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PubMed2003

Thioredoxin h overexpressed in barley seeds enhances selenite resistance and uptake during germination and early seedling development.

The uptake, distribution and metabolism of selenite were examined in germinating homozygous barley ( Hordeum vulgare L.) grain with thioredoxin h overexpressed in starchy endosperm. Results were related to the null segregant in which the transgene had segregated out during crossing. Compared with the null segregant, the homozygote showed enhanced germination and root and shoot growth in the presence of 1 and 2 mM sodium selenite. The rate of incorporation of selenite by the homozygote was approximately twice that of the null segregant. Based on X-ray absorption spectroscopy, the major products in both cases were selenomethionine-like species and the red, monoclinic form of elemental selenium, a derivative not previously reported in green plants. Selenite and selenate made up the balance. The distribution of the products formed differed as to the tissue - root, shoot, aleurone, endosperm - but the ratios were similar in the homozygote and null segregant. The results provide evidence that, in addition to the accelerated germination observed previously in water, barley grain overexpressing thioredoxin h are resistant to the inhibitory effects of selenite. These properties raise the possibility that plants overexpressing thioredoxin h could find application in the remediation of polluted environments.

Adaptation, Physiological↗
PubMed2005

Developmental responses of a terrestrial insect detritivore, Megaselia scalaris (Loew) to four selenium species.

Megaselia scalaris (Loew) (Diptera: Phoridae) is an important and ubiquitous terrestrial detritivore that consumes both animal and plant material. Because both plants and animals convert selenium pollutants into various forms, the relative toxicities of ecologically relevant concentrations of sodium selenate, sodium selenite, seleno-L-methionine, and Se-(methyl) selenocysteine hydrochloride to larvae were assessed in diet bioassays. In addition, ovipositional preferences of adults and developmental effects on the eggs and larvae were measured. With chronic exposure selenocysteine was the most toxic of the selenium species to the larvae (LC50: 83 microg/g wet weight), followed by seleno-L-methionine (LC50: 130 microg/g), selenate (LC50: 258 microg/g), and selenite (LC50: 392 microg/g). Ovipositing females did not discriminate between the highest treatment concentrations of any of the pollutants as compared to the controls, indicating a lack of avoidance behavior. Larval development time was significantly increased with exposure to selenate at 100 microg/g wet weight and above, selenite at 300 microg/g and above, and at 50 microg/g and 25 microg/g and above for seleno-L-methionine and selenocysteine respectively. Pupal development was not affected by any of the selenium treatments. Significant differences between male and female adult eclosion times were observed, with females eclosing later than males as selenium concentrations increased. Significant decreases in larval survival relative to controls occurred at the lowest treatment tested (100 microg/g) for both selenate and selenite and at 100 microg/g for seleno-L-methionine, and 50 microg/g for selenocysteine. The population level implications of lack of avoidance of contaminated food, and the effects of increased development times, reduced survivorship, and non-synchronized male and female emergence are discussed.

Animals↗
PubMed1997

Gas-liquid chromatographic properties of positional isomers of methyl thia, selena, and tellura laurate analogs.

Gas-liquid chromatographic analyses of three complete series of synthetic positional isomers of methyl thia, selena, and tellura laurate analogs were carried on a nonpolar (SE-30) and a polar (SP-2330) stationary phase. The average ECL (equivalent chain length) values of the thia, selena, and tellura laurate on SE-30 stationary phase were 13.8, 14.8, and 15.7, respectively, while on SP-2330 the average values for the same series were 17.1, 19.0, and 19.1, respectively. Positional isomers with the heteroatom at the 2-position exhibited the lowest ECL values, while those with the heteroatom at the omega-1 position gave the highest ECL values and were readily separated from the other positional isomers of the same series of analogs by this technique.

Chromatography, Gas↗
PubMed1989

Rat liver ATP-sulfurylase: purification, kinetic characterization, and interaction with arsenate, selenate, phosphate, and other inorganic oxyanions.

