Longitudinal selenium status in healthy British adults: assessment using biochemical and molecular biomarkers

Abstract

Human selenium (Se) requirements are currently based on biochemical markers of Se status. In rats, tissue glutathione peroxidase-1 (Gpx1) mRNA levels can be used effectively to determine Se requirements; blood Gpx1 mRNA levels decrease in Se-deficient rats, so molecular biology-based markers have potential for human nutrition assessment. To study the efficacy of molecular biology markers for assessing Se status in humans, we conducted a longitudinal study on 39 subjects (age 45 +/- 11) in Reading, UK. Diet diaries (5 day) and blood were obtained from each subject at 2, 8, 17 and 23 weeks, and plasma Se, glutathione peroxidase (Gpx3) enzyme activity, and selenoprotein mRNA levels were determined. There were no significant longitudinal effects on Se biomarkers. Se intake averaged 48 +/- 14 microg/d. Plasma Se concentrations averaged 1.13 +/- 0.16 micromol/l. Plasma Se v. energy-corrected Se intake (ng Se/kJ/d) was significantly correlated, but neither Gpx3 activity v. Se intake (ng Se/kJ/d) nor Gpx3 activity v. plasma Se was significantly correlated. Collectively, this indicates that subjects were on the plateaus of the response curves. Selenoprotein mRNAs were quantitated in total RNA isolated from whole blood, but mRNA levels for Gpx1, selenoprotein H, and selenoprotein W (all highly regulated by Se in rodents), as well selenoprotein P, Gpx3, and phospholipid hydroperoxide glutathione peroxidase were also not significantly correlated with plasma Se. Thus selenoprotein molecular biomarkers, as well as traditional biochemical markers, are unable to further distinguish differences in Se status in these Se replete subjects. The efficacy of molecular biomarkers to detect Se deficiency needs to be tested in Se-deficient populations.

Fig. 1

Correlations between plasma Gpx3 enzyme activity and dietary Se intake in Reading subjects (●) expressed as μg Se/d (A) or as dietary Se intake adjusted for energy consumption ng Se/kJ/d (B). Values are the means for each subject (n = 39) of values determined at weeks 2, 8, 17 and 23. Shown in each panel is the linear regression line (–), the correlation coefficient and P-value for significance. Also plotted for reference are values from two US volunteers (male: ▼, female: ■) whose blood was assayed at the same time and used for standardization. In addition, a simple hyperbolic fit for the Reading data (…) is also plotted as described in the text.

Fig. 2

Correlations between plasma Se concentration and dietary Se intake (●) expressed as μg Se/d (A) or as dietary Se intake adjusted for energy consumption as ng Se/kJ/d (B). Plasma Se values are the means for each subject (n = 20) of values determined at weeks 2 and 23; dietary Se values are means for each subject of values determined at weeks 2, 8, 17 and 23. Shown in each panel is the linear regression line (–), the correlation coefficient and P-value for significance. Also plotted for reference are values from two US volunteers (male: ▼, female: ■), and a simple hyperbolic fit for the Reading data (…).

Fig. 3

Correlation between plasma Gpx3 enzyme activity and plasma Se concentration (●). Plasma Gpx3 values are means for each subject for values determined at weeks 2, 8, 17 and 23; plasma Se values are the means for each subject (n = 20) of values determined at weeks 2 and 23. Shown is the linear regression line (–), the correlation coefficient and P-value for significance. Also plotted for reference are values from two US volunteers (male: ▼, female: ■), and a simple hyperbolic fit for the Reading data (…).

Fig. 4

RPA analysis for Gpx1 mRNA, Gpx4 mRNA and Gapdh mRNA (A) and correlation between blood Gpx1 mRNA level and plasma Se concentration (B). A: Total RNA (10 μg) isolated from blood from Reading subjects was subjected to RPA analysis. Shown is a representative autoradiogram for RNA isolated from a human male liver (Liver), from blood from 19 Reading subjects, and from blood from male and female US volunteers. Locations of the protected Gpx1, Gpx4 and Gapdh fragments are shown. tRNA lane shows control hybridization reaction with 20 μg of yeast tRNA with all of the probes. B: Resulting Gpx1 mRNA values normalized to Gapdh mRNA levels and set to 100 %, and plotted against plasma Se values (●). Shown is the linear regression line (–), the correlation coefficient and P-value for significance. Also plotted for reference are values from two US volunteers (male: ▼, female: ■) and a simple hyperbolic fit for the Reading data (…).

Fig. 5

Correlations between selenoprotein mRNA levels as determined by qRT-PCR and plasma Se concentration (●). Selenoprotein mRNA values are means for each subject for values determined at weeks 2, 8, 17 and 23, normalized to Gapdh mRNA levels and set to 100%; plasma Se values are the means for each subject (n = 20) for values determined at weeks 2 and 23. Shown is the linear regression line, the correlation coefficient and P-value for significance. Also plotted for reference are values from two US volunteers (male: ▼, female: ■).