Cardiolipin drives the catalytic activity of GPX4 on membranes: Insights from the R152H mutant

The aim of this study was to examine, in biochemical detail, the functional role of the Arg152 residue in the selenoprotein Glutathione Peroxidase 4 (GPX4), whose mutation to His is involved in Sedaghatian-type Spondylometaphyseal Dysplasia (SSMD). Wild-type and mutated recombinant enzymes with selenopcysteine (Sec) at the active site, were purified and structurally characterized to investigate the impact of the R152H mutation on enzymatic function. The mutation did not affect the peroxidase reaction's catalytic mechanism, and the kinetic parameters were qualitatively similar between the wild-type enzyme and the mutant when mixed micelles and monolamellar liposomes containing phosphatidylcholine and its hydroperoxide derivatives were used as substrate. However, in monolamellar liposomes also containing cardiolipin, which binds to a cationic area near the active site of GPX4, including residue R152, the wild-type enzyme showed a non-canonical dependency of the reaction rate on the concentration of both enzyme and membrane cardiolipin. To explain this oddity, a minimal model was developed encompassing the kinetics of both the enzyme interaction with the membrane and the catalytic peroxidase reaction. Computational fitting of experimental activity recordings showed that the wild-type enzyme was surface-sensing and prone to "positive feedback" in the presence of cardiolipin, indicating a positive cooperativity. This feature was minimal, if any, in the mutant. These findings suggest that GPX4 physiology in cardiolipin containing mitochondria is unique, and emerges as a likely target of the pathological dysfunction in SSMD.

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.

Redox Pioneer: Professor Regina Brigelius-Flohé

Dr. Regina Brigelius-Flohé (PhD 1978) is recognized here as redox pioneer because she has published an article on redox biology, as first author, that has been cited >1000 times, plus four articles cited >500 times, and a total of 30 articles cited >100 times. She obtained her doctorate in biochemistry at the Institute of Biochemistry of the University of Münster, Germany. She held positions in both, academia (Münster, Munich, Düsseldorf, Hannover, and Potsdam, Germany) and industry (Aachen, Germany). Dr. Brigelius-Flohé is the pioneer who, as head of the department of biochemistry of micronutrients of the German Institute of Human Nutrition (DIfE; Potsdam-Rehbrücke, Germany), worked out the metabolism of tocopherols and tocotrienols ("Key Finding 1"). She was the first to sequence glutathione peroxidase 4 (GPx4) ("Key Finding 2"), and unraveled the role of selenium, in particular of GPxs, in inflammation and carcinogenesis ("Key Finding 3"). Her contributions, thus, focused on serious biomedical problems such as nutrition, inflammation, and carcinogenesis. She has been a member of the scientific advisory board of the German Society of Biochemistry and Molecular Biology for 6 years and was president of SFRR-Europe in 2005-2006. She edited several books and serves on the editorial board of journals in the fields of nutrition, free radicals, and redox regulation. Antioxid. Redox Signal. 35, 595-601.

Production and purification of homogenous recombinant human selenoproteins reveals a unique codon skipping event in E. coli and GPX4-specific affinity to bromosulfophthalein

Selenoproteins are translated via animal domain-specific elongation machineries that redefine dedicated UGA opal codons from termination of translation to selenocysteine (Sec) insertion, utilizing specific tRNA species and Sec-specific elongation factors. This has made recombinant production of mammalian selenoproteins in E. coli technically challenging but recently we developed a methodology that enables such production, using recoding of UAG for Sec in an RF1-deficient host strain. Here we used that approach for production of the human glutathione peroxidases 1, 2 and 4 (GPX1, GPX2 and GPX4), with all these three enzymes being important antioxidant selenoproteins. Among these, GPX4 is the sole embryonically essential enzyme, and is also known to be essential for spermatogenesis as well as protection from cell death through ferroptosis. Enzyme kinetics, ICP-MS and mass spectrometry analyses of the purified recombinant proteins were used to characterize selenoprotein characteristics and their Sec contents. This revealed a unique phenomenon of one-codon skipping, resulting in a lack of a single amino acid at the position corresponding to the selenocysteine (Sec) residue, in about 30% of the recombinant GPX isoenzyme products. We furthermore confirmed the previously described UAG suppression with Lys or Gln as well as a minor suppression with Tyr, together resulting in about 20% Sec contents in the full-length proteins. No additional frameshifts or translational errors were detected. We subsequently found that Sec-containing GPX4 could be further purified over a bromosulfophthalein-column, yielding purified recombinant GPX4 with close to complete Sec contents. This production method for homogenously purified GPX4 should help to further advance the studies of this important selenoprotein.

Aerobic pyruvate metabolism sensitizes cells to ferroptosis primed by GSH depletion

Ferroptosis is a non-accidental, regulated form of cell death operated by lipid peroxidation under strict control of GPx4 activity. This is consistent with the notion that lipid peroxidation is initiated by radicals produced from decomposition of traces of pre-existing lipid hydroperoxides. The question, therefore, emerges about the formation of these traces of lipid hydroperoxides interacting with Fe²⁺. In the most realistic option, they are produced by oxygen activated species generated during aerobic metabolism. Screening for metabolic sources of superoxide supporting ferroptosis induced by GSH depletion, we failed to detect, in our cell model, a role of respiratory chain. We observed instead that the pyruvate dehydrogenase complex -as other α keto acid dehydrogenases already known as a major source of superoxide in mitochondria- supports ferroptosis. The opposite effect on ferroptosis by silencing either the E1 or the E3 subunit of the pyruvate dehydrogenase complex pointed out the autoxidation of dihydrolipoamide as the source of superoxide. We finally observed that GSH depletion activates superoxide production, seemingly through the inhibition of the specific kinase that inhibits pyruvate dehydrogenase. In summary, this set of data is compatible with a scenario where the more electrophilic status produced by GSH depletion not only activates ferroptosis by preventing GPx4 activity, but also favors the formation of lipid hydroperoxides. In an attractive perspective of tissue homeostasis, it is the activation of energetic metabolism associated to a decreased nucleophilic tone that, besides supporting energy demanding proliferation, also sensitizes cells to a regulated form of death.

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.

A dual attack on the peroxide bond. The common principle of peroxidatic cysteine or selenocysteine residues

The (seleno)cysteine residues in some protein families react with hydroperoxides with rate constants far beyond those of fully dissociated low molecular weight thiol or selenol compounds. In case of the glutathione peroxidases, we could demonstrate that high rate constants are achieved by a proton transfer from the chalcogenol to a residue of the active site [Orian et al. Free Radic. Biol. Med. 87 (2015)]. We extended this study to three more protein families (OxyR, GAPDH and Prx). According to DFT calculations, a proton transfer from the active site chalcogenol to a residue within the active site is a prerequisite for both, creating a chalcogenolate that attacks one oxygen of the hydroperoxide substrate and combining the delocalized proton with the remaining OH or OR, respectively, to create an ideal leaving group. The “parking postions” of the delocalized proton differ between the protein families. It is the ring nitrogen of tryptophan in GPx, a histidine in GAPDH and OxyR and a threonine in Prx. The basic principle, however, is common to all four families of proteins. We, thus, conclude that the principle outlined in this investigation offers a convincing explanation for how a cysteine residue can become peroxidatic.

