Protective effect of dietary selenium supplementation on delayed cardiotoxicity of adriamycin in rat: is PHGPX but not GPX involved?

The involvement of Se enzymes in the protection against the oxidative stress induced by adriamycin (ADR) in rat heart has been studied in animals fed for 10 weeks at three different levels of Se content (low = 0.02 ppm; normal = 0.5 ppm; high = 1.0 ppm) and receiving a weekly injection of 3 mg/kg ADR for 4 weeks. ECG (QaT duration) and contractility of isolated atria were measured. The high-Se diet showed a significant protection on both parameters. To assess the hypothesis that an increase of specific activity of antioxidant Se enzymes may account for the cardioprotective effect of selenium, glutathione peroxidase (GPX), and phospholipid hydroperoxide glutathione peroxidase (PHGPX) were tested. The assays were performed on ventricles isolated from treated rats. At the end of the experimental period, GPX (cytosolic enzyme) did not show any significant difference between controls and ADR-treated at any level of Se content, thus excluding its involvement in the cardioprotection observed in high-Se ADR-treated animals. PHGPX, which is present both in cytosol and in the cell membrane, showed a trend to increase its activity in the presence of ADR treatment only in the membrane fraction; however, the statistical significance was reached only in the low-Se group (+100%). This observation suggests that membrane PHGPX might be involved in the cellular mechanism of adaptation of the heart to the toxic effects of ADR; however, the behavior of these enzymes does not seem to account for the significant protection of selenium supplementation both on ECG and on contractile indices of ADR cardiotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Ipriflavone does not alter bone apatite crystal structure in adult male rats

We have previously found that a short-term treatment with high doses of ipriflavone increased bone density and improved the biomechanical properties of adult male rat bones, without altering their mineral composition. To determine whether this effect can be associated with alterations of bone crystal structure, we have performed X-ray diffraction analysis of bones obtained from rats treated with ipriflavone at doses that were effective in inducing favorable changes on bone density and biomechanics. Eighteen-week-old male Sprague Dawley rats were treated by oral route with either ipriflavone (200 or 400 mg/kg/day), or its vehicle for 12 weeks. The treatment was well tolerated and body weight increased to the same extent in all animals. As a measure of bone crystallinity, we examined the (310) and (002) reflections of the X-ray diffraction patterns, corresponding to the directions perpendicular and parallel to the c-axis of the crystals, respectively. No major differences were observed between ipriflavone-treated and control animals for the broadening parameter beta(1/2) for (310) and (002) peaks, as well as for lattice parameters. Therefore, a 12-week treatment with ipriflavone at high doses does not induce significant modifications of bone "crystallinity." Thus, the positive effect of ipriflavone on bone mineral density appears to be associated with an increased apatite crystal formation rather than an increase of crystal size. These results provide further evidence for the safety and usefulness of ipriflavone in the treatment of osteoporotic syndromes.

Kinetic analysis of lipid-hydroperoxides in plasma

We increased the precision of chemiluminescent procedure for measuring lipid hydroperoxides in plasma or lipoproteins by (i) escaping from extraction and chromatography of lipids, (ii) using detergent dispersed lipids, and (iii) calculating the results by fitting the photon emission rate with the integrated equation, which describes the model of the series of reactions. The use of kinetics instead of the crude integration of cps increases precision because at each measurement the correct reaction pathway is tested. This was relevant for the optimization of the analytical procedure, contributing to the elimination of possible side reactions. The relationship between lipid hydroperoxide content in the sample and cps is not linear; thus, the calculation of results through internal calibration is carried out using an exponential equation. This is in agreement with the reaction mechanism and raises the point of the linear calibration previously reported in other chemiluminescent procedures. Although sensitive and precise, this procedure suffers for being time consuming, requiring approximately 30 min per sample. Moreover, since no chromatography is used, information about the hydroperoxides in different lipid classes is missing. Obviously this will be solved when a validated procedure for quantitatively extracting lipid hydroperoxides is available.

Antioxidant defences of rabbit alveolar lining fluid

In the lower respiratory tract, alveolar cells are exposed to an oxidative challenge related to the exposure to both high levels of molecular oxygen and oxidants generated by activated phagocytes. The antioxidant defence system of alveolar cells has been thoroughly investigated, but some reports also suggest the presence of antioxidants in the layer of fluid lining the alveoli. In this report we present our studies on the antioxidant activities present in the bronchoalveolar lavage of adult rabbits. We studied total radical-trapping antioxidant capacity of surfactant and the activity of antiperoxidant enzymes. Although previous reports suggested the presence of radical scavengers, we did not find any antioxidant activity in purified surfactant. On the other hand the alveolar-lining fluid seems to contain superoxide dismutase, catalase and glutathione peroxidase, but not appreciable amounts of ferroxidase activity, as previously suggested. These enzymes could protect alveolar cells by catalyzing the dismutation of superoxide and hydrogen peroxide. The presence of glutathione peroxidase in the alveolar space seems to be physiologically relevant since the alveolar lining fluid also contains millimolar amounts of glutathione. Our studies support the concept that the alveolar lining fluid contains an active defence system against products of partial reduction of oxygen, but not chain-breaker antioxidants.

