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PubMed1999

Trace 5-methylaminomethyl-2-selenouridine in bovine tRNA and the selenouridine synthase activity in bovine liver.

We measured the amount of Se in bovine liver tRNA. tRNA was chromatographed on a BD-cellulose column and Se-rich tRNA was eluted from the column in front of a main tRNA peak. There was 0.3 mmol Se/mol of tRNA and this level is about one tenth that of Escherichia coli tRNA. This suggests the presence of an enzyme that modifies tRNA with Se in bovine liver. We isolated the activity of this enzyme (selenouridine synthase) by chromatography of bovine liver extracts on a DEAE-cellulose column. ATP and selenophosphate synthetase, as well as selenouridine synthase and tRNA, were necessary for the reaction. 75Se was used to label the reaction products, which were analyzed by TLC after digestion with ribonuclease T2. The position of the 75Se-nucleotide on a TLC plate was identical to that of the Se-nucleotide, 5-methylaminomethyl-2-seleno-Up, prepared from 75Se-tRNA in E. coli.

Animals↗
PubMed2005

Selenoprotein deficiency and high levels of selenium compounds can effectively inhibit hepatocarcinogenesis in transgenic mice.

The micronutrient element selenium (Se) has been shown to be effective in reducing the incidence of cancer in animal models and human clinical trials. Selenoproteins and low molecular weight Se compounds were implicated in the chemopreventive effect, but specific mechanisms are not clear. We examined the role of Se and selenoproteins in liver tumor formation in TGFalpha/c-Myc transgenic mice, which are characterized by disrupted redox homeostasis and develop liver cancer by 6 months of age. In these mice, both Se deficiency and high levels of Se compounds suppressed hepatocarcinogenesis. In addition, both treatments induced expression of detoxification genes, increased apoptosis and inhibited cell proliferation. Within low-to-optimal levels of dietary Se, tumor formation correlated with expression of most selenoproteins. These data suggest that changes in selenoprotein expression may either suppress or promote tumorigenesis depending on cell type and genotype. Since dietary Se may have opposing effects on cancer, it is important to identify the subjects who will benefit from Se supplementation as well as those who will not.

Animals↗
PubMed1998

Speciation of selenium in the mammalian organism.

As almost all of the selenium present in the mammalian organism is protein-bound, speciation is mostly concerned with the determination of the different selenium-containing proteins. Information on their distribution and their concentrations in the different tissues of the rat was obtained by means of tracer procedures which, after application of 75Se-selenite with a very high specific activity to selenium-depleted animals and electrophoretic separation of the labelled proteins, allow the determination of these compounds in the pmol to fmos range. A method was developed for the determination of selenocysteine and selenomethionine in the selenium-containing proteins. The identification of specific selenoproteins was achieved by analysis of their selenoamino acid residues and by studies on their characteristics and possible biological functions. This is being followed by the development of methods for the quantitative analysis of the selenoproteins in questions in the tissues of animals and man. In this paper the strategies and procedures used in the identification, characterization and determination of the selenium species present in the mammalian organism will be discussed.

Animals↗
PubMed1999

Incorporation of selenium into selenoprotein P and extracellular glutathione peroxidase: HPLC-ICPMS data with enriched selenite.

The metabolic turnover of selenoprotein P (Sel P) and extracellular glutathione peroxidase (eGPx) in plasma was examined by HPLC-ICPMS, with the use of enriched Se, [82Se]selenite. [82Se]selenite was injected intravenously into rats at a dose of 25 micrograms 82Se kg-1 body weight, and the concentrations of labeled 82Se and naturally occurring 77Se in the serum, liver and kidneys were determined in samples obtained 1, 3, 6, 9, 12, 18 and 24 h after the injection. The distributions of both exogenous (labeled) 82Se and endogenous (naturally occurring) 77Se in serum, and supernatant fractions of the liver and kidneys were determined on a gel filtration column by HPLC-ICPMS. This dose was shown not to affect the constitutive levels of cellular GPx (cGPx), eGPx and Sel P. The labeled Se in Sel P increased soon after the injection, peaked at 6-9 h and then decreased, whereas that in eGPx continued increasing after 6 h post-injection and then throughout the remaining observation period in the bloodstream. These observations demonstrated the rapid and efficient incorporation of Se into Sel P in the liver and excretion into the plasma followed by the slow and steady incorporation of Se into eGPx in the kidneys and excretion into the plasma, with a minimal response of cGPx to selenite injection.

Animals↗
PubMed1992

Determination of selenium in human diets by radiochemical neutron activation analysis.

