Research needs for human nutrition in the post-genome-sequencing era.

The sequencing and annotation of the human genome and the genomes of other model organisms offer new tools and new opportunities for human nutrition research in the 21st century. Basic research continues to be the key foundation for formulating solid nutrition recommendations for the public, but the basis for establishing human nutrient recommendations today suffers because of lack of good biomarkers and because of weak federal funding for nutrition research. In the context of this post-human genome-sequencing era, tantalizing opportunity exists in seven areas-four basic and three applied: 1) identification of molecular biomarkers for nutrient status; 2) characterization of single polynuclear polymorphisms (SNPs) associated with nutrition; 3) development of a national genome array nutrition database; 4) use of models at all phylogenetic levels for nutrition research; 5) application of post-genome-sequencing tools to study diet and human health, including phytonutrients and genetically modified foods; 6) application of these tools to the role of nutrition in pathogenesis of human disease; and 7) development of a funding base for research on exercise and human health.

Selenium regulation of transcript abundance and translational efficiency of glutathione peroxidase-1 and -4 in rat liver.

Glutathione peroxidase (GPX)1 mRNA in rat liver falls dramatically during Se deficiency to levels that are approx. 10% of Se-adequate levels. This regulation is mediated by mRNA stability, and is hypothesized to involve nonsense-mediated mRNA decay. mRNA levels for GPX4 and other selenoproteins are much less regulated by Se status. To evaluate the relative contribution of mRNA abundance versus translational efficiency to overall regulation of GPX1 expression, we quantified GPX1, GPX4 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) transcripts per cell in rat liver. Surprisingly, we found that GPX1 transcripts in Se deficiency are moderately abundant and similar in abundance to GAPDH and other selenoprotein mRNAs; Se supplementation increases GPX1 mRNA so that it is 30-fold higher than GAPDH mRNA. Translational efficiency of GPX1 mRNA is half of that of GPX4. Translational efficiency of GPX1 mRNA increases approx. 20-fold with Se supplementation and appears to switch GPX1 mRNA from nonsense-mediated degradation to translation. This regulatory switch can explain why GPX1 expression is an excellent parameter for assessment of Se status.

UGA codon position affects the efficiency of selenocysteine incorporation into glutathione peroxidase-1.

A UGA codon and a selenocysteine insertion sequence in the 3'-untranslated region are the only established mRNA elements necessary for selenocysteine (Sec or U) incorporation during translation. These two elements, however, do not universally confer efficient Sec incorporation. The objective of this study was to systematically examine the effect of UGA codon position on efficiency of Sec insertion. In a glutathione peroxidase-1 (F-GPX1) expression vector, the UGA at the native position (U47) was mutated to a cysteine codon, and codons for Ser-7, Ser-12, Ser-18, Ser-29, Ser-45, Ser-93, Cys-154, Val-172, Ser-178, and Ser-195 were individually mutated to UGA and transiently expressed in COS-7 cells. 75Se incorporation at the 11 positions was 31, 72, 54, 105, 90, 100, 146, 135, 13, 11, and 43%, respectively, of 75Se incorporation at U47, suggesting that Sec is more efficiently incorporated at UGA codons positioned in the middle of the coding region rather than close to the 5' or 3' ends. Ribonuclease protection showed that these differences were not due to differences in mRNA level. When the green fluorescence protein (GFP) coding region was placed in-frame at the 5' or 3' ends of the coding region in F-GPX1 to produce chimeric 50-51-kDa GFP/GPX1 proteins, Sec incorporation at UGA codons, formerly close to the 5' or 3' ends, was increased to levels comparable to the UGA at U47. Insertion of GFP after the UAA-stop was just as effective in increasing Sec insertion efficiency as GFP inserted before the stop. These studies used a recombinant expression model that incorporated Sec at non-native UGA codons at rates equal to those of endogenous glutathione peroxidase-1 and showed that the efficiency of Sec incorporation can be modulated by UGA position; Sec incorporation at high efficiency appears to require that the UGA be >21 nucleotides from the AUG-start and >204 nucleotides from the selenocysteine insertion sequence element.

