The sites of synthesis of extracellular (E) glutathione peroxidase (GPX), a unique selenoglycoprotein present in plasma, are not known. To investigate the possibility that the kidney is the main source for the plasma GPX, we examined GPX activities and selenium concentrations in the plasma of patients with renal failure on dialysis and nephrectomized patients before and after kidney transplantation. Plasma GPX activities in these patients were 42, 22, and 180% of normal EGPX activity, respectively, whereas plasma Se levels were within the normal range. Twenty-four hours after nephrectomy of anesthetized rats, plasma GPX activity was 30.0 +/- 6.4% of the activity at zero time. Northern hybridization analysis of eight human tissues probed with EGPX and cellular glutathione peroxidase (CGPX) cDNA revealed that the ratio of EGPX to CGPX was highest in the kidney. cRNA in situ hybridization studies on kidney slices showed that only proximal tubular epithelial cells and parietal epithelial cells of Bowman's capsule contained EGPX transcripts. Caki-2, a proximal tubular renal carcinoma cell line, makes and actively secretes EGPX. Taken together, these results strongly suggest that kidney proximal tubular cells are the main source for GPX activity in the plasma.

The official first action AOAC fluorometric method for selenium in plants, 3.074–3.078, has been modified to simplify the method and to make it more accurate. The digestion time has been increased from 15 to 30 min past the appearance of perchloric acid fumes to better assure complete oxidation of all forms of selenium to selenite. Preparation of the 2,3-diaminonaphthalene solution, the reagent used for fluorometric analysis, has been changed so that the reagent is stable for several weeks; in the previous writeup, this solution had to be prepared daily. Special equipment (micro-Kjeldahl flasks with ground glass joints) has been eliminated and cyclohexane has been substituted for decahydronaphthalene. The modified method is convenient and applicable to a wide range of materials; it yields results comparable to those from the official method.

No abstract

Plasma selenium concentration is decreased in patients with cirrhosis and, based on this finding, it has been suggested that patients with cirrhosis are selenium deficient. We measured plasma selenium concentration and the two plasma selenoproteins, glutathione peroxidase (GSHPx-3) and selenoprotein P, in the plasma of patients with cirrhosis of Child classes A, B, and C and in control subjects. Plasma selenium declined in proportion to the severity of the cirrhotic condition, as indicated by the Child class. Selenoprotein P, which originates largely in the liver, declined in a similar manner. Plasma glutathione peroxidase activity increased, and GSHPx-3 originates in the kidney. Selenium in the non-selenoprotein pool, shown by others to be largely selenomethionine in albumin, declined. Thus, although plasma selenium is decreased in patients with cirrhosis, the increase in plasma glutathione peroxidase activity, which occurs in them, suggests that patients with cirrhosis do not have selenium deficiency. (HEPATOL-OGY 1998;27:794-798.) Selenium exerts biological effects as an essential constituent of selenoproteins. 1 Pathological conditions, such as selenium deficiency and organ damage, which are severe enough to alter selenium metabolism, can be expected to affect the concentrations of selenoproteins and to have biochemical and pathological consequences. Several groups have reported that patients with cirrhosis have plasma selenium concentrations lower than those of healthy controls. [2][3][4][5][6] The pathogenesis of those depressed selenium levels is unknown; therefore it is not known whether a remediable state of selenium deficiency exists in cirrhotics.Virtually all the selenium in plasma is present in the form of seleno-amino acids in the primary structure of proteins. One of these amino acids is selenocysteine, the physiologically active form of the element, which is synthesized in animal cells and is present in stoichiometric amounts in selenoproteins. Glutathione peroxidase (isoform GSHPx-3) and selenoprotein P are the only selenoproteins that have been identified in plasma. 7,8 The other amino acid form of selenium in plasma is selenomethionine, 9 an amino acid that is synthesized by plants and is incorporated randomly into animal proteins as a constituent of the methionine pool.Proteins that contain selenomethionine do not contain the element in stoichiometric amounts and are often referred to as selenium-containing proteins to distinguish them from true selenoproteins. Small molecule forms of uncharacterized selenium are involved in homeostasis of the element. Those forms account for approximately 3% of plasma selenium under steady state conditions. 10

Plasma glutathione peroxidase (p-GSHPx) is a unique selenoglycoprotein. A hepatic cell line synthesizes both this extracellular form for secretion and the cellular form that remains within the cells. Because the two forms could be a result of post-translational modifications of a product of a single gene, we partially sequenced p-GSHPx. Purified p-GSHPx was trypsin digested, and three of the peptides were sequenced. Only one of the peptide sequences was partially homologous to a sequence found in human cellular glutathione peroxidase. Because p-GSHPx is a secreted enzyme, we determined whether GSHPx in milk (another extracellular fluid) is due to this form of the enzyme. Ninety percent of human milk GSHPx activity could be precipitated by anti-p-GSHPx-immunoglobulin G. Thus, most, if not all, GSHPx activity in milk is due to the plasma selenoprotein form of the enzyme. In milk of two North American women, 3.6% and 14.3% of selenium was associated with GSHPx.