Selenium metabolism in the chick and effects of Vitamin E and selenium on heat stress Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7h149s45k

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  • An in vitro study was conducted to investigate the metabolism and distribution of Se⁷⁵-selenite and Se⁷⁵-selenomethionine (SEM) in chick blood. Se⁷⁵-selenite is taken up by RBC (13% within 20 minutes) and expelled into the plasma to become bound to proteins. In contrast, Se⁷⁵-SEM showed a more gradual and continuous buildup in RBC with time, according to a hyperbolic type function. Chromatography of RBC lysates on Sephadex G200 established that Se⁷⁵ from selenite is more specifically incorporated into GSH-Px (MW 150,000), whereas Se⁷⁵ from SEM is mostly incorporated into Hb (MW 66,000). The elution pattern of plasma Se⁷⁵-selenoproteins from Se⁷⁵-selenite or Se⁷⁵-SEM on Sephadex G200 showed that radioactivity is associated with two peaks corresponding to MW >440,000 and 89,000. Binding of Se⁷⁵, from selenite or SEM, to plasma proteins was dependent on the presence of RBC. Addition of GSH and GSSG reductase to plasma gave the same effects as RBC on binding of Se⁷⁵ from selenite, but not from SEM, to plasma proteins. To study the nature of this bond, the resistance of plasma protein bound Se⁷⁵ (from selenite or SEM) to extraction by TCA or exhaustive dialysis was studied and indicated that the element was tightly bound to plasma proteins. Treatment with β-mercaptoethanol followed by TCA, however, caused release of most of the Se⁷⁵ plasma selenoproteins (from both Se sources), suggesting that Se⁷⁵ is present in proteins as a selenotrisulfide bond (Pr-S-Se⁷⁵-S-Pr). This was confirmed by chromatography of the released radioactivity on an amino acid analyzer. Based on these results, an in vitro model of selenium metabolism in chick blood is postulated. After chicks were intubated with Se⁷⁵-selenite or Se⁷⁵-selenomethionine (SEM), the distribution and metabolism of radioactivity, as judged by incorporation into cytosolic proteins of various tissues, were studied. Gel filtration studies indicated that Se⁷⁵ from selenite is more specifically incorporated into erythrocyte GSH-Px (MW 150,000) than Se⁷⁵-SEM. It was observed that chick hemoglobin (Hb, MW 66,000) bound Se⁷⁵ from both sources, and possessed peroxidase activity. A mechanism was postulated to explain Hb peroxidase activity. Results of chromatographing plasma on Sephadex G200 indicated that proteins corresponding to MW of 338,000 are the major carriers of Se⁷⁵ from selenite or SEM 6 hours after dosing, however proteins possessing a MW of 89,000 are the important Se⁷⁵ carriers 96 hours after intubation. Compared to tissues, the liver and kidney contained the highest concentrations of Se⁷⁵. Gel filtration chromatography of tissue cytosols on Sephadex G200 indicated that the majority (34-80%) of Se⁷⁵, from selenite or SEM, is associated with the enzyme GSH-Px 24 hours after dosing. Pancreatic cytosol of chicks dosed with Se⁷⁵-SEM had the majority of radioactivity associated with a peak corresponding to MW 44,500. Evidence for non-glutathione peroxidase selenoproteins were detected in various tissues. These included high molecular weight (>440,000) selenoproteins in cytosols of all tissues examined except in bone marrow, a peak (MW 44,500) specific to selenite but not SEM in liver cytosols, a low molecular weight (MW 25,100) protein in kidney and spleen cytosols from both Se⁷⁵ treatments, and two distinct testicular selenoproteins (MW 35,000 and 263,000) from both Se⁷⁵ treatments. A hypothetical pathway for the incorporation of Se⁷⁵ into selenocysteine-containing proteins (i.e., GSH-Px) is presented to account for these observations. During a study on the effects of high levels of Se and/or Vitamin E on performance under heat stress, a significant depression in gain and feed consumption was found, regardless of treatment. However, feed efficiency significantly improved in heat stressed chicks fed diets high in selenium and Vitamin E when compared to feed efficiency in animals on other dietary regimes. Significantly lower hematocrit levels were found in heat stressed chicks, as compared to control chicks, which were raised under conventional conditions. Se levels in chick blood, liver, kidney, and pancreas, and plasma Vitamin E levels were directly related to dietary Se or Vitamin E content. Dietary Vitamin E supplementation counteracted the effect of heat stress in depressing blood selenium levels, however, the opposite trend was observed in the other organs studied. The pancreas in heat stressed chicks was larger than chicks raised at conventional temperatures.
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