Evidence for two selenium-binding proteins distinct from glutathione peroxidase in mouse liver

Tissue-specific modification of selenium concentration by acute and chronic dexamethasone administration in mice

Several clinical reports have shown changes in plasma Se concentration with corticosteroid treatments, but the results have been inconsistent. Few experimental studies have been done on this subject. In the present study the effect of dexamethasone (DEX) treatment on Se concentrations and activities of Se-dependent glutathione peroxidase (EC 1.11.1.9; SeGPx) were examined in adult male ICR mice. In the first experiment, DEX was given via drinking water containing 5 or 50 mg DEX/l. At 1 or 3 weeks of DEX treatment, mice were dissected and the Se concentrations as well as SeGPx activities in various tissues, including plasma, were determined. At 1 week the DEX-treated groups had significantly lower hepatic Se concentrations and significantly higher plasma and cerebral concentrations than the control group. The DEX-treated groups showed lower SeGPx activities in the hepatic cytosol and higher SeGPx activities in the plasma than the saline (9 g NaCl/l)-treated group, in parallel with the changes in Se concentrations. At 3 weeks, neither hepatic nor plasma Se concentrations showed a significant change. In the second experiment, mice were injected subcutaneously with DEX and, thereafter, mice were food-deprived. The DEX-injected groups had higher plasma Se concentrations. A similar finding was obtained also when the DEX- or saline-injected mice were not food-deprived. Thus, the difference between the DEX-treated and control groups was possibly caused by redistribution of tissue Se. These results suggested that the effects of DEX on Se concentrations were tissue dependent and that the higher plasma Se observed in DEX-treated groups might be explained by the release of tissue Se into plasma as plasma SeGPx.

Isolation, characterization, and chromosomal mapping of a novel cDNA clone encoding human selenium binding protein

We have isolated the full-length human 56 kDa selenium binding protein (hSP56) cDNA clone, which is the human homolog of mouse 56 kDa selenium binding protein. The cDNA is 1,668 bp long and has an open reading frame encoding 472 amino acids. The calculated molecular weight is 52.25 kDa and the estimated isoelectric point is 6.13. Using Northern blot hybridization, we found that this 56 kDa selenium binding protein is expressed in mouse heart with an intermediate level between those found in liver/lung/kidney and intestine. We have also successfully expressed hSP56 in Escherichia coli using the expression vector-pAED4. The hSP56 gene is located at human chromosome 1q21-22).

A new selenoprotein from human lung adenocarcinoma cells: purification, properties, and thioredoxin reductase activity

We report the isolation and characterization of a new selenoprotein from a human lung adenocarcinoma cell line, NCI-H441. Cells were grown in RPMI-1640 medium containing 10% (vol/vol) fetal bovine serum and 0.1 microM [75Se]selenite. A 75Se-labeled protein was isolated from sonic extracts of the cells by chromatography on DE-23, phenyl-Sepharose, heparin-agarose, and butyl-Sepharose. The protein, a homodimer of 57-kDa subunits, was shown to contain selenium in the form of selenocysteine; hydrolysis of the protein alkylated with either iodoacetate or 3-bromopropionate yielded Se-carboxymethyl-selenocysteine or Se-carboxyethyl-selenocysteine, respectively. The selenoprotein showed two isoelectric points at pH 5.2 and pH 5.3. It was distinguished from selenoprotein P by N-glycosidase assay and by the periodate-dansylhydrazine test, which indicated no detectable amounts of glycosyl groups on the protein. The selenoprotein contains FAD as a prosthetic group and catalyzes NADPH-dependent reduction of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), and reduction of insulin in the presence of thioredoxin (Trx). The specific activity was determined to be 31 units/mg by DTNB assay. Apparent Km values for DTNB, Escherichia coli Trx, and rat Trx were 116, 34, and 3.7 microM, respectively. DTNB reduction was inhibited by 0.2 mM arsenite. Although the subunit composition and catalytic properties are similar to those of mammalian thioredoxin reductase (TR), the human lung selenoprotein failed to react with anti-rat liver TR polyclonal antibody in immunoblot assays. The selenocysteine-containing TR from the adenocarcinoma cells may be a variant form distinct from rat liver TR.

Protein synthesis is not required for the inhibitory effect of selenite on cell colony formation and RNA synthesis

Selenite has been shown to undergo intracellular metabolism that results in its conversion to other low molecular weight Se-containing species and also to its incorporation into a selenocysteine residue in selenoprotein. In order to investigate whether the incorporation into protein is required for the cytotoxic effects of selenite, we have examined whether inhibition of protein synthesis prevents the inhibitory effect of selenite on the ability of cells to form colonies or to synthesize RNA. We have found that treatment of HeLa cells with cycloheximide inhibited protein synthesis by > 90% but had no effect on the inhibitory effect of selenite on cell colony formation or RNA synthesis. Since protein synthesis is not necessary for these cytotoxic effects of selenite they are unlikely to result from an increase in the synthesis of selenoproteins.

Purification, antibody production, and partial amino acid sequence of the 58-kDa acetaminophen-binding liver proteins

Immunochemical analysis of electrophoretically resolved liver proteins from mice administered hepatotoxic doses of acetaminophen has identified two proteins of 44 and 58 kDa as major targets for acetaminophen arylation. In the present study the 58-kDa acetaminophen-binding protein (58-ABP) was purified from mouse liver cytosol by gel permeation chromatography, preparative isoelectric focusing, and polyacrylamide gel electrophoresis. The acetaminophen adducts were visualized on immunoblots using affinity-purified anti-acetaminophen antibodies after each step of the purification. Gel permeation chromatography, under nondenaturing conditions, indicated that the protein is a monomer. Two-dimensional gel electrophoresis demonstrated that the 58-ABP consists of a cluster of four immunochemically reactive isoforms with isoelectric points ranging from 6.2 to 6.6. V-8 protease digestion of the isoforms suggested that they contained similar peptide fragments. The purified 58-ABP was utilized to produce polyclonal antibodies and to determine the amino acid composition and partial sequence of the protein. These antibodies revealed a protein cluster of similar molecular weight and isoelectric points in the cytosol of a human liver specimen. Amino acid analysis of the purified protein indicated that it contains eight cysteine residues (about 1.4% by weight). This low cysteine content raises the possibility that at hepatotoxic doses acetaminophen may also bind to non-thiol sites on the protein. The amino acid sequence of two cyanogen bromide/tryptic peptide fragments revealed that the major immunochemically detectable acetaminophen target in the cytosol is homologous to a selenium-binding protein which has been recently sequenced.