Graduate Thesis Or Dissertation
 

A metabolic function of selenium, assessed with rat liver homogenates

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  • In three separate trials rats were fed either a basal diet deficient in selenium and vitamin E or the same diet supplemented with one or both of these factors. In addition to these treatments in two of the trials, ethoxyquin was included to provide adequate lipid antioxidant protection and to alleviate the primary need for selenium and vitamin E in the role of a lipid antioxidant. On termination of either five- or eleven-week feeding trials, the rats were killed, their livers removed and prepared for in vitro studies. These liver preparations (either homogenates or mitochondria preparations) were used to study the metabolic activities of selenium and vitamin E in a NAD- and flavoprotein-dependent system, using pyruvate or succinate as the substrate. Results showed that selenium function was associated with the oxidation of pyruvate by the liver preparations, but not with oxidation of succinate. The inclusion of vitamin E along with selenium in the diet did not significantly increase the oxygen utilization of the pyruvate by animals which were on the test diets for only five weeks. However, by extending the feeding period to eleven weeks and including ethoxyquin in the diets, the combination of both selenium and vitamin E was required to increase the rate of pyruvate oxidation. This suggests that both selenium and vitamin E are biologically necessary in the oxidation of pyruvate and have a function other than that of a lipid antioxidant. This interaction between selenium and vitamin E could not be explained by the lipid antioxidant properties of vitamin E. The presence of ethoxyquin in these diets would be expected to alleviate further requirements of a lipid antioxidant by the animal tissue. In addition, dietary supplementation with vitamin E did not have the same response on pyruvate oxidation as noted by the combination of vitamin E and selenium since the oxygen utilization values with vitamin E lone were not significantly different from the deficient group. If further antioxidants were required for the body tissue, supplementation with vitamin E would be expected to reflect this in the oxidation values. For further proof of selenium function in the oxidative pathways involving pyruvate, sodium malonate was included in the incubation medium. This inhibited the flavoprotein-dependent system and made it possible to observe only the influence of selenium on the NAD-dependent oxidative system. The results from animals receiving ethoxyquin in their diet indicate that supplementation with selenium did not significantly increase the oxidative rate. A slight increase in pyruvate oxidation was noted following the combination of selenium and vitamin E in the diet, but this was not significantly different from the groups not supplemented with both. An investigation into the cause of the non-significant difference due to the inclusion of sodium malonate in the incubation medium revealed that oxaloacetic acid was not in sufficient supply. This probably limited the oxidation of pyruvate to the availability of oxaloacetic acid to combine with the acetyl CoA to form citric acid and did not reflect the extent of the response of selenium on the oxidation of pyruvate. As noted in the first trial, vitamin E exhibited a significant increase in the oxygen uptake values with succinate as the substrate for the liver homogenates. In later trials this influence was suggested to be not a direct one on the oxidative process involving succinate oxidation since through the inclusion of amytal to isolate the flavoprotein-dependent oxidative system, no difference in the succinate oxidation was noted. The suggestion that certain end-products may have masked the response of vitamin E was discounted since addition of sodium fumarate to the medium containing the amytal did not decrease the oxidation of succinate.
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