Nucleotide sequence of cDNA for rabbit glutathione peroxidase

Down-regulation of GPx1 mRNA and the loss of GPx1 activity causes cellular damage in the liver of selenium-deficient rabbits

The effects of 10 weeks of dietary selenium and/or vitamin E deficiency (< 0.03 mg Se and 1.5 mg vitamin E per kg diet) on body Se and vitamin E stores and on the down-regulation of liver cellular glutathione peroxidase (GPx1) and plasma glutathione peroxidase (GPx3) were examined in growing female New Zealand White rabbits in comparison to Se (+ 0.40 mg Se/kg diet) and/or vitamin E (+ 150 I.U./kg diet) supplemented controls. Additionally plasma lactate dehydrogenase (LDH) activity, liver thiobarbituric acid-reactive substances (TBA-RS) and liver protein carbonyls were measured to assess the development of oxidative stress during an alimentary Se and/or vitamin E deficiency. Significantly decreased concentrations of Se and vitamin E in plasma (Se: - 70%; vitamin E: - 87%) and liver (Se: - 90%; vitamin E: - 95%) indicated an efficacious Se and vitamin E depletion of the rabbits within 10 weeks. GPx1 messenger RNA levels (GPx1 mRNA) in the livers of Se-depleted rabbits were down-regulated to 1/3-1/8 of the Se supplemented controls. GPx1 enzyme activity in the livers of Se-deficient rabbits declined to 10% of the Se-supplied control rabbits. A significantly elevated LDH activity in the blood plasma of Se- and vitamin E-deficient rabbits indicated a general impairment of tissues. Markedly increased TBA-RS concentrations and protein carbonyl contents in the livers of Se- and vitamin E-deficient rabbits gave further evidence for severe oxidative damage of cellular lipids and proteins during an alimentary Se and/or vitamin E deficiency. Both a full expresssion of GPx1 attained by dietary Se supplementation and dietary vitamin E supply effected an almost complete protection against oxidative cellular damage of the liver.

The Gpx1 gene encodes mitochondrial glutathione peroxidase in the mouse liver

Mitochondria have GPX and PHGPX activity. It has been an unsettled issue whether mitochondrial GPX is encoded by Gpx1. Unlike the Gpx4 gene which encodes PHGPX with alternative transcription and translation start sites determining the subcellular localization of PHGPX, the Gpx1 gene appears to have a single translation start site. Additionally, mitochondrial GPX has been shown to have different chromatographic and kinetic properties from the cytosolic GPX1. We studied mouse liver mitochondrial GPX activity in homozygous Gpx1-knockout mice. Mitochondria were enriched at the density of 1.10 g/ml in the Percoll gradients as shown by electron microscopy. The H2O2-reducing GPX activity in the highly enriched mitochondrial fraction of wild-type mouse liver is 2700 mU/mg which is about one-half of specific activity found in cytosol. There is less than 0.5% GPX activity in the cytosol and no GPX activity in the mitochondria of Gpx1-knockout mouse liver compared to the cytosol of wild-type mouse liver using H2O2 or cumene hydroperoxide as the substrate. The fact that the knockout mice express normal levels of plasma GPX as well as testis and liver PHGPX activity indicates that animals are not selenium-deficient. Based on these observations, we concluded that mitochondrial GPX is the product of the Gpx1 gene.

Production and characterization of two monoclonal antibodies to human glutathione peroxidase

Glutathione peroxidase (GSH-Px) is an important selenium-containing enzyme which protects cells from oxidative damage. Two hybridoma clones (GPX-121 and GPX-347), producing mouse IgG1 monoclonal antibodies specific for GSH-Px, were established. Immunoblot analysis revealed that GPX-347 was specific for human GSH-Px, while GPX-121 cross-reacted with human, rat, mouse and rabbit GSH-Px. Correlation between GSH-Px content and its enzymatic activity was investigated in erythrocytes of 76 humans and in human lung adenocarcinoma PC-9 cells by using a sandwich type ELISA. The results indicated that GSH-Px activity was expressed higher than expected from GSH-Px content especially in the range of low GSH-Px concentration. PC-9 cells selenium depleted medium did not stain but the cytoplasm of PC-9 cells grown in medium supplemented with selenium stained strongly.