Effect of plasma on the degradation of hydroperoxides of unesterified linoleic acid and copper-peroxidized LDL

The determination of lipid hydroperoxides in plasma and lipoproteins recently reached a clinical relevance in disorders such as atherosclerosis, where oxidative reactions have been suggested to play a fundamental pathogenetic role. The peroxide content of lipoproteins is usually measured after ultracentrifugation and extraction. During this procedure, some peroxides might decompose causing a too low recovery. To screen this possibility, the disappearance, in the presence of human plasma, of hydroperoxides of linoleic acid and Cu-oxidized low density lipoprotein (LDL) have been investigated, using both a iodometric titration and an enzymatic assay. While only in the presence of GSH plasma decomposes linoleic acid hydroperoxides quite rapidly, peroxides in Cu-oxidized LDL were stable both in presence as well as in absence of GSH. This indicated that lipid hydroperoxides are stable in plasma and that peroxides of Cu-oxidized LDL are not substrate for the glutathione-dependent peroxidase activity in plasma. The relevant decrease of the iodometric titre of LDL peroxides observed in the presence of elevated amounts of plasma was shown to be artifactual, since some compounds extracted from plasma do react with iodine generated by peroxides. Whole plasma itself, indeed, has been shown to reduce back to I- appreciable amount of free iodine.

Attempt to differentiate between individual glutathione peroxidases in biological samples

Routine determinations of glutathione peroxidases (GPx) still suffer from poor standardization and usually from lack of specification. The different types of glutathione peroxidases present in cellular homogenates may be differentially estimated by their distinct substrate specificities. Complex lipid hydroperoxides required for this approach, however, are not generally available nor easily standardized due to their molecular heterogeneity and tendency to form micelles. We therefore developed a simple procedure for the differential estimation of the major cellular types of GPx, the cytosolic GPx (cGPx) and the phospholipid hydroperoxide glutathione peroxidase (PHGPx) taking advantage of the peculiar susceptibility of PHGPx to deoxycholate. It proved to reliably determine the activities of both purified cGPx and PHGPx, in mixtures thereof, and in homogenates of tissue samples (e.g., testes), and some (e.g. ECV 304) but not all (e.g. THP-1) cultured cell lines. The method allows the differential estimation of cGPx and PHGPx, if the samples do not contain further types of GPx.

Phospholipid hydroperoxide glutathione peroxidase of rat testis. Gonadotropin dependence and immunocytochemical identification

A high glutathione peroxidase activity toward phospholipid hydroperoxides is present in rat testis. The attribution of this activity to the selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPX) was supported by cross-reactivity with antibodies raised against pig heart PHGPX which had been purified and characterized. Rat testis PHGPX is partially cytosolic and partially linked to nuclei and mitochondria. The soluble and organelle-bound enzymes appear identical by Western blot analysis. PHGPX, but neither selenium-dependent nor non-selenium-dependent glutathione peroxidase activity, is expressed in testes only after puberty, disappears after hypophysectomy, and is partially restored by gonadotropin treatment. Specific immunostaining of testes by antiserum against PHGPX appears as a fine granular brown pattern localized throughout the cytoplasm in more immature cells but is confined to the peripheral part of the cytoplasm, the nuclear membrane, and mitochondria in maturating spermatogenic cells. As expected, immunostaining of spermatogenic cells in hypophysectomized animals was negative, but gonadotropin treatment only marginally increased the immunoreactivity. The expression of PHGPX in testes is consistent with the previously described specific requirement for selenium for synthesis of a 15-20-kDa selenoprotein which is related to the production of functional spermatozoa.

Protective action of phospholipid hydroperoxide glutathione peroxidase against membrane-damaging lipid peroxidation. In situ reduction of phospholipid and cholesterol hydroperoxides

