The effects of zinc supplementation on the copper status of healthy adult men, as assessed by the activities of the copper-metalloenzymes, plasma ferroxidase (ceruloplasmin), and erythrocyte Cu,Zn-superoxide dismutase, were determined. The subjects were given either two daily doses of 25 mg zinc or placebo for 6 wk. No significant differences in the plasma copper levels or the ferroxidase activities between the supplemented and control groups could be detected at 2, 4, or 6 wk. Plasma zinc increased and erythrocyte Cu,Zn-superoxide dismutase decreased in the supplemented group, the difference between the groups becoming significant at 6 wk (p less than 0.05). This suggested that the zinc supplements decreased the copper status of the experimental group.
Everted duodenal segments, tied into sacs, taken from animals fed different amounts of zinc were used to investigate the antagonistic effect of dietary zinc on copper absorption. The intestinal segments taken from animals fed low amounts of zinc transferred more copper from a nutrient medium across the mucosal cells than did intestines from rats fed high levels of zinc. The mucosal cells from animals fed low amounts of zinc retained less copper than the cells from animals fed high amounts of the element. This retained copper was bound to a protein fraction having a molecular weight similar to that of metallothionein. The data suggest that zinc exerts its antagonistic effect by inducing the synthesis of a copper-binding ligand, probably a thionein, in the mucosal cells which sequesters copper from the nutrient medium, making it unavailable for serosal transfer. This may be a possible mechanism by which dietary zinc decreases copper absorption and leads to a decreased copper status.
Sprague-Dawley rats were fed a basal AIN-76 diet containing 80, 200, 350, 500 or 650 mg of magnesium per kilogram of diet for 6 wk. Ventricular slices, as well as microsomal fractions, were prepared from the hearts and were used to determine sodium-potassium pump activity. Sodium-potassium pump activity was assessed in the microsomal membranes by determining the ouabain-inhibitable Na+, K+-ATPase activity and [3H]ouabain binding, and in the ventricular slices, by determining ouabain-sensitive 86Rb uptake under K+-free conditions. The ATPase activity increased with increasing dietary magnesium, so that in the hearts of those animals that were fed 500 and 650 mg of magnesium/kg diet, it was significantly greater than the activity in the hearts of the animals fed 80 and 200 mg/kg diet. Similarly, 86Rb uptake by heart slices from rats fed 500 and 650 mg of magnesium/kg diet was significantly greater than the uptake by heart slices from animals fed 80 and 200 mg/kg diet. [3H]Ouabain binding did not change with increasing dietary magnesium. Thus, magnesium deficiency appears to have no effect on the number of sodium-potassium pump sites, but does decrease the activity of the pump. It is suggested that this leads to an increase in intracellular Na+, resulting in a change in the membrane potential, and may contribute to the arrhythmias associated with magnesium deficiency.
An earlier acid digestion determination of iodine in foods was modified to provide an improved detection limit and to allow for the analysis of a greater variety and larger amounts of foods. The organic material in the sample was oxidized overnight by concentrated nitric acid, followed by digestion in a mixture of concentrated sulfuric and 70% perchloric acid. The iodine was determined by an automated colorimetric method based on the iodide-catalyzed reduction of Ce+4 by As+3. The method had an average relative standard deviation of 3.1% for the samples analyzed, and a detection limit of 0.1 ng/mL in the digested solution and 5 ng/g in a 2 g sample prior to digestion. The recovery of added iodine ranged from 90.3 to 101.3%, using external standards. Samples analyzed included NBS Standard Reference Material 1549, and composites of a variety of dairy products, meat, eggs and fish, cereals, and potatoes. The iodine detected in these samples ranged from 9 ng/g for the potato group to 3360 ng/g for the standard reference material.
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