Selenium-dependent glutathione peroxidases from ovine and bovine erythrocytes occur as longer chain forms than previously recognized

From the cDNA sequence of bovine glutathione peroxidase [G. T. Mullenbach, A. Tabrizi, B. D. Irvine, G. I. Bell, J. A. Tainer, and R. A. Hallewell (1988) Protein Eng. 2, 239-246], it is known that the full transcript represents a 205-residue protein with the N-terminal sequence of MCAAQRSAAALAAAAPRTV. However, a protein primary structure determination on what is believed to be the mature protein showed an N-terminal sequence of AAALAAAAPRTV [W. A. Günzler, G. J. Steffens, A. Grossmann, S. A. Kim, F. Otting, A. Wendel, and L. Flohe (1984) Hoppe-Seyler's Z. Physiol. Chem. 365, 195-212], suggesting processing of the N-terminal 7 residues to give the mature protein. It is shown here that the normal form of bovine erythrocyte glutathione peroxidase is most likely the full transcript lacking the N-terminal methionine, and that subsequent proteolysis during isolation results in formation first of the form characterized by Günzler et al. and then of two slightly smaller species, the smaller of which appears to be similar to that examined crystallographically [O. Epp, R. Ladenstein, and A. Wendel (1983) Eur. J. Biochem. 133, 51-69]. For ovine erythrocyte glutathione peroxidase the same behavior is seen, with initial isolation of a high-molecular-weight form that is subsequently proteolyzed to two intermediate forms and a final form that migrates at the same position on acrylamide gels as the lowest-molecular-weight form of the bovine enzyme. The contaminating protease can be inhibited by the addition of 10 mM EDTA, suggesting that it is a metalloprotease. Activity measurements on the intact and processed forms of the ovine enzyme show no significant differences with both hydrogen peroxide and t-butyl hydroperoxide as substrates. These results suggest that both bovine and ovine erythrocyte glutathione peroxidases consist of a globular core that is relatively resistant to proteolysis and an N-terminal tail of approximately 17 residues that seems to be exposed and is very sensitive to proteolysis. This tail, which contains many hydrophobic residues, is predicted to be largely alpha-helical and may be involved in anchoring the enzyme at or close to the membrane surface.

Isolation and characterization of a plant cDNA showing homology to animal glutathione peroxidases

A cDNA library from freshly isolated protoplasts was differentially screened using cDNAs from mesophyll cells, stressed leaf strips and cell suspension cultures. One of the selected clones, 6P229, turned out to encode a putative polypeptide showing homology to the btuE periplasmic protein of Escherichia coli and to animal selenium-dependent glutathione peroxidases. A major difference was that the putative selenocysteine in the active site was not encoded by the termination codon TGA. The 6P229 gene was found to be expressed in germinating seeds, in apex and in flowers, as well as in stressed tissues. This pattern of expression would be consistent with a key role in cellular metabolism such as defense against oxidative stresses.

Phospholipid hydroperoxide glutathione peroxidase is a selenoenzyme distinct from the classical glutathione peroxidase as evident from cDNA and amino acid sequencing

The primary structure of phospholipid hydroperoxide glutathione peroxidase (PHGPx) was partially elucidated by sequencing peptides obtained by cyanogen bromide cleavage and tryptic digestion and by isolating and sequencing corresponding cDNA fragments covering about 75% of the total sequence. Based on these data PHGPx can be rated as a selenoprotein homologous, but poorly related to classical glutathione peroxidase (GPx). Peptide loops constituting the active site in GPx are, however, strongly conserved in PHGPx. This suggests that the mechanism of action involving an oxidation/reduction cycle of a selenocysteine residue is essentially identical in PHGPx and GPx.