Liver selenium and testis phospholipid hydroperoxide glutathione peroxidase are associated with growth during selenium repletion of second-generation Se-deficient male rats.

We have previously shown that changes in glutathione peroxidase-1 (GPX1; H2O2:oxidoreductase, EC 1.11.1.9), plasma thyroid hormone and glutathione-S-transferase were not associated with changes in growth observed in second-generation (F2) severely Se-deficient rats; we also found that liver phospholipid hydroperoxide glutathione peroxidase (GPX4; EC 1.11.1.12) activity falls in first-generation (F1) Se-deficient rats to 41% of levels in Se-adequate rats. The purposes of this study were to determine the effect of F2 Se deficiency on GPX4 and to detect early changes in Se parameters associated with growth after single, small Se injections. Se-deficient male F2 weanling rats were randomly divided into two groups and fed a Se-deficient crystalline amino acid (0.003 microg Se/g diet; -Se) diet or that diet supplemented for 14 d with 0.2 microg Se/g diet (+Se) as Na2SeO3. Growth of -Se rats was 55% of the rate of +Se rats. Liver Se, GPX1 activity, GPX4 activity and testis GPX4 activity in -Se rats at 14 d were 1, 2, 23 and 13%, respectively, of levels in +Se rats. In a series of experiments, additional F2 male weanling rats were fed the -Se diet for 14 d and then were given an intraperitoneal single saline injection of 0, 1 or 5 microg Se/100 g body weight (BW) as Na2SeO3 and killed 1 or 7 d later. Rats injected with 1 or 5 microg Se/100 g BW grew 36 or 48%, respectively, above the rate of saline-injected rats. Liver Se concentration increased 367% and testis GPX4 activity doubled in rats 1 d after injection of 1 microg Se/100 g BW compared with saline-injected rats; these parameters were further elevated with 5 microg Se/100 g BW injections. Increases in liver Se and testis GPX4 activity were the parameters best associated with improved growth after Se injection, but the molecular role for Se in growth remains unclear.

Cis-acting elements are required for selenium regulation of glutathione peroxidase-1 mRNA levels.

Classical glutathione peroxidase (GPX1) mRNA levels can decrease to less than 10% in selenium (Se)-deficient rat liver. The cis-acting nucleic acid sequence requirements for Se regulation of GPX1 mRNA levels were studied by transfecting Chinese hamster ovary (CHO) cells with GPX1 DNA constructs in which specific regions of the GPX1 gene were mutated, deleted, or replaced by comparable regions from unregulated genes such as phospholipid hydroperoxide glutathione peroxidase (GPX4). For each construct, stable transfectants were pooled two weeks after transfection, divided into Se-deficient (2 nM Se) or Se-adequate (200 nM Se) medium, and grown for an additional four days. On day of harvest, Se-deficient GPX1 and GPX4 activities averaged 13 +/- 2% and 15 +/- 2% of Se adequate levels, confirming that cellular Se status was dramatically altered by Se supplementation. RNA was isolated from replicate plates of cells and transfected mRNA levels were specifically determined by RNase protection assay. Analysis of chimeric GPX1/GPX4 constructs showed that the GPX4 3'-UTR can completely replace the GPX1 3'-UTR in Se regulation of GPX1 mRNA. We did not find any GPX1 coding regions that could be replaced by the corresponding GPX4 coding regions without diminishing or eliminating Se regulation of the transfected GPX1 mRNA. Further analysis of the GPX1 coding region demonstrated that the GPX1 Sec codon (UGA) and the GPX1 intron sequences are required for full Se regulation of transfected GPX1 mRNA levels. Mutations that moved the GPX1 Sec codon to three different positions within the GPX1 coding region suggest that the mechanism for Se regulation of GPX1 mRNA requires a Sec codon within exon 1. Lastly, we found that addition of the GPX1 3'-UTR to beta-globin mRNA can convey significant Se regulation to beta-globin mRNA levels when a UGA codon is placed within exon 1. We conclude that Se regulation of GPX1 mRNA requires a functional selenocysteine insertion sequence (SECIS) in the 3'-UTR and a Sec codon followed by an intron.