The general reactivity of membrane lipid hydroperoxides (LOOHs) with the selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPX) has been investigated. When human erythrocyte ghosts (lipid content: 60 wt % phospholipid; 25 wt % cholesterol) were treated with GSH/PHGPX subsequent to rose bengal-sensitized photoperoxidation, iodometrically measured LOOHs were totally reduced to alcohols. Similar treatment with the classic glutathione peroxidase (GPX) produced no effect unless the peroxidized membranes were preincubated with phospholipase A2 (PLA2). However, under these conditions, no more than approximately 60% of the LOOH was reduced; introduction of PHGPX brought the reaction to completion. Thin layer chromatographic analyses revealed that the GPX-resistant (but PHGPX-reactive) LOOH was cholesterol hydroperoxide (ChOOH) consisting mainly of the 5 alpha (singlet oxygen-derived) product. Membrane ChOOHs were reduced by GSH/PHGPX to species that comigrated with borohydride reduction products (diols). Sensitive quantitation of PHGPX-catalyzed ChOOH reduction was accomplished by using [14C]cholesterol-labeled ghosts. Kinetic analyses indicated that the rate of ChOOH decay was approximately 1/6 that of phospholipid hydroperoxide decay. Photooxidized ghosts underwent a large burst of free radical-mediated lipid peroxidation when incubation with ascorbate/iron or xanthine/xanthine oxidase/iron. These reactions were only partially inhibited by PLA2/GSH/GPX treatment, but totally inhibited by GSH/PHGPX treatment, consistent with complete elimination of LOOHs in the latter case. These findings provide important clues as to how ChOOHs are detoxified in cells and add new insights into PHGPX's protective role.

Phospholipid hydroperoxide glutathione peroxidase is the 18-kDa selenoprotein expressed in human tumor cell lines

Human tumor cell lines cultured in 75Se-containing media demonstrate four major 75Se-labeled cellular proteins (57, 22, 18, and 12 kDa) on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Among these selenoproteins, an enzymatic activity is known only for the 22-kDa protein, since this protein has been identified as the monomer of glutathione peroxidase. However, all tested cell lines also contained a peroxidase activity with phospholipid hydroperoxides that is completely accounted for by the other selenoenzyme, phospholipid hydroperoxide glutathione peroxidase (PHGPX) (Ursini, F., Maiorino, M., and Gregolin, C. (1985) Biochim. Biophys. Acta 839, 62-70). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography of 75Se-labeled proteins separated by gel permeation chromatography supported the identification of PHGPX as the monomeric protein matching the 18 kDa band. This paper is the first report on the identification of PHGPX in human cells.

Phospholipid-hydroperoxide glutathione peroxidase. Genomic DNA, cDNA, and deduced amino acid sequence

The complete amino acid sequence of the selenoprotein phospholipid-hydroperoxide glutathione peroxidase (PHGPX) from pig heart has been deduced from the corresponding genomic DNA, the cDNA covering the coding region, and by sequencing the N terminus of the protein. The maximum length of the peptide chain derived from the cDNA amounts to 170 amino acid residues. By protein sequencing the N-terminal residues methionine and cysteine of the deduced sequence were found to be cleaved. The molecular mass of 19,671 Da obtained by laser desorption mass spectroscopy, however, significantly exceeds the mean molecular mass of 19,257.09 calculated for the sequence 3-170 of PHGPX, thus indicating posttranscriptional modification. In contrast to glutathione peroxidase (GPX) the coding area of the PHGPX gene is composed of seven exons. Only the amino acid sequences encoded by the third and fifth exon are highly homologous to GPX sequences. The amino acid residues selenocysteine, tryptophan, and glutamine forming the catalytic site in bovine GPX are conserved in homologous positions of PHGPX, whereas the arginine residues presumed to bind GSH in GPX are not. Gaps in the PHGPX sequence correspond to subunit interaction sites of the tetrameric GPX. The data suggest an identical catalytic mechanism of the selenoperoxidases, a less stringent substrate specificity of PHGPX, and explain the monomeric nature of PHGPX. As in other selenoproteins, the selenocysteine residue of PHGPX is encoded by UGA. The 3'-untranslated region (UTR) of the PHGPX shows a limited consensus with that of GPX and 5'-deiodinase, where it was shown to be responsible for the decoding of UGA as selenocysteine. The 3'-UTR of PHGPX can form a stem/loop as in other mammalian selenoprotein genes. The 5'-UTR and the first intron of the PHGPX gene contain a variety of putative regulatory elements indicating hormonal control.

Product of the Schistosoma mansoni glutathione peroxidase gene is a selenium containing phospholipid hydroperoxide glutathione peroxidase (PHGPx) sharing molecular weight and substrate specificity with its mammalian counterpart

In the blood fluke Schistosoma mansoni a functionally active, monomeric, phospholipid hydroperoxide glutathione peroxidase (PHGPx) has been purified and characterized. This enzyme contains a catalytically active selenocysteine. The protein has been shown to be the product of a cloned gene, previously referred to as a glutathione peroxidase gene. S. mansoni PHGPx has been found 5 times more abundant in female than in male worm extract. As in vertebrate PHGPx, homology alignment indicates that the residues involved in the glutathione binding by the tetrametric cellular glutathione peroxidase are mutated in the S. mansoni enzyme. Thus, this aspect appears a landmark of the PHGPx-type of glutathione peroxidases, which might be of functional relevance.

