Comparative Biochemistry and Physiology Part B 136 (2003) 921–934 1096-4959/03/$ - see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S1096-4959(03)00288-4 Influence of selenium sources on age-related and mild heat stress- related changes of blood and liver glutathione redox cycle in broiler chickens (Gallus domesticus) Kamel Z. Mahmoud *, F.W. Edens a, b Department of Animal Production, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan a Department of Poultry Science, North Carolina State University, Raleigh, NC 27695-7635, USA b Received 11 June 2003; received in revised form 10 September 2003; accepted 11 September 2003 Abstract Selenium is an essential trace element that up-regulates a major component of the antioxidant defense mechanism by controlling the body’s glutathione (GSH) pool and its major Se-containing antioxidant enzyme, glutathione peroxidase (GPX). Evidence has emerged suggesting that organic selenium, natural seleno-amino acids found in plants, grains and selenized yeast, maintains the antioxidant defense system more efficiently than inorganic selenium. Inorganic selenium is a pro-oxidant, whereas organic selenium possesses antioxidant properties itself. As a pro-oxidant, inorganic selenium is not suitable for animals or humans. Therefore, we examined the GSH–GPX system in broiler chickens and determined that organic selenium was indeed more beneficial than inorganic selenium. Chickens fed the organic selenium as Sel- Plex , a selenized yeast, had elevated GPX activity in both blood and liver in a thermoneutral environment and after  heat distress. More importantly, the ability to reduce the oxidized glutathione (GSSG to 2 GSH) was enhanced and facilitated by maintenance of glutathione reductase activity. Organic selenium-fed chickens were less affected by mild heat distress than inorganic selenium-fed chickens, and this assessment was based upon less induction of heat shock protein 70 (hsp70) in organic selenium-fed chickens. Our results clearly show that heat distress, a potent inducer of oxidative stress and hsp70, can be partially ameliorated by feeding organic selenium. We attribute this observation to an enhanced GSH–GPX antioxidant system in organic selenium-fed chickens.  2003 Elsevier Inc. All rights reserved. Keywords: Antioxidant; Glutathione; Selenium; Broiler; hsp70; Heat stress 1. Introduction Selenium is an essential trace element that Abbreviations: GPX, glutathione peroxidase; GR, glutathi- one reductase; G6PDH, glucose-6-phosphodehydogenase; GSH, reduced glutathione; GSSG, oxidized glutathione: organ- ic selenium: Sel-Plex ; BS, basal diet with no supplemental  selenium; IS, inorganic selenium (sodium selenite); hsp, heat shock proteins; hsp70, heat shock protein 70. *Corresponding author. Tel.: q962-2-720-1000x22212; fax: q962-2-709-5069. E-mail address: kmahmoud@just.edu.jo (K.Z. Mahmoud). regulates a major component of the antioxidant defense mechanism in all living tissues (Goehring et al., 1984; Gerloff, 1992) by controlling the body’s glutathione (GSH) and its major Se-con- taining antioxidant enzymes, glutathione peroxi- dase (GPX) and thioredoxin reductase. Glutathione and GPX have the capacity to protect the integrity of unsaturated bonds of membrane phospholipids by extinguishing free radical attacks capable of initiating and propagating lipid oxidation (Meister and Anderson, 1983; Deleve and Kaplowitz, 1991;