Selenium-dependent enzymes

Gladyshev VN, SV Khangulov, TC Stadtmann (1994) Nicotinic acid hydrolase from Clostridium barkeri electron paramagnetic studies show that selenium is coordinated with molybdenum in the catalytically active selenium-dependent enzyme. Proc Natl Acad USA 91 232-236. 

Imhoff D, JR Andreesen (1979) Nicotinic acid hydroxylase from Clostridium barkeri selenium-dependent formation of active enzyme. FEMS Microbiol Lett 5 155-158. 

Hu, M.-L., Dillard, C.J. and Tappel, A.L. (1988) Aurofhioglucose effect on sulfhydryls and glutathione-metabolizing enzymes in vivo inhibition of selenium-dependent glutathione peroxidase. Research Communications in Chemical Pathology and Pharmacology, 59,... 

Glutathione peroxidase is a selenium-dependent enzyme, which rapidly detoxifies hydrogen peroxide and various hydroperoxides. Suttorp et al. [67] showed that the impairment of glutathione cycle resulted in an increase in the injury of pulmonary artery endothelial cells. Glutathione cycle protected against endothelial cell injury induced by 15-HPETE, an arachi-donate metabolite produced by 15-lipoxygenase-catalyzed oxidation [68]. 

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

One underlying question remains why does this small class of microorganisms require a labile selenium cofactor in these enzymes. Few have speculated on this in the published literature. Yet one key comparison between selenium and non-selenium-dependent hydroxylases may be quite telling. The well-studied bovine XDH has a turnover rate of approximately 5 while the PH enzyme from C. purinolyticum has a far... 

Table 12.1. Selenium-dependent enzymes from Clostridiaceae.

Another selenium-containing molybdenum hydroxylase that has been isolated from Clostridium barkeri (identical to Eubacterium barkeri) is nicotinic acid hydroxylase (NAH). Clostridium barkeri was isolated initially as a fermentor of nicotinic acid and thus NAH is a key enzyme in the efficient fermentation of nicotinic acid as a source of carbon and energy. NAH contained selenium when purified from cells labeled with Se-selenite. However, this label was lost during denaturing gel electrophoresis and also on heating of the enzyme (Dilworth 1982). Exhaustive analysis of selenium-labeled alkylation products of NAH under various conditions revealed selenium was bound as a labile cofactor (Dilworth 1982), and not as seleno-cysteine. This report was the first to describe a selenium-dependent enzyme that did not contain selenium in the form of selenocysteine. 

In addition to the molybdenum hydroxylases mentioned above, a new selenium-dependent hydroxylase with specificity for purine and hypoxan-thine as substrates, termed purine hydroxylase, was uncovered during purification of XDH from C. purinolyticum (Self and Stadtman 2000). Purified PH was labeled with Se and was not reduced in the presence of xanthine as a substrate. As with other selenium-dependent molybdenum hydroxylases, selenium was removed by treatment with cyanide with parallel loss in catalytic activity. Selenium was also efficiently removed in the presence of low ionic strength buffer during final dialysis of PH, indicating that ionic strength affects the stability of the labile selenium cofactor in this enzyme. 

Many proteins, including many enzymes, contain hghtly bound metal ions. These may be inhmately involved in enzyme catalysis or may serve a purely structural role. The most common tightly bound metal ions found in metalloproteins include copper (Cu+ and Cu +), zinc (Zn +), iron (Fe + and Fe +), and manganese (Mn +). Other proteins may contain weakly bound metal ions that generally serve as modulators of enzyme activity. These include sodium (Na+), potassium (K+), calcium (Ca +), and magnesium (Mg +). There are also exotic cases for which enzymes may depend on nickel, selenium, molybdenum, or silicon for activity. These account for the very small requirements for these metals in the human diet. 

This selenium-dependent enzyme [EC 1.11.1.9] catalyzes the reaction of two molecules of glutathione with hydrogen peroxide to produce glutathione disulfide and two water molecules. Hydrogen peroxide can be replaced by steroid and lipid hydroperoxides, albeit not as effectively (nevertheless, this enzyme is not identical with phospholipid-hydroperoxide glutathione peroxidase [EC 1.11.1.12]). However, the hydroperoxy products formed by the action of lipoxygenase [EC 1.13.11.12] are not substrates. 

Considerable interest was generated when it was shown that substantial GSHPx activity exists in liver of selenium-depleted rats when LOOH is used as substrate but not H2O2 [20]. The enzyme was subsequently shown to be identical to a member of the glutathione-S-transferase family of enzymes [20,21,48], The enzyme was for a time called selenium-independent , or non-selenium dependent GSHPx and this nomenclature, which is confusing, has persisted in the literature. The role of GSH-S-transferases in the overall protection of cells against the harmful potential of LOOH has not been determined but it undoubtedly must form part of that protective mechanism. 

Beran, M Vyzkumny Ustav Potravinarsky Evaluation of combined supplementation with selenium and iodine on levels of selenium-dependent enzymes, thyroidal hormones and other biochemical parameters. CRIS/UDSA 2001... 

StadtmanTC. 1980. Selenium-dependent enzymes. Annu Rev Biochem 49 93-110. 

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