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In some protein families, (seleno)cysteine residues react with hydroperoxides with very high rate constants.

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In GPx, DFT models of the oxidation of the catalytic site support a two-step mechanism for the H₂O₂ reduction.

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This mechanism is here found to operate in other thiol-based enzymes, i.e. OxyR, GAPDH and Prx.

Lack of glutathione peroxidase-8 in the ER impacts on lipid composition of HeLa cells microsomal membranes

GPx8 is a glutathione peroxidase homolog inserted in the membranes of endoplasmic reticulum (ER), where it seemingly plays a role in controlling redox status by preventing the spill of H₂O₂. We addressed the impact of GPx8 silencing on the lipidome of microsomal membranes, using stably GPx8-silenced HeLa cells. The two cell lines were clearly separated by Principal Component Analysis (PCA) and Partial Least Square Discriminant analysis (PLS-DA) of lipidome. Considering in detail the individual lipid classes, we observed that unsaturated glycerophospholipids (GPL) decreased, while only in phosphatidylinositols (PI) a substitution of monounsaturated fatty acids (MUFA) for polyunsaturated fatty acids (PUFA) was observed. Among sphingolipids (SL), ceramides (CER) decreased while sphingomyelins (SM) and neutral glycophingolipids (nGSL) increased. Here, in addition, longer chains than in controls in the amide fatty acid were present. The increase up to four folds of the CER (d18:1; c24:0) containing three hexose units, was the most remarkable species increasing in the differential lipidome of siGPx8 cells. Quantitative RT-PCR complied with lipidomic analysis specifically showing an increased expression of: i) acyl-CoA synthetase 5 (ACSL5); ii) CER synthase 2 and 4; iii) CER transporter (CERT); iv) UDP-glucosyl transferase (UDP-GlcT), associated to a decreased expression of UDP-galactosyl transferase (UDP-GalT). A role of the unfolded protein response (UPR) and the spliced form of the transcription factor XBP1 on the transcriptional changes of GPx8 silenced cells was ruled-out. Similarly, also the involvement of Nrf2 and NF-κB. Altogether our results indicate that GPx8-silencing of HeLa yields a membrane depleted by about 24% of polyunsaturated GPL and a corresponding increase of saturated or monounsaturated SM and specific nGSL. This is tentatively interpreted as an adaptive mechanism leading to an increased resistance to radical oxidations. Moreover, the marked shift of fatty acid composition of PI emerges as a possibly relevant issue in respect to the impact of GPx8 on signaling pathways.

Insight into the mechanism of ferroptosis inhibition by ferrostatin-1

Ferroptosis is a form of cell death primed by iron and lipid hydroperoxides and prevented by GPx4. Ferrostatin-1 (fer-1) inhibits ferroptosis much more efficiently than phenolic antioxidants. Previous studies on the antioxidant efficiency of fer-1 adopted kinetic tests where a diazo compound generates the hydroperoxyl radical scavenged by the antioxidant. However, this reaction, accounting for a chain breaking effect, is only minimally useful for the description of the inhibition of ferrous iron and lipid hydroperoxide dependent peroxidation. Scavenging lipid hydroperoxyl radicals, indeed, generates lipid hydroperoxides from which ferrous iron initiates a new peroxidative chain reaction. We show that when fer-1 inhibits peroxidation, initiated by iron and traces of lipid hydroperoxides in liposomes, the pattern of oxidized species produced from traces of pre-existing hydroperoxides is practically identical to that observed following exhaustive peroxidation in the absence of the antioxidant. This supported the notion that the anti-ferroptotic activity of fer-1 is actually due to the scavenging of initiating alkoxyl radicals produced, together with other rearrangement products, by ferrous iron from lipid hydroperoxides. Notably, fer-1 is not consumed while inhibiting iron dependent lipid peroxidation. The emerging concept is that it is ferrous iron itself that reduces fer-1 radical. This was supported by electroanalytical evidence that fer-1 forms a complex with iron and further confirmed in cells by fluorescence of calcein, indicating a decrease of labile iron in the presence of fer-1. The notion of such as pseudo-catalytic cycle of the ferrostatin-iron complex was also investigated by means of quantum mechanics calculations, which confirmed the reduction of an alkoxyl radical model by fer-1 and the reduction of fer-1 radical by ferrous iron. In summary, GPx4 and fer-1 in the presence of ferrous iron, produces, by distinct mechanism, the most relevant anti-ferroptotic effect, i.e the disappearance of initiating lipid hydroperoxides.

GPx4, Lipid Peroxidation, and Cell Death: Discoveries, Rediscoveries, and Open Issues

SIGNIFICANCE

Iron-dependent lipid peroxidation is a complex oxidative process where phospholipid hydroperoxides (PLOOH) are produced in membranes and finally transformed into a series of decomposition products, some of which are endowed with biological activity. It is specifically prevented by glutathione peroxidase 4 (GPx4), the selenoenzyme that reduces PLOOH by glutathione (GSH). PLOOH is both a product and the major initiator of peroxidative chain reactions, as well as an activator of lipoxygenases. α-Tocopherol both specifically breaks peroxidative chain propagation and inhibits lipoxygenases. Thus, GPx4, GSH, and α-tocopherol are integrated in a concerted anti-peroxidant mechanism. Recent Advances: Ferroptosis has been recently identified as a cell death subroutine that is specifically activated by missing GPx4 activity and inhibited by iron chelation or α-tocopherol supplementation. Ferroptosis induction may underlie spontaneous human diseases, such as major neurodegeneration and neuroinflammation, causing an excessive cell death. The basic mechanism of ferroptosis, therefore, fits the features of activation of lipid peroxidation.

CRITICAL ISSUES

Still lacking are convincing proofs that lipoxygenases are involved in ferroptosis. Also, unknown are the molecules eventually killing cells and the mechanisms underlying the drop of the cellular anti-peroxidant capacity.

FUTURE DIRECTIONS

Molecular events and mechanisms of ferroptosis to be unraveled and validated on animal models are GPx4 inactivation, role of GSH concentration, increased iron availability, and membrane structure and composition. This is expected to drive drug discovery that is aimed at halting cell death in degenerative diseases or boosting it in cancer cells. Antioxid. Redox Signal. 29, 61-74.

The Oxygen Paradox, the French Paradox, and age-related diseases

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.

