Effect of Selenium Depletion on Oxidative Stress in Mature Horses

Abstract

s / Journal of Equine Veterinary Science 31 (2011) 230-356 257 [5] Bryden WL. Amino acid requirements of horses estimated from tissue composition. Proc Nutr Soc 1991;16:53. [6] Wilson JA, Graham-Thiers PM. Muscle and plasma amino acids in pregnant/lactating mares and their weanling foals. J Equine Vet Sci 2009;29:370. [7] Nutrient Requirements of Horses. 6th ed. Washington, DC: The National Academies Press; 2007. [8] Waggoner JW, LOxest CA, Mathis CP, Hallford DM, Petersen MK. Effects of rumen-protected methionine supplementation and bacterial lipopolysaccharide infusion on nitrogen metabolism and hormonal responses of growing beef steers. J Anim Sci 2009;87:681. Effect of Selenium Depletion on Oxidative Stress in Mature Horses M. Brummer, S. Hayes, J.E. Earing, S.M. McCown, and L.M. Lawrence University of Kentucky, Lexington, KY, USA Introduction: Glutathione peroxidase (GPx) plays an important role in the neutralization of hydroperoxides andhydrogenperoxide. Hydrogen peroxide can generate hydroxyl radicals which are regarded as a very harmful reactive oxygen species [1]. Selenium (Se) deficiency can result in decreased GPx activity; therefore our aim was to investigate the effect of low Se status on oxidative stress in mature idle horses. We hypothesized that Se depletion would result in a decrease in GPx activity, serum total antioxidant capacity (TAC) and an increase in oxidative stress, measured as serum malondialdehyde concentration (MDA). Type I deiodinase, is a selenoenzyme responsible for the conversion of inactive T4 to active T3. Therefore as additional indicators of Se status we measured triiodothyronine (T3), thyroxine (T4), and the T3:T4 ratio as changes in these hormones or in their ratio has been evaluated in horses fed varying levels of Se before [2]. Materials and Methods: Twenty eight mature horses (6 24 yr) were used in this study. The 28 horses included 8 geldings and 20 mares that had resided on the farm for at least 60 days prior to the study. The study period was 28 wks. Horses were kept on pasture ( .05). MDA concentrationwas not different between AS and LS at the initial or final time point, howeverMDAdid increaseover time (P< .05) in LS (from3.40 .86 nmol/mL to 4.04 .89 nmol/mL) while it remained similar in AS (4.68 1.78 nmol/mL to 5.09 1.09 nmol/mL). Two traditional indicators of Se status were measured: whole blood Se concentration and GPx activity. In addition the serum T3/ T4 ratio was determined. Type I deiodinase, the enzyme responsible for converting T4 to T3, has a greater ranking in terms of priority of Se supply than cytosolic GPx [2]. A change in this ratio is generally not observed during a marginal Se deficiency [2], so the increased ratio of T3/T4 in the LS horses along with the changes in whole blood Se and GPx suggests that the LS horses were at, or approaching, deficient Se status. TAC was expected to decrease in LS horses, but it was unaffected. The lack of change in TAC may point towards a compensatory mechanism within the antioxidant system, or lack of sensitivity of the assay as a measure of the GPx-associated antioxidant capacity in serum. GPx is only one of several compounds that contribute to total antioxidant capacity. MDA is a byproduct of lipid peroxidation [4]. We hypothesized that MDA concentrations would increase with decreasing Se status. Although MDA concentrations increased over time in the LS horses, the magnitude of the changes were relatively small. Much larger changes in MDA have been reported in response to exercise or disease in other studies [4] and some type of oxidative challenge might be necessary to produce more marked differences in MDA in the horses studied here. Conclusion: In this study feeding a low Se diet for 28 wks altered indicators of Se status, but appears to have hadminimal effects on the indicator of oxidative damage. However, more marked effects might have been observed if the horses had been challenged with an oxidative stressor such as exercise or disease. Further research in this area may provide better insight into the importance of Se in oxidative stress.