography. This scheme has allowed us to identify a 120-kDa polypeptide that co-elutes with SBP2 binding activity from wild-type but not mutant RNA affinity columns. A characterization of SBP2 biochemical properties reveals that SBP2 binding is sensitive to oxidation and the presence of heparin, rRNA, and poly(G). SBP2 activity elutes with a molecular mass of approximately 500 kDa during gel filtration chromatography, suggesting the existence of a large functional complex. Direct cross-linking and competition experiments demonstrate that the minimal phospholipid hydroperoxide glutathione peroxidase 3' UTR binding site is between 82 and 102 nucleotides, which correlates with the minimal sequence necessary for translational read-through. SBP2 also interacts specifically with the minimally functional 3' UTR of another selenoprotein mRNA, deiodinase 1.

Mitochondrial phospholipid hydroperoxide glutathione peroxidase plays a major role in preventing oxidative injury to cells

Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is synthesized as a long form (L-form; 23 kDa) and a short form (S-form; 20 kDa). The L-form contains a leader sequence that is required for transport to mitochondria, whereas the S-form lacks the leader sequence. A construct encoding the leader sequence of PHGPx tagged with green fluorescent protein was used to transfect RBL-2H3 cells, and the fusion protein was transported to mitochondria. The L-form of PHGPx was identified as the mitochondrial form of PHGPx and the S-form as the non-mitochondrial form of PHGPx since preferential enrichment of mitochondria for PHGPx was detected in M15 cells that overexpressed the L-form of PHGPx, whereas no similar enrichment was detected in L9 cells that overexpressed the S-form. Cell death caused by mitochondrial injury due to potassium cyanide (KCN) or rotenone (chemical hypoxia) was considerably suppressed in the M15 cells, whereas the L9 cells and control RBL-2H3 cells (S1 cells, transfected with the vector alone) succumbed to the cytotoxic effects of KCN. Flow cytometric analysis showed that mitochondrial PHGPx suppressed the generation of hydroperoxide, the loss of mitochondrial membrane potential, and the loss of plasma membrane integrity that are induced by KCN. Mitochondrial PHGPx might prevent changes in mitochondrial functions and cell death by reducing intracellular hydroperoxides. Mitochondrial PHGPx failed to protect M15 cells from mitochondrial injury by carbonyl cyanide m-chlorophenylhydrazone, which directly reduces membrane potential without the generation of hydroperoxides. M15 cells were more resistant than L9 cells to cell death caused by direct damage to mitochondria and to extracellular oxidative stress. L9 cells were more resistant to tert-butylhydroperoxide than S1 cells, whereas resistance to t-butylhydroperoxide was even more pronounced in M15 cells than in L9 cells. These results suggest that mitochondria might be a target for intracellular and extracellular oxidative stress and that mitochondrial PHGPx, as distinct form non-mitochondrial PHGPx, might play a primary role in protecting cells from oxidative stress.

Cloning and expression of mitochondrial capsule selenoprotein gene in the golden hamster

Mitochondrial capsule selenoprotein (MCS) has been known as a structural protein of the mitochondrial sheath in spermatoza. In the present study, a full-length cDNA encoding the MCS was first isolated from the testes of 10-week-old golden hamsters using a RACE (rapid amplification of cDNA ends) technique and its mRNA expression pattern was investigated from the hamster tissues by reverse transcription-polymerase chain reaction (RT-PCR) analysis. Hamster MCS cDNA was 820 bp long, including 24 bp of the 5'-untranslated region (UTR) and 243 bp of the 3'-UTR, and showed identity of 75.6% and 73.9% with mouse and rat MCS. According to the deduced amino acid (aa) sequence analysis, hamster MCS encoded a polypeptide of 184 aa, including a cysteine- and proline-rich domain which is the characteristic sequences of MCS, and contained 2 in-frame UGA codons for selenocysteine. Hamster MCS also shared aa identity of 64.4% with mouse MCS and contained an Arg-Lys-Ser-Thr-rich region in the N-terminus similar to the mitochondrial targeting signal. On the other hand, according to the RT-PCR analysis using the specific primers for hamster MCS, hamster MCS mRNA was expressed in various tissues as well as the testes. This finding indicates that MCS in hamster may have more than just a function of mitochondrial sheath formation of spermatozoa.