Glutathione peroxidase 4-catalyzed reduction of lipid hydroperoxides in membranes: The polar head of membrane phospholipids binds the enzyme and addresses the fatty acid hydroperoxide group toward the redox center

GPx4 is a monomeric glutathione peroxidase, unique in reducing the hydroperoxide group (-OOH) of fatty acids esterified in membrane phospholipids. This reaction inhibits lipid peroxidation and accounts for enzyme's vital role. Here we investigated the interaction of GPx4 with membrane phospholipids. A cationic surface near the GPx4 catalytic center interacts with phospholipid polar heads. Accordingly, SPR analysis indicates cardiolipin as the phospholipid with maximal affinity to GPx4. Consistent with the electrostatic nature of the interaction, KCl increases the K_D. Molecular dynamic (MD) simulation shows that a -OOH posed in the core of the membrane as 13 - or 9 -OOH of tetra-linoleoyl cardiolipin or 15 -OOH stearoyl-arachidonoyl-phosphaphatidylcholine moves to the lipid-water interface. Thereby, the -OOH groups are addressed toward the GPx4 redox center. In this pose, however, the catalytic site facing the membrane would be inaccessible to GSH, but the consecutive redox processes facilitate access of GSH, which further primes undocking of the enzyme, because GSH competes for the binding residues implicated in docking. During the final phase of the catalytic cycle, while GSSG is produced, GPx4 is disconnected from the membrane. The observation that GSH depletion in cells induces GPx4 translocation to the membrane, is in agreement with this concept.

Selenoprotein Gene Nomenclature

The human genome contains 25 genes coding for selenocysteine-containing proteins (selenoproteins). These proteins are involved in a variety of functions, most notably redox homeostasis. Selenoprotein enzymes with known functions are designated according to these functions: TXNRD1, TXNRD2, and TXNRD3 (thioredoxin reductases), GPX1, GPX2, GPX3, GPX4, and GPX6 (glutathione peroxidases), DIO1, DIO2, and DIO3 (iodothyronine deiodinases), MSRB1 (methionine sulfoxide reductase B1), and SEPHS2 (selenophosphate synthetase 2). Selenoproteins without known functions have traditionally been denoted by SEL or SEP symbols. However, these symbols are sometimes ambiguous and conflict with the approved nomenclature for several other genes. Therefore, there is a need to implement a rational and coherent nomenclature system for selenoprotein-encoding genes. Our solution is to use the root symbol SELENO followed by a letter. This nomenclature applies to SELENOF (selenoprotein F, the 15-kDa selenoprotein, SEP15), SELENOH (selenoprotein H, SELH, C11orf31), SELENOI (selenoprotein I, SELI, EPT1), SELENOK (selenoprotein K, SELK), SELENOM (selenoprotein M, SELM), SELENON (selenoprotein N, SEPN1, SELN), SELENOO (selenoprotein O, SELO), SELENOP (selenoprotein P, SeP, SEPP1, SELP), SELENOS (selenoprotein S, SELS, SEPS1, VIMP), SELENOT (selenoprotein T, SELT), SELENOV (selenoprotein V, SELV), and SELENOW (selenoprotein W, SELW, SEPW1). This system, approved by the HUGO Gene Nomenclature Committee, also resolves conflicting, missing, and ambiguous designations for selenoprotein genes and is applicable to selenoproteins across vertebrates.

Redox status in a model of cancer stem cells

Reversible oxidation of Cys residues is a crucial element of redox homeostasis and signaling. According to a popular concept in oxidative stress signaling, the oxidation of targets of signals can only take place following an overwhelming of the cellular antioxidant capacity. This concept, however, ignores the activation of feedback mechanisms possibly leading to a paradoxical effect. In a model of cancer stem cells (CSC), stably overexpressing the TAZ oncogene, we observed that the increased formation of oxidants is associated with a globally more reduced state of proteins. Redox proteomics revealed that several proteins, capable of undergoing reversible redox transitions, are indeed more reduced while just few are more oxidized. Among the proteins more oxidized, G6PDH emerges as both more expressed and activated by oxidation. This accounts for the observed more reduced state of the NADPH/NADP⁺ couple. The dynamic redox flux generating this apparently paradoxical effect is rationalized in a computational system biology model highlighting the crucial role of G6PDH activity on the rate of redox transitions eventually leading to the reduction of reversible redox switches.

Redox homeostasis: The Golden Mean of healthy living

The notion that electrophiles serve as messengers in cell signaling is now widely accepted. Nonetheless, major issues restrain acceptance of redox homeostasis and redox signaling as components of maintenance of a normal physiological steady state. The first is that redox signaling requires sudden switching on of oxidant production and bypassing of antioxidant mechanisms rather than a continuous process that, like other signaling mechanisms, can be smoothly turned up or down. The second is the misperception that reactions in redox signaling involve “reactive oxygen species” rather than reaction of specific electrophiles with specific protein thiolates. The third is that hormesis provides protection against oxidants by increasing cellular defense or repair mechanisms rather than by specifically addressing the offset of redox homeostasis. Instead, we propose that both oxidant and antioxidant signaling are main features of redox homeostasis. As the redox shift is rapidly reversed by feedback reactions, homeostasis is maintained by continuous signaling for production and elimination of electrophiles and nucleophiles. Redox homeostasis, which is the maintenance of nucleophilic tone, accounts for a healthy physiological steady state. Electrophiles and nucleophiles are not intrinsically harmful or protective, and redox homeostasis is an essential feature of both the response to challenges and subsequent feedback. While the balance between oxidants and nucleophiles is preserved in redox homeostasis, oxidative stress provokes the establishment of a new radically altered redox steady state. The popular belief that scavenging free radicals by antioxidants has a beneficial effect is wishful thinking. We propose, instead, that continuous feedback preserves nucleophilic tone and that this is supported by redox active nutritional phytochemicals. These nonessential compounds, by activating Nrf2, mimic the effect of endogenously produced electrophiles (parahormesis). In summary, while hormesis, although globally protective, results in setting up of a new phenotype, parahormesis contributes to health by favoring maintenance of homeostasis.

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Redox homeostasis is the continuously challenged oxidative/nucleophilic balance.

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Rheostatic redox signaling enzymes maintain oxidative/nucleophilic homeostasis.

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Phytochemicals assist redox homeostasis through oxidative feedback (parahormesis).

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Adaptation and hormesis while protective establish a new phenotype and set point.

Selenocysteine oxidation in glutathione peroxidase catalysis: an MS-supported quantum mechanics study

Glutathione peroxidases (GPxs) are enzymes working with either selenium or sulfur catalysis. They adopted diverse functions ranging from detoxification of H(2)O(2) to redox signaling and differentiation. The relative stability of the selenoenzymes, however, remained enigmatic in view of the postulated involvement of a highly unstable selenenic acid form during catalysis. Nevertheless, density functional theory calculations obtained with a representative active site model verify the mechanistic concept of GPx catalysis and underscore its efficiency. However, they also allow that the selenenic acid, in the absence of the reducing substrate, reacts with a nitrogen in the active site. MS/MS analysis of oxidized rat GPx4 complies with the predicted structure, an 8-membered ring, in which selenium is bound as selenenylamide to the protein backbone. The intermediate can be re-integrated into the canonical GPx cycle by glutathione, whereas, under denaturing conditions, its selenium moiety undergoes β-cleavage with formation of a dehydro-alanine residue. The selenenylamide bypass prevents destruction of the redox center due to over-oxidation of the selenium or its elimination and likely allows fine-tuning of GPx activity or alternate substrate reactions for regulatory purposes.