Selenium regulation of selenium-dependent glutathione peroxidases in animals and transfected CHO cells.

Glutathione peroxidase (GPX1) was the first identified selenium-dependent enzyme, and this enzyme has been most useful as a biochemical indicator of selenium (Se) status and the parameter of choice for determining Se requirements. We have continued to study Se regulation of GPX1 to better understand the underlying mechanism and to gain insight into how cells themselves regulate nutrient status. In progressive Se deficiency in rats, GPX1 activity, protein and mRNA all decrease in a dramatic, coordinated and exponential fashion such that Se-deficient GPX1 mRNA levels are 6-15% of Se-adequate levels. mRNA levels for other Se-dependent proteins are far less decreased in the same animals. The mRNA levels for a second Se-dependent peroxidase, phospholipid hydroperoxide glutathione peroxidase (GPX4), are little affected by Se deficiency, demonstrating that Se regulation of GPX1 is unique. Se regulation of GPX1 activity in growing male and female rats shows that the Se requirement is 100 ng/g diet, based on liver GPX1 activity; use of GPX1 mRNA as the parameter indicates that the Se requirement is nearer to 50 ng Se/g diet in both male and female rats. This approach will readily detect an altered dietary Se requirement, as shown by the incremental increases in dietary Se requirement by 150, 100 or 50 ng Se/g diet in Se-deficient rat pups repleted with Se for 3, 7 or 14 d, respectively. Studies with CHO cells stably transfected with recombinant GPX1 also show that overexpression of GPX1 does not alter the minimum level of media Se necessary for Se-adequate levels of GPX1 activity or mRNA. We hypothesize that classical GPX1 has an integral biological role in the mechanism used by cells to regulate Se status, making GPX1 an especially useful and effective parameter for determining Se requirements in animals.

Selenium regulation of classical glutathione peroxidase expression requires the 3' untranslated region in Chinese hamster ovary cells.

Classical glutathione peroxidase (GPX) mRNA levels fall dramatically in selenium (Se)-deficient animals, but it is not known whether this mechanism is related to the mRNA 3' untranslated region (3'UTR) sequences that have been shown to direct Se incorporation. In this study, we used recombinant GPX constructs to investigate the role of the GPX 3'UTR in Se regulation of GPX mRNA levels in Chinese hamster ovary (CHO) cells. The CHO cells were transfected with GPX (pRc/GPX), GPX lacking the 3'UTR (pRc/Delta3'UTR) or the pRc/CMV vector alone, and GPX activity and GPX mRNA levels were determined in stable transfectants grown in low Se basal medium with a range of added Se concentrations. We identified two pRc/GPX transfectants with significantly elevated GPX activity levels compared with pRc/CMV transfectants. The elevated GPX expression did not dramatically shift the amount of Se that was sufficient for GPX activity to reach the Se-adequate plateau level (100 nmol/L added Se). As expected, GPX activity was not significantly different when pRc/Delta3'UTR transfectants were compared with pRc/CMV control transfectants. Among the wild type and transfected CHO cells, Se-deficient GPX activity levels averaged 35 +/- 5% of Se-adequate levels. Selenium-deficient levels of endogenous GPX mRNA as well as recombinant pRc/GPX mRNA averaged 54-58% of Se-adequate levels; 3-4 nmol/L added Se was sufficient for maximal GPX mRNA levels. In contrast, pRc/Delta3'UTR mRNA levels in the unsupplemented cells remained at Se-adequate levels and showed no distinct Se regulation. These studies demonstrate that the GPX 3'UTR is necessary for Se regulation of GPX mRNA levels in addition to its role in Se incorporation.