Antioxidant systems in rat epididymal spermatozoa

Mammalian caput and cauda epididymidal spermatozoa exhibit diverse stages of maturation, and their plasma membrane shows diverse composition and stability levels, thus enabling these spermatozoa to undergo the acrosomal reaction after transit through the epididymis. As a result, the study of antiperoxidative mechanisms is quite relevant, since epididymal spermatozoa must be properly protected against agents such as reactive oxygen species, which can impair the complex maturation process. We considered activities of certain enzymes (glutathione peroxidase [GPx], phospholipid hydroperoxide glutathione peroxidase [PHGPx], glutathione reductase [GR], superoxide dismutase [SOD], and catalase [CAT]) and the vitamin E content in isolated rat caput and cauda epididymidal spermatozoa. The results indicate that caput epididymidal sperm have significantly greater PHGPx (3.5x), GPx (2.4x), and SOD (1.7x) activities, as well as a greater amount of vitamin E (3.8x). There were no detectable differences in the GR and CAT activities of caput and cauda epididymidal spermatozoa. The substantial drop in PHGPx activity during epididymal transit is discussed in relation to an additional function of this enzyme: the use of caput sperm protamines as a sulfhydryl substrate. In vitro peroxidation of the two sperm populations by the free radical generator (azo-initiator) 2,2'-azobis(2-amidinopropane) dihydrochloride revealed that only about 13% of the vitamin E content of the caput epididymidal spermatozoa was consumed, which contrasts with the greater consumption (about 70%) of the vitamin in cauda epididymidal spermatozoa. Selective inhibition of PHGPx, SOD, or CAT did not change this picture. The higher susceptibility of cauda epididymidal spermatozoa to radicals is discussed in relation to the diverse enzymatic activities, vitamin E content, and peroxidative response. These factors are correlated with the different stages of sperm cell maturation, which are characterized-from caput to cauda epididymidis-by progressive destabilization of the plasma and acrosomal membranes.

Structural organization of the human selenium-dependent phospholipid hydroperoxide glutathione peroxidase gene (GPX4): chromosomal localization to 19p13.3

The primary structure of human selenium-dependent phospholipid hydroperoxide glutathione peroxidase (GPX4) was determined by genomic cloning. The gene structure of GPX4 spans only 2.8 kb and consists of 7 exons. The coding sequence resides on all 7 exons, and the mitochondrial leader sequence is contained entirely within the first exon. The selenocysteine coding nucleotide resides on the third exon. The introns all commenced with the consensus nucleotide sequence GTR and ended with the consensus nucleotide sequence YAG. Analysis of the GPX4 gene sequence identified a potential alternative tissue-specific first exon. Chromosomal FISH studies placed the GPX4 gene at 19p13.3 location, and downstream of the 23 k-Da polypeptide DNA-directed RNA polymerase gene.

Cloning and sequencing of the mouse cDNA encoding a phospholipid hydroperoxide glutathione peroxidase

Phospholipid hydroperoxide glutathione peroxidase (PHGPx), a selenoprotein, reduces the hydroperoxides of phospholipid, cholesterol, and cholesteryl ester in biomembranes. In this study, a full-length cDNA clone encoding the PHGPx was isolated from mouse testes using a RACE (rapid amplification of cDNA ends) technique. According to sequence analysis, the cDNA encodes a polypeptide of 197 amino acids (aa) that initiates the translation at ATG(145-147) and contains an inframe TGA selenocysteine codon. It also has selenocysteine insertion sequences in the 3'-UTR that are involved in the insertion of selenocysteine at an opal codon. Moreover, the mouse PHGPx contains the active-site residues Gln108 and Trp163 that interact with selenocysteine, and the N-terminal 27-aa residues that may act as a potential mitochondrial targeting signal. According to the deduced aa analysis, mouse PHGPx shares a high level of aa identity with pig (93.4%), human (92.9%), and rat (98%) PHGPxs. However, the PHGPx mRNA particularly showed a high degree of expression in testis. This suggests that the PHGPx in testis may have more than just an antioxidant function.