Understanding mammalian glutathione peroxidase 7 in the light of its homologs

The glutathione peroxidase homologs (GPxs) efficiently reduce hydroperoxides using electrons from glutathione (GSH), thioredoxin (Trx), or protein disulfide isomerase (PDI). Trx is preferentially used by the GPxs of the majority of bacteria, invertebrates, plants, and fungi. GSH or PDI, instead, is preferentially used by vertebrate GPxs that operate by Sec or Cys catalysis, respectively. Mammalian GPx7 and GPx8 are unique homologs that contain a peroxidatic Cys (CP). Being reduced by PDI and located within the endoplasmic reticulum (ER), these enzymes have been involved in oxidative protein folding. Kinetic analysis indicates that oxidation of PDI by recombinant GPx7 occurs at a much faster rate than that of GSH. Nonetheless, activity measurement suggests that, at physiological concentrations, a competition between these two substrates takes place, with the rate of PDI oxidation by GPx7 controlled by the concentration of GSH, whereas the GSSG produced in the competing reaction contributes to the ER redox buffer. A mechanism has been proposed for GPx7 involving two Cys residues, in which an intramolecular disulfide of the CP is formed with an alleged resolving Cys (CR) located in the strongly conserved FPCNQ motif (C86 in humans), a noncanonical position in GPxs. Kinetic measurements and comparison with the other thiol peroxidases containing a functional CR suggest that a resolving function of C86 in the catalytic cycle is very unlikely. We propose that GPx7 is catalytically active as a 1-Cys-GPx, in which CP both reduces H2O2 and oxidizes PDI, and that the CP-C86 disulfide has instead the role of stabilizing the oxidized peroxidase in the absence of the reducing substrate.

Glutathione peroxidase 8 is transcriptionally regulated by HIFα and modulates growth factor signaling in HeLa cells

GPx8 is a mammalian Cys-glutathione peroxidase of the endoplasmic reticulum membrane, involved in protein folding. Its regulation is mostly unknown. We addressed both the functionality of two hypoxia-response elements (HREs) within the promoter, GPx8 HRE1 and GPx8 HRE2, and the GPx8 physiological role. In HeLa cells, treatment with HIFα stabilizers, such as diethyl succinate (DES) or 2-2'-bipyridyl (BP), induces GPx8 expression at both mRNA and protein level. Luciferase activity of pGL3(GPx8wt), containing a fragment of the GPx8 promoter including the two HREs, is also induced by DES/BP or by overexpressing either individual HIFα subunit. Mutating GPx8 HRE1 within pGL3(GPx8wt) resulted in a significantly higher inhibition of luciferase activity than mutating GPx8 HRE2. Electrophoretic mobility-shift assay showed that both HREs exhibit enhanced binding to a nuclear extract from DES/BP-treated cells, with stronger binding by GPx8 HRE1. In DES-treated cells transfected with pGL3(GPx8wt) or mutants thereof, silencing of HIF2α, but not HIF1α, abolishes luciferase activity. Thus GPx8 is a novel HIF target preferentially responding to HIF2α binding at its two novel functional GPx8 HREs, with GPx8 HRE1 playing the major role. Fibroblast growth factor (FGF) treatment increases GPx8 mRNA expression, and reporter gene experiments indicate that induction occurs via HIF. Comparing the effects of depleting GPx8 on the downstream effectors of FGF or insulin signaling revealed that absence of GPx8 results in a 16- or 12-fold increase in phosphorylated ERK1/2 by FGF or insulin treatment, respectively. Furthermore, in GPx8-depleted cells, phosphorylation of AKT by insulin treatment increases 2.5-fold. We suggest that induction of GPx8 expression by HIF slows down proliferative signaling during hypoxia and/or growth stimulation through receptor tyrosine kinases.

Quantitative label-free redox proteomics of reversible cysteine oxidation in red blood cell membranes

Reversible oxidation of cysteine residues is a relevant posttranslational modification of proteins. However, the low activation energy and transitory nature of the redox switch and the intrinsic complexity of the analysis render quite challenging the aim of a rigorous high-throughput screening of the redox status of redox-sensitive cysteine residues. We describe here a quantitative workflow for redox proteomics, where the ratio between the oxidized forms of proteins in the control vs treated samples is determined by a robust label-free approach. We critically present the convenience of the procedure by specifically addressing the following aspects: (i) the accurate ratio, calculated from the whole set of identified peptides rather than just isotope-tagged fragments; (ii) the application of a robust analytical pipeline to frame the most consistent data averaged over the biological variability; (iii) the relevance of using stringent criteria of analysis, even at the cost of losing potentially interesting but statistically uncertain data. The pipeline has been assessed on red blood cell membrane challenged with diamide as a model of a mild oxidative condition. The cluster of identified proteins encompassed components of the cytoskeleton more oxidized. Indirectly, our analysis confirmed the previous observation that oxidized hemoglobin binds to membranes while oxidized peroxiredoxin 2 loses affinity.

An overview of mechanisms of redox signaling

A principal characteristic of redox signaling is that it involves an oxidation-reduction reaction or covalent adduct formation between the sensor signaling protein and second messenger. Non-redox signaling may involve alteration of the second messenger as in hydrolysis of GTP by G proteins, modification of the signaling protein as in farnesylation, or simple non-covalent binding of an agonist or second messenger. The chemistry of redox signaling is reviewed here. Specifically we have described how among the so-called reactive oxygen species, only hydroperoxides clearly fit the role of a second messenger. Consideration of reaction kinetics and cellular location strongly suggests that for hydroperoxides, particular protein cysteines are the targets and that the requirements for redox signaling is that these cysteines are in microenvironments in which the cysteine is ionized to the thiolate, and a proton can be donated to form a leaving group. The chemistry described here is the same as occurs in the cysteine and selenocysteine peroxidases that are generally considered the primary defense against oxidative stress. But, these same enzymes can also act as the sensors and transducer for signaling. Conditions that would allow specific signaling by peroxynitrite and superoxide are also defined. Signaling by other electrophiles, which includes lipid peroxidation products, quinones formed from polyphenols and other metabolites also involves reaction with specific protein thiolates. Again, kinetics and location are the primary determinants that provide specificity required for physiological signaling although enzymatic catalysis is not likely involved. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Glutathione peroxidases

BACKGROUND

With increasing evidence that hydroperoxides are not only toxic but rather exert essential physiological functions, also hydroperoxide removing enzymes have to be re-viewed. In mammals, the peroxidases inter alia comprise the 8 glutathione peroxidases (GPx1-GPx8) so far identified.

