In utero methylmercury exposure differentially affects the activities of selenoenzymes in the fetal mouse brain.

Pregnant ICR mice were subcutaneously injected with 0,5, or 3x3 mg Hg/kg of methylmercury (MeHg) on days 12,13, and 14(G12-14) of gestation and were sacrificed on G17. Activity of selenoenzymes, including glutathione peroxidase (GPx) and 5'- or 5-iodothyronine deiodinases (5'-DI, 5-DI), was determined in fetal brain and placenta. MeHg did not affect the concentration of Se in these tissues, while it significantly inhibited the activity of GPx in the fetal brain and placenta, but not in the maternal brain. Although the levels of thyroid hormones in the maternal and fetal plasma were not affected by MeHg, 5-DI decreased and 5'-DI increased in the fetal brain, as if they had responded to hypothyroidism. Because the level of T4 in the fetal plasma was not affected by MeHg, these changes in enzymatic activities may result in a harmful excess of T3 in the fetal brain. In addition, 5-DI activity was increased in the placenta of MeHg-treated mice. These effects of prenatal MeHg exposure on fetal and placental DIs differed from those of dietary-induced Se deficiency, where the activities of DIs were decreased or not affected. Further evaluation of the effect of MeHg on selenoenzymes, especially 5-DIs, is warranted.

In utero exposure to methylmercury and Se deficiency converge on the neurobehavioral outcome in mice.

Pregnant female ICR mice, maintained on torula-based diets containing various amounts of Se (0.02, 0.05, or 0.4 mg/kg diet), were given methyl-mercury (MeHg; 0, 5, or 9 mg Hg/kg in total) on the 12-14th days of gestation. The neurobehavioral function of the offspring born to these dams was evaluated with respect to reflex and motor development, thermal preference, and open-field activity. Se deficiency per se as well as exposure to MeHg exerted additive or synergistic effects on the neurobehavioral functions examined. The group of mice most affected was the group given the lowest amount of Se and the highest dose of MeHg. Thus, the neurobehavioral outcome of in utero MeHg exposure and Se deficiency converged. Although the dietary level of Se did not affect the Hg concentration in the fetal brain, the Se concentration and the activity of glutathione peroxidase, a selenoenzyme, were severely depressed by MeHg in the neural tissue. The possibility that functional Se deficiency by MeHg exposure partly accounts for the neurobehavioral toxicity of MeHg is discussed.

Separation of selenium-containing proteins in human and mouse plasma using tandem high-performance liquid chromatography columns coupled with inductively coupled plasma-mass spectrometry.

An analytical method that uses two different high-performance liquid chromatography (HPLC) columns in tandem has been developed that separates three major selenium-containing proteins (albumin, glutathione peroxidase, and selenoprotein P) found in human blood plasma. The first column was a heparin affinity column and the second was a gel filtration column whose outlet was directly connected to an inductively coupled plasma-mass spectrometer. The method successfully separated plasma selenium into the three selenium-containing proteins and revealed the preferential retention of selenium in the form of selenoprotein P in a selenium-deficient human and in selenium-deficient mice. Our results also confirm the results of previous studies that showed a preference for supplemented selenium to be taken up as selenoprotein P in rats. Advantages of the tandem column method are that it allows rapid and convenient analyses of the distribution of plasma selenium, and that it is suitable for stable isotope tracer studies and metal interaction studies.

Effects of mild chronic heat exposure on the concentrations of thiobarbituric acid reactive substances, glutathione, and selenium, and glutathione peroxidase activity in the mouse liver.

To determine whether mild and chronic heat stress leads to oxidative stress and to differentiate such effects of different exposure periods, we kept male ICR-mice at an ambient temperature of either 35 degrees C or 25 degrees C for 6 hours, 3 days, or 7 days and measured the concentrations of thiobarbituric acid reactive substances (TBARS), glutathione (GSH), selenium (Se), and glutathione peroxidase (GSH-Px) activities in the liver. Since the food consumption of the heat-exposed group was only half that of the control, we prepared pair-fed groups, which were kept at 25 degrees C and whose food consumption were limited to those of the heat-exposed group for the 3-day and the 7-day exposure. TBARS concentrations of the liver was significantly higher in the heat group than the control after the 3-day exposure, while there was no significant difference among the groups after the 7-day exposure. There was no significant difference in GSH concentrations between the heat-exposed group and the control after the 7-day exposure, when the GSH concentration of the pair-fed group was significantly lower than that of the control. Hepatic cytosolic Se GSH-Px activity in the heat group was significantly less than that in the control group after the 6-hour exposure and it tended to be lower in the heat group than that of the control group after the 7-day exposure, while there was no difference in the total GSH-Px activity among the three groups. Our results showed that mild and chronic heat exposure may cause oxidative damage to organisms and that GSH-related anti-oxidative systems would play an important role to defensive reaction.

Deficiency of selenium enhances the K+-induced release of dopamine in the striatum of mice.

To determine whether a selenium (Se) deficiency in the brain leads to a functional change in dopaminergic transmission in the striatum, in vivo microdialysis was conducted in mice fed a low-Se diet. After 11-13 weeks of the diet regimen, the activity of glutathione peroxidase (GPx) in the Se-deficient brain was reduced to 60% of the control brain. A high K+ perfusion (100 mM) increased the level of dopamine in the dialysate to 67 +/- 16 times the basal level; the increase was significantly greater than that observed in the control group (28 +/- 4 times). Such a between-group difference was not observed after 4-5 weeks of the Se-diet. These results indicated that prolonged Se deficiency altered the function of striatal dopaminergic neurons in mice. A possible contribution of enhanced oxidative stress due to the reduced GPx activity is discussed.

Distribution of selenium in human plasma detected by high performance liquid chromatography-plasma ion source mass spectrometry.

The distribution of selenium in human plasma has been investigated by high performance liquid chromatography (HPLC) connected directly to inductively coupled plasma mass spectrometry (ICP-MS). Human plasma was loaded on to a size exclusion column and eluted with 0.01 M sodium phosphate buffer (pH 7.0) at a flow rate of 0.6 ml/min. Four peaks of selenium were detected in the chromatogram. The first selenium peak was obtained in the void volume. The retention time of the third peak was in accord with that of bovine serum albumin as a standard. The forth peak was thought to be a ghost. The method was applied to identify the chemical form of selenium in blood plasma immediately after intestinal absorption. The chromatographic pattern of selenium in postprandial human plasma was compared with that in fasting plasma. The first and third peaks in the postprandial plasma sample were slightly higher than those in the fasting plasma sample. This finding suggests that absorbed selenium is associated with the high molecular weight fraction and mercaptalbumin in blood plasma.