A post-irradiation radiochemical separation technique was tested for the determination of selenium levels in diet samples, collected by using a duplicate portion technique, from both rural and urban population groups in Turkey. The technique involved sample irradiation, acid digestion, selective distillation, precipitation and filtration steps. During the separations it was possible to determine the yield of each sample using a stable selenium carrier. An average chemical yield of 71 +/- 3% was obtained for the radiochemical neutron activation analysis. For samples from urban and rural regions, the average selenium concentrations obtained were 0.14 +/- 0.04 and 0.07 +/- 0.02 mg kg-1, respectively. It was also possible to determine daily dietary selenium intakes, which were found to be 81 +/- 41 micrograms and 23 +/- 11 micrograms for the urban and rural groups, respectively. Although daily selenium intakes were found for a small number of subjects in this study, the separation technique developed can be used for determination of the selenium status in larger population groups.

Food Analysis↗
PubMed1995

Studies on the distribution and characteristics of new mammalian selenium-containing proteins.

It has now been established that the essential effects of selenium in mammals are owing to the presence of several biologically active selenium compounds. Seleno-enzymes identified so far include several glutathione peroxidases and the type 1 iodothyronine de-iodinase. Some other selenoproteins have been sequenced and characterized. After in vivo labelling of rats with 75Se and protein separation using gel electrophoretic methods, more than 25 selenium-containing proteins or protein sub-units were detected. Some of the results of the investigations on these compounds are summarized and discussed here. By determining the pattern in a large number of tissues information on the distribution of the selenium-containing proteins was obtained. Their biological significance is not yet known but several findings indicate that some of these proteins may have important functions, especially in the brain and the endocrine and reproductive organs. More detailed information is already available on a 34 kDa-protein found in the testis and spermatozoa. Studies on the effects of dosage and chemical form of dietary selenium indicated that the tissue levels of the seleno-enzymes are homeostatically controlled and cannot be increased by additional supply. The increase in the tissue selenium observed with high selenium intake was found to be mainly caused by the non-specific incorporation of the element into a large number or proteins. The formation of most of the other selenium-containing proteins has priority over that of the cytosolic and plasma glutathione peroxidases. Thus the selenium requirement, which was calculated for optimum plasma glutathione peroxidase activity, also covers the amounts needed for normal levels of the other biologically important selenium compounds.

Animals↗
PubMed1973

Binding of parathyroid hormone to bovine kidney-cortex plasma membranes.

1. Plasma membranes were purified from bovine kidney cortex, with a fourfold increase in specific activity of parathyroid hormone-sensitive adenylate cyclase over that in the crude homogenate. The membranes were characterized by enzyme studies. 2. Parathyroid hormone was labelled with (125)I by an enzymic method and the labelled hormone shown to bind to the plasma membranes and to be specifically displaced by unlabelled hormone. Parathyroid hormone labelled by the chloramine-t procedure showed no specific binding. (75)Se-labelled human parathyroid hormone, prepared in cell culture, also bound to the membranes. 3. Parathyroid hormone was shown to retain biological activity after iodination by the enzymic method, but no detectable activity remained after chloramine-t treatment. 4. High concentration of pig insulin inhibited binding of labelled parathyroid hormone to plasma membranes and partially inhibited the hormone-sensitive adenylate cyclase activity in a crude kidney-cortex preparation. 5. EDTA enhanced and Ca(2+) inhibited binding of labelled parathyroid hormone to plasma membranes. 6. Whereas rat kidney homogenates were capable of degrading labelled parathyroid hormone to trichloroacetic acid-soluble fragments, neither crude homogenates nor purified membranes from bovine kidney showed this property. 7. Binding of parathyroid hormone is discussed in relation to metabolism and initial events in hormone action.

Adenylyl Cyclases↗
PubMed1992

Some properties of murine selenocysteine synthase.

