In the genetic code, UGA serves as a stop signal and a selenocysteine codon, but no computational methods for identifying its coding function are available. Consequently, most selenoprotein genes are misannotated. We identified selenoprotein genes in sequenced mammalian genomes by methods that rely on identification of selenocysteine insertion RNA structures, the coding potential of UGA codons, and the presence of cysteine-containing homologs. The human selenoproteome consists of 25 selenoproteins.

/pdf/characterization-of-mammalian-selenoproteomes-296pbto9lm.pdf

Characterization of Mammalian Selenoproteomes

Tissue-specific functions of individual glutathione peroxidases.

Food systems need to produce enough of the essential trace element Se to provide regular adult intakes of at least 40 microg/d to support the maximal expression of the Se enzymes, and perhaps as much as 300 microg/d to reduce risks of cancer. Deprivation of Se is associated with impairments in antioxidant protection, redox regulation and energy production as consequences of suboptimal expression of one or more of the Se-containing enzymes. These impairments may not cause deficiency signs in the classical sense, but instead contribute to health problems caused by physiological and environmental oxidative stresses and infections. At the same time, supranutritional intakes of Se, i.e. intakes greater than those required for selenocysteine enzyme expression, appear to reduce cancer risk. The lower, nutritional, level is greater than the typical intakes of many people in several parts of the world, and few populations have intakes approaching the latter, supranutritional, level. Accordingly, low Se status is likely to contribute to morbidity and mortality due to infectious as well as chronic diseases, and increasing Se intakes in all parts of the world can be expected to reduce cancer rates.

/pdf/selenium-in-global-food-systems-23a5koo10v.pdf

Selenium in global food systems.

There are several proteins in mammalian cells that can metabolize hydrogen peroxide and lipid hydroperoxides. These proteins include four selenium-containing glutathione peroxidases that are found in different cell fractions and tissues of the body. This review considers the structure and distribution of the selenoperoxidases and how this relates to their biological function. The functions of the selenoperoxidases were originally studied in systems where their activity was manipulated by changing dietary selenium levels. More recently, molecular techniques have allowed overexpression of selenoperoxidases in cell lines and animals. Additionally, cellular glutathione peroxidase knockout mice have been used to investigate the functions of this protein. From this work it is clear that the selenoperoxidases are involved in cell antioxidant systems. However, they also have more subtle functions in ensuring the regulation and formation of arachadonic acid metabolites that are derived from hydroperoxide intermediates. The range of biological processes, which are potentially dependent on optimal selenoperoxidase activity in mammals, emphasises the importance of achieving adequate selenium intake in the diet.

The glutathione peroxidases.

Although thyroxine (3,5,3',5'-tetraiodothyronine, T4) is the principal secretory product of the vertebrate thyroid, its essential metabolic and developmental effects are all mediated by 3,5,3'-triiodothyronine (T3), which is produced from the prohormone by 5'-deiodination. The type-I iodothyronine deiodinase, a thiol-requiring propylthiouracil-sensitive oxidoreductase, is found mainly in liver and kidney and provides most of the circulating T3(1) but so far this enzyme has not been purified. Using expression cloning in the Xenopus oocyte, we have isolated a 2.1-kilobase complementary DNA for this deiodinase from a rat liver cDNA library. The kinetic properties of the protein expressed in transient assay systems, the tissue distribution of the messenger RNA, and its changes with thyroid status, all confirm its identity. We find that the mRNA for this enzyme contains a UGA codon for selenocysteine which is necessary for maximal enzyme activity. This explains why conversion of T4 to T3 is impaired in experimental selenium deficiency and identifies an essential role for this trace element in thyroid hormone action.

Type I iodothyronine deiodinase is a selenocysteine-containing enzyme

Evidence for specific selenium target tissues and new biologically important selenoproteins

We investigated the incorporation of Se into the proteins of liver and muscle, the two main Se pools, during replenishment of Se-deficient rats with normal or large doses of 75Se-labeled selenite and selenomethionine, doses equivalent to the amounts ingested from a diet with 02 or 2 mg Se/kg With the higher intake, Se levels were elevated More Se was retained from selenomethionine than from selenite After separation of the labeled proteins, it was apparent that the higher tissue Se contents were mainly due to nonspecific incorporation into a large number of proteins We observed no differences between the two chemical forms with regard to the formation of the specific selenoproteins The 10-fold increase in the Se supply led to a relatively small rise in the levels of these compounds The results indicate that after ingestion of normal amounts of selenite nearly all of the element is present in the specific selenoproteins With increasing doses a part is also incorporated nonspecifically into numerous other proteins In the case of selenomethionine, a part of the element follows the same metabolic pathways, but a percentage is also deposited directly and nonspecifically into proteins in place of methionine

Effects of Chemical Form and Dosage on the Incorporation of Selenium into Tissue Proteins in Rats

The Se-containing proteins in 27 tissues of the rat were investigated by in vivo labeling with75Se-selenite, separation of the tissue homogenate proteins by SDS-polyacrylamide gel electrophoresis, and determination of the labeled proteins by autoradiography. By using Se-depleted rats and a75Se-tracer with a high specific activity, Se compounds present at only very low concentrations could be detected. Besides the 13 Se-containing proteins previously described, for which apparent molecular masses of 12, 15, 18, 20, 22, 25, 28, 34, 56, 60, 65, 70, and 75 kD have been found here, a further 1575Se-labeled bands, with apparent molecular masses of 8, 10, 15.5, 16.5, 24, 32, 34.5, 38, 40, 41, 44, 45, 46.5, 53 and 116 kD could be distinguished. Two-dimensional separation of the kidney homogenate proteins showed that some of the Se-containing bands could be resolved into several labeled spots. Most of the newly found compounds were present in various tissues, but with some the enrichment in certain tissues suggested specific sites of action.

Newly found selenium-containing proteins in the tissues of the rat.

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 of 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.

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

Type I iodothyronine deiodinase (I-D), which catalyzes the production of the thyroid hormone 3,3',5-triiodothyronine from thyroxine, has recently been identified as a selenoenzyme. It is therefore of interest to investigate the relationships between selenium and iodine metabolism. In the livers of Se-deficient rats I-D activity was inhibited; the production of 3,3',5-triiodothyronine and 3,3'-diiodothyronine from added thyroxine was decreased by greater than 95% relative to Se-adequate controls. The hepatic I-D activity was also reduced in rats fed a diet with a low iodine concentration. Unaltered glutathione peroxidase activities in liver and plasma of these rats suggest, however, that with normal Se intake this metabolic pathway of Se is not affected by iodine depletion. When rats were administered 75Se-labeled selenium at levels equal to the amounts ingested from diets with Se concentrations of 0.3 or 2 mg Se/kg, greater Se concentrations were found in the thyroid and liver of the animals receiving the higher dosage. The thyroidal 3,3',5-triiodothyronine and thyroxine concentrations, however, were comparable in rats fed diets with 0.3 mg Se/kg diet as selenite and 2 mg Se/kg as selenite or L-selenomethionine. The measurement of the hepatic I-D and glutathione peroxidase activities in these animals showed that excessive Se supply does not elevate the activities of the two enzymes but might even have the opposite effect. At high Se intake tissue Se concentration cannot therefore be used as indicator of the selenoenzyme activities.

Hildegard Gessner

Papers

Evidence for specific selenium target tissues and new biologically important selenoproteins

Effects of Chemical Form and Dosage on the Incorporation of Selenium into Tissue Proteins in Rats

Newly found selenium-containing proteins in the tissues of the rat.

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

Type I iodothyronine deiodinase activity after high selenium intake, and relations between selenium and iodine metabolism in rats.