| Summary: | Thesis (Ph.D.) -- Memorial University of Newfoundland, 1983. Biochemistry Bibliography : leaves 248-262. The gastrointestinal uptake of Zn2+ was studied in the winter flounder using an in situ technique, an equilibrium dialysis technique and a non-absorbed marker technique. -- The entire digestive tract of the flounder was capable of absorbing Zn2+, with the uppermost portion of the intestine having the highest and the stomach the lowest capacity. A seasonal study revealed that the capacity to absorb Zn2+ was greatest during the summer feeding period. Zn2+ absorption appeared to involve at least two steps, the first a rapid accumulation of Zn2+ by the tissue and the second, a slower transfer of Zn2+ into the body. The total amount of Zn2+ absorbed increased with increasing loads of Zn2+ in the lumen; the transfer mechanism did not appear to be saturated at the highest Zn2+ loads tested. However, Zn2+ uptake was inhibited by several other metals and by the amino acid, histidine. -- The capacity of the digestive tract to absorb Zn2+ was not affected by feeding the flounder a high-Zn2+ diet or by increasing the body Zn2+ load by parenteral injections. In the event of exposure to elevated levels of Zn2+ in the diet, it is suggested that elimination mechanisms may play a greater role in Zn2+ homeostasis in the winter flounder than limitation of gastrointestinal uptake. -- The dynamics of Zn2+ turnover in the flounder was investigated by examining the distribution of 65Zn in the tissues following single intramuscular injections of the radiotracer. The tissues exhibited different rates of accumulation and loss of 65Zn, the most rapid being in tissues such as the kidney, liver, gill and gastrointestinal tract. -- Retention of 65Zn was examined in live flounder using a whole-body detector. The loss of 65Zn appeared to vary seasonally; when flounder were monitored during the summer feeding period the rate of 65Zn loss (monitored in the area of the peritoneal cavity) increased over that seen in the winter non-feeding period (i.e. TB1/2 = 223 and 1510 days, respectively). Whole-body 65Zn retention patterns were similar in flounder injected with saline or a load of stable Zn2+. Under these experimental conditions the rate of Zn2+ loss was not affected by an excess of stable Zn2+. -- Experiments conducted to determine the possible site(s) of Zn2+ excretion into the digestive tract following i.v. injections of 65Zn indicated that Zn2+ could be "secreted" into the lumen contents all along the tract. Other possible sites of Zn2+ elimination include the gills, kidney and body surface. -- Chromatographic techniques were its ed. to examine the Zn2+-binding proteins in the cytosols of several tissues of the winter flounder. A low molecular weight 65Zn (Zn2+)-binding protein, with properties characterising it as metallothionein, was isolated from the mucosal and liver cytosols of Zn2+-injected flounder. Metallothionein did not appear to be serving the same function(s) in the intestine of the flounder as commonly hypothesized for mammals. The presence of the protein in the mucosal cytosol did not appear to be associated with any enhancement or depression of Zn2+ absorption. The role of metallothionein in the normal metabolism of Zn2+ in the flounder remains to be resolved. That it could play a role in homeostasis is suggested by the presence of a low molecular weight Zn2+-binding protein, with elution characteristics similar to metallothionein, in the tissues of normal flounder.
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