ATP-sulfurylase (ATP:sulfate adenylyltransferase; EC 2.7.7.4), the first enzyme of the two-step sulfate activation sequence, was purified extensively from rat liver cytosol. The enzyme has a native molecular mass of 122 +/- 12 kDa and appears to be composed of identical 62 +/- 6-kDa subunits. At 30 degrees C and pH 8.0 (50 mM Tris-Cl buffer containing 5 mM excess Mg2+), the best preparations have "forward reaction" specific activities of about 20 and 2 units X mg protein-1 with MoO4(2-) and SO4(2-), respectively. The reverse (ATP synthesis) specific activity is about the same as the forward molybdolysis activity. The kinetic constants under the above conditions are as follows: KmA = 0.21 mM, Kia = 0.87 mM, KmB = 0.18 mM, KmQ = 0.65 microM, Kiq = 0.11 microM, and KmP = 5.0 microM where A = MgATP, B = SO4(2-), Q = APS, and P = total PPi at 5 mM Mg2+. PPi is a mixed-type inhibitor with respect to MgATP and SO4(2-). SeO4(2-) is an alternative inorganic substrate with a Vmax about 20% that of SO4(2-). The product, APSe, is unstable. But in the presence of a sufficient excess of APS kinase, APSe is completely converted to PAPSe. The rate constant for nonenzymatic PAPSe hydrolysis was determined from measurements of the final steady-state reaction rate in the presence of limiting initial SeO4(2-) and a large excess of MgATP, ATP sulfurylase, APS kinase, and the other coupling enzymes and their cosubstrates. The results yielded a k of 2.4 +/- 0.5 X 10(-3) sec-1 (t1/2 ca. 5 min). Phosphate is an effective buffer for enzyme purification and storage but inhibits catalytic activity, particularly at low substrate concentrations. In the presence of buffer levels of Pi, the MgATP reciprocal plot of the SO4(2-)-dependent reaction is concave-up. Inorganic monovalent oxyanions are dead end inhibitors competitive with SO4(2-) and apparently uncompetitive with respect to MgATP. The relative potencies are in the order ClO3- greater than ClO4- greater than FSO3- greater than NO3-. Thiosulfate is also competitive with SO4(2-) but noncompetitive with respect to MgATP. Several divalent oxyanions (MoO4(2-), WO4(2-), CrO4(2-), and HAsO4(2-] promote the enzyme-catalyzed cleavage of MgATP to AMP and MgPPi. The ratio Vmaxf/KmA ranged from 0.7 to 200 for various reactive inorganic substrates. The cumulative results suggest the random binding of MgATP and the inorganic substrate but the ordered release of MgPPi before APS.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Phosphosulfate↗
PubMed1988

Kinetic mechanism and substrate specificity of glutathione peroxidase activity of ebselen (PZ51).

The glutathione peroxidase activity of ebselen (PZ51) was studied using different hydroperoxidic substrates. The single progression curves obtained in the spectrophotometric test were processed by a computer to fit the integrated rate equation that describes the ping pong reaction of the Se glutathione peroxidase. Ebselen catalyzes the GSH peroxidase reaction with a mechanism that appears kinetically identical to the mechanism of the enzymes. The inactivation of the catalytic properties of ebselen by iodoacetate suggests that a selenol moiety is involved. Among the substrates tested, the best hydroperoxidic substrates are the hydroperoxy derivatives of phosphatidyl choline. Ebselen is active also on membrane hydroperoxides as does phospholipid hydroperoxide glutathione peroxidase but not glutathione peroxidase.

Antioxidants↗
PubMed1991

Zinc containing projections to the bed nucleus of the stria terminalis.

A retrograde tracing method that selectively labels the perikarya of zinc-containing neurons was used to identify the neurons that supply zinc-containing fibers to the bed nucleus of the stria terminalis in the rat. In agreement with prior lesion studies, retrograde tracing indicates that neurons in amygdalar and periamygdalar regions are the major sources of the zinc-containing innervation of the bed nucleus complex. Zinc-containing neurons in the presubiculum and prosubiculum were also retrogradely labeled from the BNST, whereas cells of the subiculum proper did not label. Light and occasional retrograde labeling of some CA1 and CA2 neurons and limbic cortical neurons was also observed, but the possibility of transport from regions bordering BNST injections (septum, caudate-putamen) could not be excluded in the latter cases.

Animals↗
PubMed1991

Dealkylation rates of O6-alkyldeoxyguanosine, O4-alkylthymidine and related compounds in an alkyl-transfer system.