The Gpx1 gene encodes mitochondrial glutathione peroxidase in the mouse liver

Mitochondria have GPX and PHGPX activity. It has been an unsettled issue whether mitochondrial GPX is encoded by Gpx1. Unlike the Gpx4 gene which encodes PHGPX with alternative transcription and translation start sites determining the subcellular localization of PHGPX, the Gpx1 gene appears to have a single translation start site. Additionally, mitochondrial GPX has been shown to have different chromatographic and kinetic properties from the cytosolic GPX1. We studied mouse liver mitochondrial GPX activity in homozygous Gpx1-knockout mice. Mitochondria were enriched at the density of 1.10 g/ml in the Percoll gradients as shown by electron microscopy. The H2O2-reducing GPX activity in the highly enriched mitochondrial fraction of wild-type mouse liver is 2700 mU/mg which is about one-half of specific activity found in cytosol. There is less than 0.5% GPX activity in the cytosol and no GPX activity in the mitochondria of Gpx1-knockout mouse liver compared to the cytosol of wild-type mouse liver using H2O2 or cumene hydroperoxide as the substrate. The fact that the knockout mice express normal levels of plasma GPX as well as testis and liver PHGPX activity indicates that animals are not selenium-deficient. Based on these observations, we concluded that mitochondrial GPX is the product of the Gpx1 gene.

Effects of dietary selenium and vitamin E concentrations on phospholipid hydroperoxide glutathione peroxidase expression in reproductive tissues of pubertal maturing male rats

Phospholipid hydroperoxide glutathione peroxidase (PHGPX) is the second intracellular selenium (Se)-dependent glutathione peroxidase (GSH-Px) identified in mammals. Our objectives were to determine the effect of dietary vitamin E and Se levels on PHGPX activity expression in testis, epididymis, and seminal vesicles of pubertal maturing rats, and the relationship of PHGPX expression with testicular development and sperm quality. Forty Sprague-Dawley male weanling rats (21-d old), were initially fed for 3 wk a torula yeast basal diet (containing 0.05 mg Se/kg) supplemented with marginal levels of Se (0.1 mg/kg as Na2SeO3) and vitamin E (25 IU/kg as all-rac-alpha-tocopheryl acetate). Then, rats were fed the basal diets supplemented with 0 or 0.2 mg Se/kg and 0 or 100 IU vitamin E/kg diet during the 3-wk period of pubertal maturing. Compared with the Se-supplemented rats, those fed the Se-deficient diets retained 31, 88, 67, and 50% of Se-dependent GSH-Px activities in liver, testis, epididymis, and seminal vesicles, respectively. Testes and seminal vesicles had substantially higher (5- to 20-fold) PHGPX activity than liver. Dietary Se deficiency did not affect PHGPX activities in the reproductive tissues, but reduced PHGPX activity in liver by 28% (P < 0.0001). Dietary vitamin E supplementation did not affect PHGPX activity in liver, whereas it raised PHGPX activity in seminal vesicles by 43% (P < 0.005). Neither dietary vitamin E nor Se levels affected body weight gains, reproductive organ weights, or sperm counts and morphology. In conclusion, expression of PHGPX activity in testis and seminal vesicles was high and regulated by dietary Se and vitamin E differently from that in liver.