The selenium requirement for glutathione peroxidase mRNA level is half of the selenium requirement for glutathione peroxidase activity in female rats.

To determine critically the selenium (Se) requirement for weanling female rats, we used glutathione peroxidase (GSH: H2O2 oxidoreductase, EC 1.11.1.9) (GPX) mRNA and a number of other parameters to assess Se status. Rats were fed a basal torulayeast diet (0.007 micrograms Se/g) supplemented with Se as Na2SeO3 in graded levels from 0 to 0.3 micrograms Se/g diet for 32 d (3 rats/group). Selenium supplementation had no effect on growth, showing that the Se requirement for growth is less than 0.007 micrograms Se/g diet, whereas other parameters showed significant increases with Se supplementation. In rats fed the Se-deficient basal diet, liver Se concentration was 4 +/- 0%, plasma GPX activity was 8 +/- 1%, erythrocyte GPX activity was 40 +/- 3%, liver GPX activity was 2 +/- 1%, and liver GPX mRNA levels were 11-17% of the levels in rats fed 0.1 micrograms Se/g diet. Liver Se concentration and GPX activity in plasma, erythrocytes and liver all reached a plateau breakpoint at or near 0.1 micrograms Se/g diet, indicating that the dietary Se requirement for maximal GPX activity in growing female rats is 0.1 micrograms Se/g diet. Liver GPX mRNA levels reached the plateau breakpoint at 0.05 micrograms Se/g diet, showing that the minimum dietary Se requirement for maximal GPX mRNA levels in female rats is half of the Se requirement for maximal GPX activity. This experiment demonstrates that GPX mRNA can be used to determine the dietary Se requirement; the gap between the dietary Se necessary for maximal GPX mRNA and that for maximal GPX activity may represent an evolutionarily derived biological margin of safety.

Selenoprotein gene expression during selenium-repletion of selenium-deficient rats.

Selenium repletion of selenium-deficient rats with 20 micrograms selenium / kg body weight as Na2SeO3 was used as a model to investigate the mechanisms that control the distribution of the trace element to specific selenoproteins in liver and thyroid. Cytosolic glutathione peroxidase (cGSHPx), phospholipid hydroperoxide glutathione peroxidase (PHGSHPx), and iodothyronine 5'-deiodinase (IDI) activities were all transiently increased in liver 16 to 32 h after ip injection with selenium. However, only cGSHPx and PHGSHPx activities increased in the thyroid where IDI activity was already increased by selenium deficiency. These responses were owing to synthesis of the seleoproteins on newly synthesised and/or existing mRNAs. The selenoprotein mRNAs in the thyroid gland were increased two- and threefold after the transitory increases in selenoprotein activity. In contrast, there were parallel changes in selenoprotein mRNAs and enzyme activities in the liver, with no prolonged rises in mRNA levels. The organ differences suggest that increased thryotrophin (TSH) concentrations, which are known to induce thyrodial IDI and mRNA, may control the mRNAs for all the thyroidal selenoproteins investigated and be a major mechanism for the preservation of thyroidal selenoproteins when selenium supplies are limited.

Tissue-specific regulation of selenoenzyme gene expression during selenium deficiency in rats.