SCOPE OF THE REVIEW

Since GPxs have recently been reviewed under various aspects, we here focus on novel findings considering their diverse physiological roles exceeding an antioxidant activity.

MAJOR CONCLUSIONS

GPxs are involved in balancing the H2O2 homeostasis in signalling cascades, e.g. in the insulin signalling pathway by GPx1; GPx2 plays a dual role in carcinogenesis depending on the mode of initiation and cancer stage; GPx3 is membrane associated possibly explaining a peroxidatic function despite low plasma concentrations of GSH; GPx4 has novel roles in the regulation of apoptosis and, together with GPx5, in male fertility. Functions of GPx6 are still unknown, and the proposed involvement of GPx7 and GPx8 in protein folding awaits elucidation.

GENERAL SIGNIFICANCE

Collectively, selenium-containing GPxs (GPx1-4 and 6) as well as their non-selenium congeners (GPx5, 7 and 8) became key players in important biological contexts far beyond the detoxification of hydroperoxides. This article is part of a Special Issue entitled Cellular functions of glutathione.

Relationship of baseline HbA1c, HbA1c change and HbA1c target of < 7% with insulin analogues in type 2 diabetes: a meta-analysis of randomised controlled trials

AIM

We performed a meta-analysis of randomised controlled trials (RCTs) with insulin analogues in type 2 diabetes utilising a least-squared regression model in order to assess the relationship between baseline HbA1c, the magnitude of HbA1c decrease and attainment of HbA1c target of < 7%.

METHODS

Randomised controlled trials involving insulin regimens (basal, prandial, biphasic and basal-bolus) were identified through electronic searches (MEDLINE, EMBASE, CINAHL and The Cochrane Library) through September 2010. We included any study arm of RCTs if they were at least 12 weeks in duration; the number of patients in any arm was more than 30 and reported the baseline HbA1c and change from baseline HbA1c.

RESULTS

We found 87 studies, with a total of 135 arms, and 38,803 patients. The weighted R(2) values for the overall analysis assessing the association between baseline HbA1c and absolute change in HbA1c or the proportion of patients at target were 0.485 (p < 0.001) and 0.146 (p < 0.001), respectively. Subanalyses of insulin regimens for the association between basal HbA1c and absolute decrease of HbA1c produced weighted R(2), which were significant for all insulin regimens with the highest association for basal-bolus (R(2) = 0.719, p < 0.001).

CONCLUSIONS

The strong positive relationship between baseline HbA1c and the magnitude of HbA1c change we found in RCTs using insulin analogues in type 2 diabetes should be considered when assessing the clinical efficacy of insulin therapies.

Cysteine mutant of mammalian GPx4 rescues cell death induced by disruption of the wild-type selenoenzyme

Selenoproteins are expressed in many organisms, including bacteria, insects, fish, and mammals. Yet, it has remained obscure why some organisms rely on selenoproteins while others, like yeast and plants, express Cys-containing homologues. This study addressed the possible advantage of selenocysteine (Sec) vs. Cys in the essential selenoprotein glutathione peroxidase 4 (GPx4), using 4-hydroxy-tamoxifen-inducible Cre-excision of loxP-flanked GPx4 alleles in murine cells. Previously, it was shown that GPx4 disruption caused rapid cell death, which was prevented by α-tocopherol. Results presented herein demonstrate that the expression of wild-type (WT) GPx4 and its Sec/Cys (U46C) mutant rescued cell death of GPx4(-/-) cells, whereas the Sec/Ser (U46S) mutant failed. Notably, the specific activity of U46C was decreased by ∼90% and was indistinguishable from U46S-expressing and mock-transfected cells. Hence, the U46C mutant prevented apoptosis despite hardly measurable in vitro activity. Doxycycline-inducible expression revealed that minute amounts of either U46C or WT GPx4 prevented cell death, albeit WT GPx4 was more efficient. Interestingly, at the same expression level, proliferation was promoted in U46C-expressing cells but attenuated in WT-expressing cells. In summary, both catalytic efficiency and the expression level of GPx4 control the balance between cell survival and proliferation.

Redox pioneer: Professor Leopold Flohé

Leopold Flohé is recognized here as a Redox Pioneer because has published a article on antioxidant/redox biology, as first author, that has been cited more than 1,000 times, and more than 20 articles have been cited more than 100 times. He obtained the medical doctorate at the Institute of Pharmacology and Toxicology at the University of Tübingen, Germany, in 1968. He held positions in both Academia (Tübingen, Aachen, and Braunschweig, Germany) and industry (Aachen). He is now operating the biotech company MOLISA in Magdeburg, Germany, while teaching as guest professor at the local university. Dr. Flohé is the pioneer who established the selenoprotein nature of glutathione peroxidase (GPx), the first and, for almost 10 years, the only selenoprotein known in animals. His work was pivotal to link the essential trace element selenium to metabolic processes, which led the Food and Drug Administration (FDA) to approve selenium supplementation for humans in 1980, and stimulated selenium biochemistry in general. In recent years, he embarked on investigating how pathogens protect themselves from oxidative killing. His inseminating studies on the thiol-dependent hydroperoxide metabolism of trypanosomatids and mycobacteria defined molecular drug targets, paving the way to new therapeutic strategies for neglected diseases affecting the people of developing countries.

Signaling functions of reactive oxygen species

We review signaling by reactive oxygen species, which is emerging as a major physiological process. However, among the reactive oxygen species, H(2)O(2) best fulfills the requirements of being a second messenger. Its enzymatic production and degradation, along with the requirements for the oxidation of thiols by H(2)O(2), provide the specificity for time and place that are required in signaling. Both thermodynamic and kinetic considerations suggest that among possible oxidation states of cysteine, formation of sulfenic acid derivatives or disulfides can be relevant as thiol redox switches in signaling. In this work, the general constraints that are required for protein thiol oxidation by H(2)O(2) to be fast enough to be relevant for signaling are discussed in light of the mechanism of oxidation of the catalytic cysteine or selenocysteine in thiol peroxidases. While the nonenzymatic reaction between thiol and H(2)O(2) is, in most cases, too slow to be relevant in signaling, the enzymatic catalysis of thiol oxidation by these peroxidases provides a potential mechanism for redox signaling.

Signaling functions of reactive oxygen species

We review signaling by reactive oxygen species, which is emerging as a major physiological process. However, among the reactive oxygen species, H(2)O(2) best fulfills the requirements of being a second messenger. Its enzymatic production and degradation, along with the requirements for the oxidation of thiols by H(2)O(2), provide the specificity for time and place that are required in signaling. Both thermodynamic and kinetic considerations suggest that among possible oxidation states of cysteine, formation of sulfenic acid derivatives or disulfides can be relevant as thiol redox switches in signaling. In this work, the general constraints that are required for protein thiol oxidation by H(2)O(2) to be fast enough to be relevant for signaling are discussed in light of the mechanism of oxidation of the catalytic cysteine or selenocysteine in thiol peroxidases. While the nonenzymatic reaction between thiol and H(2)O(2) is, in most cases, too slow to be relevant in signaling, the enzymatic catalysis of thiol oxidation by these peroxidases provides a potential mechanism for redox signaling.