Bacterial O6-alkylguanine-DNA alkyltransferase (AGT) removes alkyl group from O6-alkylguanine and O4-alkylthymine residues in DNA, both of which are considered to be DNA damages most related to the induction of cancer and/or mutation. The repair process involves alkyl-transfer of an O-alkyl group to the active site of the enzyme, where an SH-group of cysteine residue plays the role of alkyl acceptor. In order to elucidate the chemical characteristics of substrates for this enzyme, dealkylation rates of O6-alkyldeoxyguanosine, O4-alkylthymidine and related compounds were measured using an alkyl-transfer system. Thiophenol-triethylamine system was employed as an alkyl acceptor and twenty-one O-alkyl compounds were tested. Dealkylation proceeded with pseudo first order kinetics. The half-life of O6-methyldeoxyguanosine (MedG) was 122 h and no remarkable dependence on N-9 substituents (H, CH3 and deoxyribose) was observed. A compound lacking 2-NH2 group underwent demethylation about three times faster than O6-methylguanines did, while, a compound lacking imidazole moiety underwent demethylation about 2.5 times more slowly. The half-life of O4-methylthymidine (MedT) was 38 h and no remarkable dependence on N-1 (H, CH3 and deoxyribose) and C-5 (H and CH3) substituents was observed. Deethylation proceeded much more slowly than demethylation. Substitution of selenophenol for thiophenol resulted in a 4.5 times faster MedG demethylation rate. Demethylation rates were moderately correlated with values for NMR chemical shift of CH3 group, an indicator of electron density, although the correlation curves of a series of MedG and MedT derivatives were quite different. This result suggests that some different rate-determining factors other than electron density are playing a role. These findings may be of help in resolving the details of the mechanisms of enzymic repair by bacterial and mammalian AGT.

Alkylation↗
PubMed1989

Diastolic dysfunction of perfused rat hearts induced by hydrogen peroxide. Protective effect of selenium.

Oxidant substances such as hydrogen peroxide are postulated to cause cardiac dysfunction and injury in a number of pathological conditions. Selenium is an essential nutrient which serves as an oxidant defense through the selenoenzyme glutathione peroxidase. This enzyme metabolizes hydrogen peroxide; its activity in rat heart is reduced to 5% of control by selenium deficiency. Left ventricular function of selenium-deficient and control rat hearts was studied in a Langendorff preparation under isovolumic conditions. A stabilization period of 20 min was followed by a 70 min infusion of hydrogen peroxide at 375 or 1500 nmol/min. When no hydrogen peroxide was infused, perfusion for 90 min had no effect on systolic or diastolic function and no effect of selenium deficiency was detected. Hydrogen peroxide infusion into selenium-deficient hearts at 375 nmol/min led to impaired isovolumic relaxation and a substantial increase in end-diastolic pressure after 45 min which worsened progressively until the experiment was terminated. By contrast no effect was observed on systolic contractile function as assessed by peak pressure or developed pressure. Infusion of this dose of hydrogen peroxide into control hearts had no significant effect on diastolic or systolic function. However, infusion of 1500 nmol hydrogen peroxide/min into control hearts caused diastolic dysfunction after 30 min without affecting systolic function. These results indicate that hydrogen peroxide injury to the perfused rat heart is manifested by diastolic dysfunction before systolic dysfunction occurs. Selenium deficiency lowers the dose of hydrogen peroxide needed to cause diastolic dysfunction. This suggests that the selenoenzyme glutathione peroxidase protects the heart against hydrogen peroxide injury.

Animals↗
PubMed1988

Dietary inhibitors of mutagenesis and carcinogenesis.

Dietary inhibitors of mutagenesis and carcinogenesis are of particular interest because they may be useful for human cancer prevention. Several mutagenesis inhibitors have been demonstrated to be carcinogenesis inhibitors also, e.g., ellagic acid, palmitoleic acid, and N-acetylcysteine. This means that the search for mutagenesis inhibitors may be useful for discovering anticarcinogenic agents. Many mutagenesis inhibitors have been discovered by the use of short-term assays, particularly the Ames Salmonella test. This simple in vitro system has provided opportunities to elucidate the mechanisms of inhibition. The elucidation of the mechanism may allow us to infer the possible anticarcinogenic activity of the reagent. In this chapter, inhibitors of mutagenesis and carcinogenesis that can arise as components of diet have been reviewed. Most of the inhibitors have been demonstrated to be effective against a specific class of mutagens or carcinogens. Therefore, it may be argued that these inhibitors are antagonistic only to those particular agents. Here again, understanding of the mechanisms of these inhibitions is necessary for the assessment. Dietary inhibitors reviewed in this article include: (1) as inhibitors of mutagenesis: porphyllins, fatty acids, vitamins, polyphenols, and sulfhydryl compounds, (2) as inhibitors of carcinogenesis: vitamins A, E and C, ellagic acid, sulfhydryl compounds, fats, selenium, calcium, and fiber. Further studies in this area of science appear to help establish the recipe of a healthy diet.

Animals↗