Probing the presumed catalytic triad of a selenium-containing peroxidase by mutational analysis

Glutathione peroxidases (GPx) are characterized by a catalytically active selenium which forms the center of a strictly conserved triad composed of selenocysteine, glutamine, and tryptophan. In order to check the functional relevance of this structural peculiarity, six molecular mutants of phospholipid hydroperoxide glutathione peroxidase (PHGPx) were designed, isolated, and investigated kinetically. Replacement of the selenocysteine in position 46 by cysteine decreased k + 1, i.e., the reaction rate of reduced enzyme with hydroperoxide, by three orders of magnitude. The rate of regeneration of the reduced enzyme by glutathione (k' + 2) was similarly affected. Additional substitution of Gln81 or Trp136 by acid residues resulted in a further decrease of k + 1 by three orders of magnitude, whereas histidine or neutral residues in these positions proved to be less deleterious. The data support the hypothesis that the typical triad of selenocysteine, glutamine, and tryptophan is indeed a novel catalytic center in which the reactivity of selenium is optimized by hydrogen bonding provided by the adjacent glutamine and tryptophan residues.

LDL- is a lipid hydroperoxide-enriched circulating lipoprotein

A subclass of LDL described on the basis of its greater electronegativity and oxidative status is further characterized using a new, highly sensitive single photon counting technique to measure lipid hydroperoxides. We describe in this report that these particles, which we refer to as LDL-, are enriched in lipid peroxides and other peroxidation products as compared to the bulk of the unmodified, normal LDL (nLDL) recovered from human plasma. This chemiluminescence-based, single photon counting technique has unique advantages in that analyses are performed on whole LDL, thus avoiding artifactual lipid peroxidation during lipid extraction. Evidence for increased amounts of lipid hydroperoxides in LDL- versus nLDL are in agreement with other analytical methods such as measurement of conjugated dienes as well as cholesterol oxidation products. LDL- also has lower proportions of polyunsaturated fatty acids than nLDL. Analysis of the amino acid composition of apoB-100 and fatty acid composition of total LDL lipids also revealed major differences between nLDL and LDL- consistent with an oxidative modification of the latter. Thus, LDL- has significantly lower proportions of the oxidizable amino acids histidine and lysine, and marked differences in other neutral and acidic amino acids. The deficit in specific amino acids is in agreement with a reduced TNBS reactivity and increased relative electrophoretic mobility of LDL-. We postulate that LDL- is a major carrier of lipid hydroperoxides associated with plasma LDL and may arise from oxidative events in the vasculature and/ or by ingestion of peroxide-enriched meals.

Phospholipid hydroperoxide glutathione peroxidase in the normal human kidney: a possible role in protecting cell membranes

Reactive oxygen species have been implicated in the pathogenesis of tissue injury. It is generally accepted that selenium-glutathione peroxidases form an integrated system defending the living organism against oxidative damage. Phospholipid hydroperoxide glutathione peroxidase (PHGPX) is thought to play a prominent role in preventing lipid peroxidation. Indeed, the function of PHGPX is to reduce the lipophilic substrates in membranes. In the present study, we evaluated the expression of PHGPX in normal human kidney by immunohistochemistry. The enzyme in glomeruli is mainly expressed in podocytes and parietal epithelial cells. In addition, PHGPX antigen was detected in tubule epithelial cells. Therefore, these results suggest that renal epithelial cells possess an important antioxidizing activity related to the presence of PHGPX.

Phospholipid hydroperoxide glutathione peroxidase (PHGPx): more than an antioxidant enzyme?

The family of glutathione peroxidases encompasses, as far, three tetrameric glutathione peroxidases (GPx) and the monomeric PHGPx. Although the overall homology between tetrameric enzymes and PHGPx is less than 30%, a pronounced similarity has been detected on clusters involved in the active site and a common catalytic triad (selenocysteine glutamine and tryptophan) has been defined by structural and kinetic data. A major peculiar feature of the reaction catalyzed by PHGPx is the possibility to accommodate large lipophilic substrates. This accounts for the observed dramatic antiperoxidant effect and the synergism with vitamin E. Moreover, the reduction of lipid hydroperoxides accounts also for the observed modulation of cycloxygenase and inhibition of 15-lipoxygenase. On the other hand, structural and kinetic data indicate that also the specificity of PHGPx for the donor substrate is not restricted to GSH and the recent observation the PHGPx binds to specific mitochondrial proteins, from which it is released by ionic strength and thiols, suggests a possible fole of this selenoenzyme in catalyzing the specific oxidation of protein thiols, thus modulating the activity of cellular regulatory elements. On this light, the selenium mojety of PHGPx, reacting much faster that thiols with a peroxide, and then oxidizing specific protein thiols, would channel the oxidation toward protein targets, thus providing, by protein-protein interaction, the specificity of the redox transition.