Identification by MS/MS of disulfides produced by a functional redox transition

Among posttranslational modifications of proteins entailed with signal transduction, the redox transition is today brought to the focus as a major biochemical event accounting for the signaling functions of reactive oxygen species. Thermodynamic and kinetic criteria highlight hydroperoxides and protein disulfides as signaling and transducer elements, respectively, and growing biochemical evidence supports this notion. The protein Cys residue involved in this function must react fast and specifically with the oxidant and then with a second accessible Cys yielding the disulfide. These kinetic and structural constraints are shared with peroxidases and peroxiredoxins, which are competitors for the signaling hydroperoxide. In this chapter, a procedure based on MS/MS analysis for inter- and intrachain disulfide assignment in proteins undergoing redox-switch is presented. While the sensitivity of the modern MS/MS instruments permits the sequencing of double peptides linked by a disulfide bond, the major pitfall of the proteomic procedure is the thiol-disulfide scrambling taking place at the alkaline pH needed for the proteolytic reaction of trypsin. Instead, the use of pepsin at acidic pH prevents the disulfide scrambling, but the specificity of the proteolytic reaction is low and thus the complexity of fragmentation increases. We succeeded to limit this problem by heuristically assuming a conserved pepsin cleavage pattern of the protein both in the oxidized and the reduced form. Asymmetric cleavage of the disulfide by collisional fragmentation further corroborated the identification. In conclusion, the use of pepsin, integrated by a minimal computation, appears suitable for positively assigning inter- and intrachain disulfides generated by a functional redox-switch.

Catalytic mechanisms and specificities of glutathione peroxidases: variations of a basic scheme

Kinetics and molecular mechanisms of GPx-type enzymes are reviewed with emphasis on structural features relevant to efficiency and specificity. In Sec-GPxs the reaction takes place at a single redox centre with selenocysteine as redox-active residue (peroxidatic Sec, U(P)). In contrast, most of the non-vertebrate GPx have the U(P) replaced by a cysteine (peroxidatic Cys, C(P)) and work with a second redox centre that contains a resolving cysteine (C(R)). While the former type of enzymes is more or less specific for GSH, the latter are reduced by "redoxins". The common denominator of the GPx family is the first redox centre comprising the (seleno)cysteine, tryptophan, asparagine and glutamine. In this architectural context the rate of hydroperoxide reduction by U(P) or C(P), respectively, is enhanced by several orders of magnitude compared to that of free selenolate or thiolate. Mammalian GPx-1 dominates H(2)O(2) metabolism, whereas the domain of GPx-4 is the reduction of lipid hydroperoxides with important consequences such as counteracting 12/15-lipoxygenase-induced apoptosis and regulation of inflammatory responses. Beyond, the degenerate GSH specificity of GPx-4 allows selenylation and oxidation to disulfides of protein thiols. Heterodimer formation of yeast GPx with a transcription factor is discussed as paradigm of a redox sensing that might also be valid in vertebrates.

The catalytic site of glutathione peroxidases

In GPxs, the redox-active Se or S, is at hydrogen bonding distance from Gln and Trp residues that contribute to catalysis. From sequence homology of >400 sequences and modeling of the DmGPx as a paradigm, Asn136 emerged as a fourth essential component of the active site. Mutational substitution of Asn136 by His, Ala, or Asp results in a dramatic decline of specific activity. Kinetic analysis indicates that k(+1), the rate constant for the oxidation of the enzyme, decreases by two to three orders of magnitude, whereas the reductive steps characterized by k'(+2) are less affected. Accordingly, MS/MS analysis shows that in Asn136 mutants, the peroxidatic Cys45 stays largely reduced also in the presence of a hydroperoxide, whereas in the wild-type enzyme, it is oxidized, forming a disulfide with the resolving Cys. Computational calculation of pK(a) values indicates that the residues facing the catalytic thiol, Asn136, Gln80, and, to a lesser extent Trp135, contribute to the dissociation of the thiol group, Asn136 being most relevant. These data disclose that the catalytic site of GPxs has to be redrawn as a tetrad, including Asn136, and suggest a mechanism accounting for the extraordinary catalytic efficiency of GPxs.

Differential liquid phase proteomic analysis of the effect of selenium supplementation in LNCaP cells

The effect of 100 nM sodium selenite supplementation was studied on LNCaP cells by a proteomic approach, on ProteomeLab PF 2D platform. Proteins were separated by liquid phase bi-dimensional chromatography and analyzed by pair-wise alignment of peaks to detect those differentially expressed. Differential expression threshold was set at a twice difference level and proteins matching this criterion were identified by MALDI-TOF and confirmed by ESI-ion trap MS/MS. Not all differentially expressed proteins found by PF 2D could be identified by MS analysis, the sensitivity of which emerging as the limiting factor. Thus, only the most abundant proteins, differently expressed following selenium supplementation, were identified. We positively showed an increase of expression of thioredoxin reductase 1, enolase 1, phosphoglycerate mutase 1, glyceraldehyde-3-phosphate dehydrogenase, heterogeneous nuclear ribonucleoprotein A2/B1, isoform A2, Ras-GTPase-activating protein SH3-domain-binding protein and Keratin 18 and a decrease of expression of peroxiredoxin 1 and heat shock protein 70, protein 8, isoform 1. Results are consistent, at least in part, with the less oxidant environment brought about by the synthesis of Se-dependent peroxidases, keeping low the steady-state concentration of hydrogen peroxide.

Effect of mercury on selenium utilization and selenoperoxidase activity in LNCaP cells

Formation of stable complexes with protein thiols is the best-known mechanism of mercury toxicity. However, the solubility product of Hg(2+) with sulfides, although very low, is higher than that with selenides, suggesting that the fully reduced form of selenium might also be a relevant target for Hg(2+). In cells, selenide is the suggested intermediate for selenoprotein biosynthesis and selenoenzymes, in turn, contain reduced selenium as the catalytic moiety. Thus, inhibition of biological functions of selenium could be seen as a different mechanism of Hg(2+) toxicity. To address this issue, we investigated selenoperoxidase (SeGPx) activity in LNCaP cells exposed to HgCl(2). Cells growing in standard medium express a low GPx activity, which increases on addition of selenium donors such as selenite, selenomethionine, or methyl-Se-cysteine. HgCl(2) added to the medium has different effects depending on the type of Se donor. A progressive decrease of SeGPx activity is observed in cells grown in standard medium exposed to HgCl(2), while coadministration of suprastoichiometric amounts of HgCl(2) prevents the increase of SeGPx activity only when selenite, but not selenomethionine or methyl-Se-cysteine, is the selenium source. From this evidence we conclude that HgCl(2): (a) does not inhibit directly SeGPxs, as confirmed on isolated enzymes; (b) does not interfere with the intermediates of the metabolic pathway of selenoprotein synthesis; and (c) decreases the bioavailability of selenium only when ionic complexes can be formed.