Selenocysteine (Scy) was synthesized on natural opal suppressor tRNA(Ser) by conversion from seryl-tRNA. We studied the mechanisms of the synthesis of mammalian Scy-tRNA using hydro[75Se]selenide (H75Se-). We found Scy synthase activity in the 105,000 g supernatant of a murine liver extract. The supernatant was chromatographed on DEAE-cellulose, and the activity was eluted at 0.12 M-KCl. The reaction mixture for synthesis of Scy-tRNA contained suppressor tRNA, serine, ATP, seryl-tRNA synthetase (SerRS), HSe- and the enzyme to synthesize Scy-tRNA. These are all essential for the synthesis of Scy-tRNA. Scy in the tRNA product was confirmed by five t.l.c. systems. The conversion from seryl-tRNA to Scy-tRNA was also confirmed with the use of [14C]- and [3H]-serine. The apparent Km values for the substrates serine, tRNA, ATP and HSe- were 30 microM, 140 nM, 2 mM and 40 nM respectively. The active eluates from DEAE-cellulose contained no tRNA kinase. This result showed that Scy-tRNA was not synthesized through phosphoseryl-tRNA. ATP was necessary when Scy-tRNA was synthesized from seryl-tRNA and HSe-. Therefore ATP is used for not only the synthesis of seryl-tRNA but also for the synthesis of Scy-tRNA from seryl-tRNA. The active fraction from DEAE-cellulose was chromatographed on Sephacryl S-300, but the activity disappeared. However, the activity was recovered by mixing the eluates corresponding to proteins of 500 kDa and 20 kDa. In order to examine the binding of HSe- to proteins, a mixture of the active fraction, H75Se- and ATP was analysed by chromatography on Sephacryl S-300. The 75Se radioactivity was found at the position of a 20 kDa protein in the presence of ATP. Thus the 20 kDa protein plays a role in binding HSe- in the presence of ATP. The 500 kDa protein must have a role in the synthesis of Scy-tRNA. There are two natural suppressor serine tRNAs, tRNA(NCA) and tRNA(CmCA), in cell cytosol. The present paper shows that the suppressor tRNA fraction, eluted later on benzoylated DEAE-(BD-)cellulose, is a better substrate with which to synthesize Scy-tRNA. Thus we consider that murine Scy-tRNA is synthesized from a suppressor seryl-tRNA on the 500 kDa protein with the activated HSe-, which is synthesized with ATP on the 20 kDa protein. This mammalian mechanism used to synthesize Scy is similar to that seen in Escherichia coli.

Animals↗
PubMed1998

Differential expression of selenoproteins by human skin cells and protection by selenium from UVB-radiation-induced cell death.

The generation of reactive oxygen species has been implicated as part of the mechanism responsible for UVB-radiation-induced skin damage. In mice, evidence suggests that increased dietary selenium intake may protect skin from many of the harmful effects of UVB radiation. We sought to determine the selenoprotein profile of cultured human skin cells and whether selenium supplementation could protect keratinocytes and melanocytes from the lethal effects of UVB radiation. Labelling experiments using [75Se]selenite showed qualitative and quantitative differences in selenoprotein expression by human fibroblasts, keratinocytes and melanocytes. This was most noticeable for thioredoxin reductase (60 kDa) and phospholipid glutathione peroxidase (21 kDa); these proteins were identified by Western blotting. Despite these differences, we found that a 24 h preincubation with sodium selenite or selenomethionine protected both cultured human keratinocytes and melanocytes from UVB-induced cell death. With primary keratinocytes, the greatest reduction in cell death was found with 10 nM sodium selenite (79% cell death reduced to 21.7%; P<0.01) and with 50 nM selenomethionine (79% cell death reduced to 13.2%; P<0.01). Protection could be obtained with concentrations as low as 1 nM with sodium selenite and 10 nM with selenomethionine. When selenium was added after UVB radiation, little protection could be achieved, with cell death only being reduced from 88.5% to about 50% with both compounds. In all of the experiments sodium selenite was more potent than selenomethionine at providing protection from UVB radiation.

Cell Division↗
PubMed1996

Plasma lathosterol as a screening test for bile acid malabsorption due to ileal resection: correlation with 75SeHCAT test and faecal bile acid excretion.

1. Plasma lathosterol concentration, known to reflect cholesterol and bile acid synthesis, was evaluated as a screening test for bile acid malabsorption, comparing it with faecal bile acid measurements, SeHCAT test and Schilling test in 22 subjects of whom six were healthy controls and 16 had Crohn's disease with ileal resections of varying length. 2. Plasma lathosterols and other non-cholesterol sterols were determined by GLC. Faecal bile acids were measured by GLC, and SeHCAT retention times by gamma camera. The study subjects were divided into two groups according to the degree of bile acid malabsorption: controls (faecal bile acids < 10 mg day-1 kg-1, n = 9) and bile acid malabsorption (faecal bile acids > 10 mg day-1 kg-1, n = 13). 3. Faecal bile acid excretion was 5.9 +/- 1.0 mg day-1 kg-1 in control subjects and 45.7 +/- 6.1 mg day-1 kg-1 in the bile acid malabsorption group. The biological half-life of 75SeHCAT (T1/2) was 95.6 +/- 16.3 h and 14.1 +/- 4.1 h, respectively. Plasma lathosterol levels were significantly elevated in patients with bile acid malabsorption (742 +/- 84 micrograms/ml compared with 400 +/- 59 micrograms/ml in control subjects) and correlated closely with faecal bile acid levels (r = 0.779, P < 0.001), with 75SeHCAT T1/2 (r = -0.524, P < 0.05) and with Schilling test (r = -0.591, P < 0.05). Significant correlations were also obtained for delta 8-cholestenol with faecal bile acids (r = 0.784, P < 0.001) and 75SeHCAT (r = -0.505, P < 0.05). The biological half-life of SeHCAT correlated with faecal bile acid excretion (r = -0.702, P < 0.001). Using mean+2 SD of lathosterol (In micrograms/ml cholesterol) as a cut-off value and 10 mg day-1 kg-1 as the upper limit for faecal bile acid excretion, the test gives 100% sensitivity and 82% specificity for plasma lathosterol determination to detect bile acid malabsorption. 4. The results indicate that both the 75SeHCAT test and plasma lathosterol detect bile acid malabsorption in patients with ileal resections for Crohn's disease. However, plasma lathosterol is a simpler and less expensive method.