Regulation of synthesis of the selenoenzymes cytosolic glutathione peroxidase (GSH-Px), phospholipid hydroperoxide glutathione peroxidase (PHGSH-Px) and type-1 iodothyronine 5'-deiodinase (5'IDI) was investigated in liver, thyroid and heart of rats fed on diets containing 0.405, 0.104 (Se-adequate), 0.052, 0.024 or 0.003 mg of Se/kg. Severe Se deficiency (0.003 mg of Se/kg) caused almost total loss of GSH-Px activity and mRNA in liver and heart. 5'IDI activity decreased by 95% in liver and its mRNA by 50%; in the thyroid, activity increased by 15% and mRNA by 95%. PHGSH-Px activity was reduced by 75% in the liver and 60% in the heart but mRNA levels were unchanged; in the thyroid, PHGSH-Px activity was unaffected by Se depletion but its mRNA increased by 52%. Thus there is differential regulation of the three mRNAs and subsequent protein synthesis within and between organs, suggesting both that mechanisms exist to channel Se for synthesis of a particular enzyme and that there is tissue-specific regulation of selenoenzyme mRNAs. During Se depletion, the levels of selenoenzyme mRNA did not necessarily parallel the changes in enzyme activity, suggesting a distinct mechanism for regulating mRNA levels. Nuclear run-off assays with isolated liver nuclei showed severe Se deficiency to have no effect on transcription of the three genes, suggesting that there is post-transcriptional control of the three selenoenzymes, probably involving regulation of mRNA stability.

Glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase are differentially regulated in rats by dietary selenium.

Phospholipid hydroperoxide glutathione peroxidase (PHGPX) and classical glutathione peroxidase (GPX1) are encoded by separate genes with only about 40% amino acid and nucleic acid sequence identity. To determine the response of tissue PHGPX expression to dietary Se level and to compare these responses with those for GPX1, weanling male rats were fed amino acid diets containing from 2 (-Se) to 130 (+Se) microgram Se/kg diet or a torula diet containing 5 and 190 micrograms Se/kg diet as Na2SeO3 for 28 d. Tissues were analyzed for PHGPX and GPX1 activity and mRNA. There was no effect of Se on growth. In -Se rats, GPX1 activity was reduced to 1% in liver and 4-9% in heart, kidney and lung compared with +Se rats; PHGPX activity was reduced only to 25-50% in these four tissues. The Se response curves indicated that the dietary Se requirement to reach plateau liver PHGPX activity was half that required for plateau GPX activity. In -Se rats, liver and heart GPX1 mRNA levels were reduced to 6 and 12%, respectively, whereas PHGPX mRNA was not significantly affected by Se deficiency. Notably, 65 micrograms Se/kg diet resulted in plateau liver GPX1 mRNA levels but not plateau GPX activity. Testis had the lowest GPX activity and GPX1 mRNA of all tissues examined, but had 15-fold higher PHGPX activity and 45-fold higher PHGPX mRNA levels when compared with liver. There was no significant effect of dietary Se on testis GPX1 and PHGPX mRNA levels. This study demonstrates that these two selenoperoxidases are differentially regulated by dietary Se.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth and plasma triiodothyronine concentrations are modified by selenium deficiency and repletion in second-generation selenium-deficient rats.

Classical glutathione peroxidase (GPX) is a useful Se-dependent parameter for determining Se status, but loss of GPX activity alone cannot explain the full effects of Se deficiency. The recent identification of type I thyroxine 5'-deiodinase as a Se-dependent enzyme provides a new potentially critical role for Se. To develop a model of impaired growth due to Se deficiency, second generation deficient weanling rats were fed a Se-deficient amino acid diet with adequate vitamin E and methionine. Initial growth rates of deficient males and females were 53 and 63%, respectively, of rats fed 0.1 micrograms Se/g diet. In short-term experiments with deficient males, liver Se and GPX activity were reduced 99%, liver glutathione-s-transferase activity was increased 114%, plasma thyroxine concentrations were increased 67%, plasma triiodothyronine was decreased 23% and the plasma triiodothyronine:thyroxine ratio was decreased 55%, compared with rats fed 0.2 micrograms Se/g diet. When deficient rats were injected on d 14 with 0, 1, 5 or 10 micrograms Se/100 g, rats grew 4.45, 7.62, 7.17 and 9.05 g/d, respectively, over the next 7 d. Injection with 10 micrograms Se/100 g restored plasma thyroxine and triiodothyronine concentrations 7 d later. Rats injected with 1 microgram Se/100 g rat had significantly altered plasma thyroxine and triiodothyronine concentrations 1 but not 7 d after injection. Infusion of Se-deficient rats with 438 ng triiodothyronine/d for 7 d restored plasma triiodothyronine concentrations but did not increase growth rate compared with rats infused with saline. This model produced a significant growth depression that was significantly reversed by as little as 1 microgram Se/100 g rat, but not by triiodothyronine infusion, suggesting that other Se-dependent roles in addition to 5'-deiodinase and GPX are necessary for adequate growth.