The Thioredoxin Specificity of Drosophila GPx: A Paradigm for a Peroxiredoxin-like Mechanism of many Glutathione Peroxidases

Some members of the glutathione peroxidase (GPx) family have been reported to accept thioredoxin as reducing substrate. However, the selenocysteine-containing ones oxidise thioredoxin (Trx), if at all, at extremely slow rates. In contrast, the Cys homolog of Drosophila melanogaster exhibits a clear preference for Trx, the net forward rate constant, k'(+2), for reduction by Trx being 1.5x10(6) M(-1) s(-1), but only 5.4 M(-1) s(-1) for glutathione. Like other CysGPxs with thioredoxin peroxidase activity, Drosophila melanogaster (Dm)GPx oxidized by H(2)O(2) contained an intra-molecular disulfide bridge between the active-site cysteine (C45; C(P)) and C91. Site-directed mutagenesis of C91 in DmGPx abrogated Trx peroxidase activity, but increased the rate constant for glutathione by two orders of magnitude. In contrast, a replacement of C74 by Ser or Ala only marginally affected activity and specificity of DmGPx. Furthermore, LC-MS/MS analysis of oxidized DmGPx exposed to a reduced Trx C35S mutant yielded a dead-end intermediate containing a disulfide between Trx C32 and DmGPx C91. Thus, the catalytic mechanism of DmGPx, unlike that of selenocysteine (Sec)GPxs, involves formation of an internal disulfide that is pivotal to the interaction with Trx. Hereby C91, like the analogous second cysteine in 2-cysteine peroxiredoxins, adopts the role of a "resolving" cysteine (C(R)). Molecular modeling and homology considerations based on 450 GPxs suggest peculiar features to determine Trx specificity: (i) a non-aligned second Cys within the fourth helix that acts as C(R); (ii) deletions of the subunit interfaces typical of tetrameric GPxs leading to flexibility of the C(R)-containing loop. Based of these characteristics, most of the non-mammalian CysGPxs, in functional terms, are thioredoxin peroxidases.

Functional Interaction of Phospholipid Hydroperoxide Glutathione Peroxidase with Sperm Mitochondrion-associated Cysteine-rich Protein Discloses the Adjacent Cysteine Motif as a New Substrate of the Selenoperoxidase

The mitochondrial capsule is a selenium- and disulfide-rich structure enchasing the outer mitochondrial membrane of mammalian spermatozoa. Among the proteins solubilized from the sperm mitochondrial capsule, we confirmed, by using a proteomic approach, the presence of phospholipid hydroperoxide glutathione peroxidase (PHGPx) as a major component, and we also identified the sperm mitochondrion-associated cysteine-rich protein (SMCP) and fragments/aggregates of specific keratins that previously escaped detection (Ursini, F., Heim, S., Kiess, M., Maiorino, M., Roveri, A., Wissing, J., and Flohé, L. (1999) Science 285, 1393-1396). The evidence for a functional association between PHGPx, SMCP, and keratins is further supported by the identification of a sequence motif of regularly spaced Cys-Cys doublets common to SMCP and high sulfur keratin-associated proteins, involved in bundling hair shaft keratin by disulfide cross-linking. Following the oxidative polymerization of mitochondrial capsule proteins, catalyzed by PHGPx, two-dimensional redox electrophoresis analysis showed homo- and heteropolymers of SMCP and PHGPx, together with other minor components. Adjacent cysteine residues in SMCP peptides are oxidized to cystine by PHGPx. This unusual disulfide is known to drive, by reshuffling oxidative protein folding. On this basis we propose that oxidative polymerization of the mitochondrial capsule is primed by the formation of cystine on SMCP, followed by reshuffling. Occurrence of reshuffling is further supported by the calculated thermodynamic gain of the process. This study suggests a new mechanism where selenium catalysis drives the cross-linking of structural elements of the cytoskeleton via the oxidation of a keratin-associated protein.

The nuclear form of phospholipid hydroperoxide glutathione peroxidase is a protein thiol peroxidase contributing to sperm chromatin stability

The selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPx) is regarded as the major molecular target of selenodeficiency in rodents, accounting for most of the histopathological and structural abnormalities of testicular tissue and male germ cells. PHGPx exists as a cytosolic form, mitochondrial form, and nuclear form (nPHGPx) predominantly expressed in late spermatids and spermatozoa. Here, we demonstrate that mice with a targeted deletion of the nPHGPx gene were, unlike mice with the full knockout (KO) of PHGPx, not only viable but also, surprisingly, fully fertile. While both morphological analysis of testis and epididymis and sperm parameter measurements did not show any apparent abnormality, toluidine blue and acridine orange stainings of spermatozoa indicated defective chromatin condensation in the KO sperm isolated from the caput epididymis. Furthermore, upon drying and hydrating, KO sperm exhibited a significant proportion of morphologically abnormal heads. Monobromobimane labeling and protein-free thiol titration revealed significantly less extensive oxidation in the cauda epididymis when compared to that in the wild type. We conclude that nPHGPx, by acting as a protein thiol peroxidase in vivo, contributes to the structural stability of sperm chromatin.

Primary structure of the nuclear forms of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in rat spermatozoa

Phospholipid hydroperoxide glutathione peroxidase is a monomeric Se-peroxidase highly expressed in mammalian male germ cells. Its nuclear form, sperm nuclei glutathione peroxidase (snGPx), has been originally identified in maturating spermatozoa as a transcription product containing an alternative exon within the phospholipid hydroperoxide glutathione peroxidase gene. In this paper, we show that this form is inconstantly detectable in rat spermatozoa where a 20.0 and 25.9 kDa major forms are detected instead. These have been conclusively characterized. The N-terminus sequence of the 20.0 kDa form confirmed that the protein is identical to cytosolic form, suggesting diffusion into the nucleus. The 25.9 kDa protein represented a truncated form of the previously described nuclear snGPx, lacking the basic nuclear localization signal. This protein is present in two forms differing from each other by the presence of an N-terminal methionine. The presence of traces of the larger snGPx form suggests that exhaustive proteolytic processing of the precursor produces the 25.9 kDa enzyme, although the alternate use of a downstream ATG, at least in rodents, could not be unequivocally ruled out.

Oral carnitine supplementation increases sperm motility in asthenozoospermic men with normal sperm phospholipid hydroperoxide glutathione peroxidase levels

OBJECTIVE

To clarify the role of carnitine supplementation in idiopathic asthenozoospermia and to look for a rationale for its use in asthenozoospermic patients.