Evidence for altered structure and impaired mitochondrial electron transport function in selenium deficiency

Selenium (Se) deficiency in the experimental models, Coturnix coturnix japonica and Corcyra cephalonica, resulted in impaired mitochondrial substrate oxidations and lowered thiol levels. Studies with respiratory inhibitors confirmed reduced mitochondrial electron transport enzyme activities, especially at cytochrome c oxidase (COX), the terminal segment. Enhanced mitochondrial lipid peroxidation in Se deficiency was more pronounced in the heart tissue of the quail compared to other tissues. Glutathione peroxidase (GSH-Px) activity toward H2O2 and cumene hydroperoxide were generally low in the insect muscle tissue and activity toward H2O2 was maximal in the quail heart mitochondria that was not very sensitive to Se status. Lowered COX activity in Se deficiency was more directly correlated with the increased level of lipid peroxidation than with the GSH-Px activity measured, suggestive of Se mediated protective mechanisms independent of GSH-Px. Electron microscopic observations revealed structural changes such as loss of cristae with proliferative and degenerative changes of the mitochondria in Se deficiency. Involvement of Se in maintaining structure and functional efficiency of mitochondria is evident from the present study.

Rat phospholipid-hydroperoxide glutathione peroxidase. cDNA cloning and identification of multiple transcription and translation start sites

Phospholipid-hydroperoxide glutathione peroxidase (PhGPx) is a selenoenzyme that reduces hydroperoxides of phospholipid, cholesterol, and cholesteryl ester. Previous studies suggested that both the mitochondrial and nonmitochondrial forms of PhGPx are approximately 170 amino acids long. In this study, we isolated a full-length cDNA clone encoding rat testis PhGPx. Based on sequence analysis, the cDNA encodes a protein of 197 amino acids, with translation initiating at AUG61. The additional 27 amino acids at the N terminus contain the features of a mitochondrial targeting sequence. In vitro translation of the full-length PhGPx mRNA initiated predominantly at AUG61. However, translation initiated at AUG141 when AUG61 was deleted. An RNase protection assay was used to map the 5'-ends of PhGPx mRNAs in rat tissues. We identified two major windows of transcription initiation that are tissue-specific. Rat testis predominantly expresses larger transcripts that encode the 197-amino acid protein containing the potential mitochondrial targeting signal. The predominant smaller transcripts in somatic tissues lack AUG61 and encode a 170-amino acid protein, which may represent the nonmitochondrial forms of PhGPx. Our results suggest that the use of alternative transcription and translation start sites determines the subcellular localization of PhGPx in different tissues.

Heterologous expression and enzymatic properties of a selenium-independent glutathione peroxidase from the parasitic nematode Brugia pahangi

A full-length cDNA from the parasitic nematode Brugia pahangi encoding a secreted homolog of glutathione peroxidase in which the codon for the active site selenocysteine is substituted naturally by a cysteine codon has been expressed in Spodoptera frugiperda (insect) cells via Autographa californica nuclear polyhedrosis virus (baculovirus). The recombinant protein was glycosylated and secreted from the cells in tetrameric form. The purified protein showed glutathione peroxidase activity with a range of organic hydroperoxides, including L-alpha-phosphatidylcholine hydroperoxide, but no significant activity against hydrogen peroxide. Glutathione was the only thiol tested that served as a substrate for the enzyme, which showed no activity with the thioredoxin system (thioredoxin, thioredoxin reductase, and NADPH). No glutathione-conjugating activity was detected against a range of electrophilic compounds that are common substrates for glutathione S-transferases. The apparent (pseudo)m for glutathione was determined as 4.9 mM at a fixed concentration of linolenic acid hydroperoxide (3 microM). The enzyme showed low affinity for hydroperoxide substrates (apparent Km for linolenic acid hydroperoxide and L-alpha-phosphatidylcholine hydroperoxide of 3.8 and 9.7 mM, respectively at a fixed glutathione concentration of 3 mM).