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Glutathione Peroxidase, Selenoprotein P and Selenium in Serum of Elderly Subjects in Relation to Other Biomarkers of Nutritional Status and Food Intake

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Abstract

The relation of the intake of selenium and different food groups to serum levels of selenium, glutathione peroxidase and selenoprotein P, and urinary selenium was studied in 50- to 69-year-old subjects (101 men, 105 women). Blood was sampled six times during the course of 1 year, and during the same time the subjects performed six 3-day weighed dietary records. The concentration of glutathione peroxidase in serum was higher among men (4.3 (0.74) mg/L) (mean (SD)) than among women (4.0 (0.76) mg/L, P < 0.03), whereas men had lower serum selenium (1.10 (0.17) μmol/L) vs. 1.17 (0.19) μmol/L, P < 0.01). The intake of selenium among men was calculated to be 36 (18–54) μg/day and among women 29 (13–48) μg/day (geometric means (90% central range)), but there was no significant gender difference when the selenium intake was corrected for energy intake. No difference between men and women was observed for serum selenoprotein P (1.47 (0.25) a.u. versus 1.47 (0.24) a.u. (mean(SD)) or urinary selenium excretion (0.31 (0.09) μmol/d vs. 0.27 (0.08) μmol/d). Serum selenium was significantly associated with selenoprotein P in both sexes but with glutathione peroxidase only in men. Serum selenium had a marked association with urinary selenium excretion. Selenium intake was significantly associated with serum selenium in men, and with selenoprotein P and urinary selenium in women. Among 11 major food groups, the intakes of fish and milk products were significantly associated with biomarkers of selenium status among women. Calculation of the association between biomarkers of selenium status and variables on intake and serum content of nutrients using multiple regression analysis resulted in most cases in low explanatory power. The variables most consistently related to more than one variable of selenium status were serum levels of retinol, α-tocopherol and eicosapentaenoic acid, and intake of protein. The study shows that individual biomarkers of selenium status are associated differently to variables on dietary intake and nutritional status.

Introduction

The concentration of selenium in plasma or blood is often used as an index of selenium status in epidemiologic and clinical studies.1, 2, 3, 4, 5, 6The plasma level responds more rapidly to changes in selenium balance than the blood level. The interpretation of such changes is complicated by the fact that the chemical composition of selenium compounds in plasma and erythrocytes is only partly known. It has been established for some time that extracellular glutathione peroxidase (eGSHPx) is a selenoprotein in plasma,[7]and recently another selenoprotein, selenoprotein P, has been found in human plasma, accounting for a large part of plasma selenium.[8]Radioimmunoassays have been developed to measure these proteins.8, 9, 10

The concentration of selenium and selenoproteins in tissues can be influenced by selenium intake,11, 12, 13but the relation of the intake of different foods to selenium and selenoprotein concentrations in humans is incompletely known. There is also a need to assess the influence of other life-style variables than dietary intake on biomarkers of selenium status. Moreover, because glutathione peroxidases and maybe other selenoproteins are believed to act as components of the oxidant defence, their interaction with other antioxidants needs further study. In the present study the mutual relationships of eGSHPx, selenoprotein P, and selenium in serum, and selenium in urine were investigated. In addition their associations to dietary intake, some life-style variables and serum antioxidant levels were studied.

Section snippets

Study Design

Nine hundred subjects (50 to 69 years old) living in Malmö were randomly selected from the population register and invited to participate in the study as described in detail elsewhere.14, 15, 16To screen the subjects for inclusion in the study, they completed a medical questionnaire and underwent a medical examination, including assays of clinical-chemical variables. Of the 900 subjects, 552 (61%) took part in this health survey.[15]Nineteen subjects were excluded for medical reasons or because

Selenium Intake and Selenium Status

The dietary selenium intake was higher in men than in women and the difference was similar at all sampling times (Table 1). The geometric mean ratio (95% CI) men/women with respect to the mean intake at six recording periods was 1.26 (1.15, 1.38), but the energy-adjusted selenium intake was not significantly different in men and women (ratio: 1.00 (0.92, 1.09)). Men had a significantly lower concentration of serum selenium, and the mean from six samplings for men was 6% lower than that of

Discussion

The established metabolic role of selenium in mammalian species is tied to its presence in selenocysteine residues in specific selenoproteins. One of them, eGSHPx, catalyzes the reduction of different hydroperoxides using glutathione or the thioredoxin and glutaredoxin systems as electron donors.7, 23The function of the other selenoprotein demonstrated in human plasma, selenoprotein P, is unknown, but it may be involved in the oxidant defense.[24]This study examines the use of the serum levels

Acknowledgements

The study was planned by a working group at the Swedish Medical Research Council, and we thank Prof. H. Danielsson for his support during the initiation of the study. Ms. E. Callmer, Ms. I. Malmquist and Ms. I. Mattisson supervised the contacts with the subjects and administered the dietary assessment procedures. Ms. B. Ekström, Ms. B. Mårtensson, and Ms. B. Mattsson gave skilful assistance in the analytical work. Dr. U. Strömberg gave much valuable statistical advice.

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    The study was supported by grants from the Swedish Medical Research Council (Projects 3968 and 7010) and was also generously supported by the Malmö General Hospital. Additional support (to B.Å.) was provided by the Swedish Council for Forestry and Agricultural Research and the Påhlsson Foundation. W.H. was supported by the Medical Faculty, University of Lund, and E.M. by the Swedish Institute.

    1

    Dr. Marchaluk’s present address: Department of Biochemistry/Chemistry, University School of Medical Science, Bydgoszcz, Poland.

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