DESIGN

Blind clinical study.

SETTING

Academic.

PATIENT(S)

Thirty asthenozoospermic patients divided in two groups according to phospholipid hydroperoxide glutathione peroxidase (PHGPx) levels.

INTERVENTION(S)

Placebo for 3 months, then oral L-carnitine (2 g/day) for 3 months; semen samples were collected at baseline, after placebo, after carnitine administration, and again after 3 months with no drugs.

MAIN OUTCOME MEASURE(S)

Evaluation of seminal parameters and determination of seminal PHGPx levels, measured as rescued activity.

RESULT(S)

When asthenozoospermic subjects were divided in two groups on the basis of PHGPx levels, we observed an improvement of mean sperm motility only in the group of patients with normal PHGPx levels.

CONCLUSION(S)

Phospholipid hydroperoxide glutathione peroxidase has an important role in male infertility, and carnitine treatment might improve sperm motility in the presence of normal mitochondrial function.

Distinct promoters determine alternative transcription of gpx-4 into phospholipid-hydroperoxide glutathione peroxidase variants

A nuclear variant of phospholipid-hydroperoxide glutathione peroxidase (PHGPx, GPx-4) was considered to be derived from alternative pre-mRNA splicing in testis and to regulate sperm maturation. The genomic sequence of rat gpx-4 was established and investigated in respect to expression into the cytosolic, mitochondrial, and nuclear forms of PHGPx. In silico analysis suggested the presence of two distinct promoter regions, the upstream one leading to transcripts translating into cPHGPx or mPHGPx and the downstream one yielding nPHGPx. The promoter activity of both regions was verified by luciferase-based reporter constructs in A7r5 and H9c2 cells. The data reveal that the formation of nPHGPx is due to alternative transcription and not to alternative splicing. Transcripts encoding nPHGPx were most abundant in testis although not restricted to this organ. This observation points to a general role of the nuclear PHGPx variant in regulating cell division.

Versatility of selenium catalysis in PHGPx unraveled by LC/ESI-MS/MS

Phospholipid hydroperoxide glutathione peroxidase (PHGPx; EC 1.11.1.12), a broad-spectrum thiol-dependent peroxidase, deserves renewed interest as a regulatory factor in various signaling cascades and as a structural protein in sperm cells. We present a first attempt to identify catalytic intermediates and derivatives of the selenoprotein by liquid chromatography coupled to electrospray tandem mass spectrometry (LC/ESI-MS/MS) and to explain observed specificities by molecular modeling. The ground state enzyme E proved to correspond to position 3-170 of the deduced porcine sequence with selenium being present as selenocysteine at position 46. The selenenic acid form, which is considered to be the first catalytic intermediate F formed by reaction with hydroperoxide, could not be identified. The second catalytic intermediate G was detected as Se-glutathionylated enzyme. This intermediate is generated in the reverse reaction where the active site selenol interacts with glutathione disulfide (GSSG). According to molecular models, specific binding of reduced glutathione (GSH) and of GSSG is inter alia facilitated by electrostatic attraction of Lys-48 and Lys-125. Polymerization of PHGPx is obtained under oxidizing conditions in the absence of low molecular weight thiols. Analysis of MS spectra revealed that the process is due to a selective reaction of Sec-46 with Cys-148' resulting in linear polymers representing dead-end intermediates (G'). FT Docking of PHGPx molecules allowed reactions of Sec-46 with either Cys-66', Cys-107', Cys-168' or Cys-148', the latter option being most likely as judged by the number of proposed intermediates with reasonable hydrogen bonds, interaction energies and interface areas. We conclude that the same catalytic principles, depending on the conditions, can drive the diverse actions of PHGPx, i.e. hydroperoxide reduction, GSSG reduction, S-derivatization and self-incorporation into biological structures.

Genetic variations of gpx-4 and male infertility in humans

Phospholipid hydroperoxide glutathione peroxidase (PHGPx), the product of gpx-4, is the major selenoprotein in sperm and is considered essential for fertilization because of its multiple roles in spermatogenesis, such as hydroperoxide detoxification, formation of the mitochondrial capsule, and chromatin condensation. Genomic DNA sequences of 3.148 kilobases covering the whole gpx-4 and its flanking regions were amplified from 63 men using the polymerase chain reaction and were analyzed for polymorphisms by direct sequencing. A total of 23 variant sites were detected; 2 were present only in control men (proven fathers; n = 21) and 10 were common to fertile controls and infertile patients (n = 42). A further 11 variant sites were seen in five of the infertile men only. Four of the gpx-4 variants were considered irrelevant to GPx-4-related fertility problems because they occurred homozygously in controls. The majority of the remaining variant sites are also of questionable relevance because they are located in introns or, as third base exchanges, do not affect the protein sequence. However, one of the exon variations leads to an Ala93-Thr exchange that reduces activity in a porcine GPx-4 homologue. Two detected promoter variations were shown by reporter gene constructs to affect transcription in somatic cell lines. These results indicate that gpx-4 polymorphism cannot generally account for the correlation of PHGPx content of sperm and fertility-related parameters, but further examination of this gene as a potential cause of infertility in particular cases is warranted.

Male fertility is linked to the selenoprotein phospholipid hydroperoxide glutathione peroxidase

The selenoprotein phospholipid hydroperoxide glutathione peroxidase (PHGPx) accounts for almost the entire selenium content of mammalian testis. PHGPx is abundantly expressed in spermatids as active peroxidase but is transformed to an oxidatively inactivated protein in mature sperm, where it is a major constituent of the mitochondrial capsule in the midpiece. Male infertility in selenium-deficient animals, which is characterized by impaired sperm motility and morphological midpiece alterations, is considered to result from insufficient PHGPx content. We studied the relationship between sperm PHGPx, measured as rescued activity, and human fertility. Sperm specimens from 75 infertile men and 37 controls were analyzed for fertility-related parameters according to World Health Organization criteria. The PHGPx protein content was estimated after reductive solubilization of the spermatozoa by measuring the rescued PHGPx activity. Rescued PHGPx activity of infertile men ranged significantly below that of controls (93.2 +/- 60.1 units/mg sperm protein vs. 187.5 +/- 55.3 units/mg) and was particularly low in oligoasthenozoospermic specimens (61.93 +/- 45.42 units/mg; P < 0.001 compared with controls and asthenozoospermic samples). Rescued PHGPx activity was correlated positively with viability, morphological integrity, and most profoundly forward motility (r = 0.35, 0.44, and 0.45, respectively). In isolated motile samples, motility decreased faster with decreasing PHGPx content. In humans, PHGPx appears to be indispensable for structural integrity of spermatozoa and to codetermine sperm motility and viability. Because the content of PHGPx, irrespective of the cause of alteration, is correlated with fertility-related parameters, PHGPx can be considered a predictive measure for fertilization capacity.