Phospholipid hydroperoxide glutathione peroxidase: full-length pig blastocyst cDNA sequence and regulation by selenium status.

We have isolated and sequenced an apparent full-length cDNA clone for phospholipid hydroperoxide glutathione peroxidase (Genbank accession number L12743) from a pig blastocyst cDNA library. The sequence encodes a polypeptide of 170 amino acids, including a TGA-encoded selenocysteine at residue 46, with a calculated M(r) of 19,492 Da. Use of this clone in Northern blot analysis of Se-deficient rat liver revealed that phospholipid hydroperoxide glutathione peroxidase mRNA levels were little affected by Se deficiency, whereas classical glutathione peroxidase mRNA levels were decreased by 90% in the same samples. Lastly, liver phospholipid hydroperoxide glutathione peroxidase mRNA levels were not elevated in female rats, in contrast to classical glutathione peroxidase.

Comparison of liver glutathione peroxidase activity and mRNA in female and male mice and rats.

1. Female and male adult mouse and rat liver was analyzed for glutathione peroxidase activity, mRNA levels, and other selected liver enzymes. 2. Species and sex differences in liver protein, total RNA and total mRNA were minor. 3. Glutathione peroxidase activity, mRNA levels, and selenium concentration was lower in male rats when compared to female rats, male mice or female mice. 4. Plasma ceruloplasmin activity, but not liver mRNA levels, were lower in mice compared to rats. 5. Cu,Zn-superoxide dismutase activity and mRNA were not greatly influenced by species or sex. 6. Glutathione transferase activity towards 1-chloro-2,4-dinitrobenzene was highest in male mice and equivalent in the other three groups.

Modulation of selenium-dependent glutathione peroxidase by perfluorodecanoic acid in rats: effect of dietary selenium.

Forty-eight male Sprague-Dawley rats fed a diet containing 0.4, 0.2 or 1.0 mg of selenium (Se)/kg of diet were injected with a single dose (35 mg/kg) of perfluorodecanoic acid (PFDA) in corn oil and killed 2 wk later. Control animals were pair-fed and treated with an equal volume of vehicle. PFDA treatment significantly increased Se-dependent glutathione peroxidase (Se-GSHPx) activity in liver cytosol of rats fed the 0.04 mg of Se/kg of diet but not in rats fed the other diets. The increase in liver cytosolic Se-GSHPx activity in rats fed 0.04 mg of Se/kg of diet paralleled increases in Se content and serum Se-GSHPx activity. Determination of Se-GSHPx by an enzyme-linked immunosorbent assay showed that PFDA caused a decrease in Se-GSHPx protein in rats fed 0.2 or 1.0 mg of Se/kg of diet but not in rats fed 0.04 mg of Se/kg of diet. Further analysis revealed that the ratio of Se-GSHPx activity to antibody-reactive protein was increased by PFDA in all three groups. The in vitro addition of PFDA directly to the assay mixture for Se-GSHPx activity did not produce any effect. Reduced glutathione was significantly increased by PFDA treatment in all three groups. These data show that PFDA affects the Se content, Se-GSHPx activity and Se-GSHPx protein in rat liver and that the effect is dependent on the dietary/hepatic Se level.

Structure of the active site of sulfite oxidase. X-ray absorption spectroscopy of the Mo(IV), Mo(V), and Mo(VI) oxidation states.