Adult↗
PubMed2000

Expression and characterization of nonmammalian selenoprotein P in the zebrafish, Danio rerio.

BACKGROUND: Selenoprotein P is a protein of considerable intrigue, due to its unusual composition and requirements for its biosynthesis. Whereas most selenoproteins contain a single selenocysteine residue, the human, bovine and rodent selenoprotein P genes encode proteins containing 10-12 selenocysteines. Selenoprotein P genes have, to date, only been reported in mammals, and the function of the protein remains elusive. RESULTS: Herein, we report the identification and characterization of nonmammalian selenoprotein P in the zebrafish Danio rerio. Sequencing of the cDNA revealed the presence of 17 selenocysteine codons, the highest number reported in any protein. Two histidine-rich regions present in the mammalian selenoprotein P sequences are conserved in the zebrafish protein, and two SECIS elements are present in the 3' untranslated region. Whole-mount in situ hybridization of zebrafish embryos revealed high levels of expression of selenoprotein P mRNA in fertilized eggs and in the yolk sac of developing embryos. Transient transfection of the cDNA in mammalian cells resulted in efficient expression of the full-length secreted selenoprotein. A single N-glycosylation site is predicted, and shown to be utilized. CONCLUSIONS: Discovery of selenoprotein P in the zebrafish opens a previously unavailable avenue for genetic investigation of the functions of this unusual protein.

Amino Acid Sequence↗
PubMed1999

Substrate-specific selenoprotein B of glycine reductase from Eubacterium acidaminophilum. Biochemical and molecular analysis.

The substrate-specific selenoprotein B of glycine reductase (PBglycine) from Eubacterium acidaminophilum was purified and characterized. The enzyme consisted of three different subunits with molecular masses of about 22 (alpha), 25 (beta) and 47 kDa (gamma), probably in an alpha 2 beta 2 gamma 2 composition. PBglycine purified from cells grown in the presence of [75Se]selenite was labeled in the 47-kDa subunit. The 22-kDa and 47-kDa subunits both reacted with fluorescein thiosemicarbazide, indicating the presence of a carbonyl compound. This carbonyl residue prevented N-terminal sequencing of the 22-kDa (alpha) subunit, but it could be removed for Edman degradation by incubation with o-phenylenediamine. A DNA fragment was isolated and sequenced which encoded beta and alpha subunits of PBglycine (grdE), followed by a gene encoding selenoprotein A (grdA2) and the gamma subunit of PBglycine (grdB2). The cloned DNA fragment represented a second GrdB-encoding gene slightly different from a previously identified partial grdBl-containing fragment. Both grdB genes contained an in-frame UGA codon which confirmed the observed selenium content of the 47-kDa (gamma) subunit. Peptide sequence analyses suggest that grdE encodes a proprotein which is cleaved into the previously sequenced N-terminal 25-kDa (beta) subunit and a 22-kDa (alpha) subunit of PBglycine. Cleavage most probably occurred at an -Asn-Cys- site concomitantly with the generation of the blocking carbonyl moiety from cysteine at the alpha subunit.

Amino Acid Oxidoreductases↗
PubMed1985

[Function tests of the lower small intestine with 75selenium-labeled homotaurocholic acid in Crohn disease and resections of the small intestine].

UNLABELLED: Faecal excretion of bile acids was investigated using 75Se-homotaurocholic acid (75SeHCAT) in a total of 75 patients: 9 with irritable colon, 58 with Crohn's disease and 8 with small-bowel resection due to various indications but without Crohn's disease. Bile acid malabsorption, taken as a pathological bile acid retention of less than 19%, was found present in 43 patients with diseased terminal ileum or with more than 20 cm resection of this organ, except in one case (12 cm). On the other hand a normal bile acid retention (21 to 27%) was found in 29 of 72 patients (including 9 patients with irritable colon) who showed either no radiologically detectable lesion of the ileum (n = 21) or changes involving an extension of up to 30 cm (inflammation or resection) in the terminal ileum (n = 8), with the exception of one case (50 cm). Three patients had a raised bile acid excretion contrary to clinical expectation: they had colitis Crohn without radiologically detectable involvement of the small intestine. Thus using the 75SeHCAT-retention test it seems possible to recognize functional disorders in diseased segments of the bowel before the appearance of radiologically detectable changes. CONCLUSION: 75SeHCAT is a suitable substance for investigations on the function of the lower small intestine.

Adolescent↗