The active site of sulfite oxidase has been investigated by X-ray absorption spectroscopy at the molybdenum K-edge at 4 K. We have investigated all three accessible molybdenum oxidation states, Mo(IV), Mo(V), and Mo(VI), allowing comparison with the Mo(V) electron paramagnetic resonance data for the first time. Quantitative analysis of the extended X-ray absorption fine structure indicates that the Mo(VI) oxidation state possesses two terminal oxo (Mo = O) and approximately three thiolate-like (Mo-S-) ligands and is unaffected by changes in pH and chloride concentration. The Mo(IV) and Mo(V) oxidation states, however, each have a single oxo ligand plus one Mo-O- (or Mo-N less than) bond, most probably Mo--OH, and two to three thiolate-like ligands. Both reduced forms appear to gain a single chloride ligand under conditions of low pH and high chloride concentration.

Purification of prosthetically intact sulfite oxidase from chicken liver using a modified procedure.

A modified procedure was used to purify sulfite oxidase (sulfite:O2 oxidoreductase; EC 1.8.3.1) from chicken liver in high yield. The modifications included dialysis of the enzyme against a buffered solution containing sodium molybdate (prior to ion-exchange chromatography), which apparently reconstituted any demolybdo enzyme present in the extract, and phenyl-Sepharose column chromatography. Analysis showed that the purified enzyme contained Mo and heme in a 1.03:1.00 ratio, indicating that the enzyme was prosthetically intact; exogenous heme and other colored proteins were absent from the final pool. Treatment of the sulfite-reduced enzyme with 50 mM cyanide at pH 8.5 resulted in a gradual loss of catalytic activity with a half-life of 19.7 min. Analysis of the cyanide-inactivated enzyme gave a Mo:heme ratio of 1.02:1.00, providing the first direct evidence that the enzyme does not lose molybdenum when inactivated with cyanide. This modified purification procedure provides enzyme in high yield which is well-suited for experiments requiring prosthetically intact enzyme and which is not contaminated with extraneous heme or with other redox active proteins.

The nature of the phosphate complex of sulphite oxidase from electron-paramagnetic-resonance studies.

The phosphate complex of sulphite oxidase in the Mo(V) oxidation state was investigated by e.p.r. spectroscopy. Third-derivative spectra reveal a wealth of structural detail previously unobserved in this spectrum. Most notable is the presence of hyperfine coupling from two inequivalent I = 1/2 nuclei, which we tentatively attribute to two 31P nuclei. Unresolved hyperfine interactions from at least one exchangeable 1H nucleus are also present.

Effect of dietary methionine on tissue selenium and glutathione peroxidase (EC 1.11.1.9) activity in rats given selenomethionine.

1. The effect of dietary methionine on the utilization of selenium from dietary selenomethionine [( Se]Met) for tissue Se deposition and for glutathione peroxidase (EC 1.11.1.9; GSH-Px) synthesis was studied in male weanling rats. 2. When rats were given 0.5 mg Se as [Se]Met/kg diet supplemented with 0, 4 or 9 g methionine/kg, Se in plasma, erythrocytes, liver and muscle increased significantly over the 20 d period for all methionine-treatment groups. The increases in erythrocyte and muscle Se, however, were significantly higher in rats fed on the methionine-deficient diet compared with the methionine-supplemented diets. 3. In contrast to the increases in tissue Se, GSH-Px activity in liver, plasma and muscle decreased in methionine-deficient rats given 0.5 mg Se as [Se]Met/kg whereas GSH-Px activity was maintained or increased in rats supplemented with methionine. 4. The percentage of tissue Se associated with GSH-Px was calculated from the measured Se concentration and GSH-Px activity. A significantly lower percentage of Se was associated with GSH-Px in methionine-deficient rats compared with methionine-supplemented rats. 5. These results show that Se from dietary [Se]Met is preferentially incorporated into body proteins rather than used for GSH-Px synthesis when methionine is limiting in the diet. 6. These results further suggest that [Se]Met might not be the optimum Se compound to use for Se supplementation because metabolism of dietary [Se]Met to a biochemically active form, such as GSH-Px, was impaired when [Se]Met was provided in diets low in methionine.