mRNA expression of antioxidant enzymes (SOD, CAT and GSH-Px) and lipid peroxidative stress in liver of Atlantic salmon (Salmo salar ) exposed to hyperoxic water during smoltification P.A. Olsvik a, * , T. Kristensen b , R. Waagbø a , B.O. Rosseland b,d , K.-E. Tollefsen b , G. Baeverfjord c , M.H.G. Berntssen a a National Institute of Nutrition and Seafood Research, PO Box 2029 Nordnes, N-5817 Bergen, Norway b Norwegian Institute for Water Research, N-0411 Oslo, Norway c Akvaforsk—The Institute of Aquaculture Research, N-6600 Sunndalsøra, Norway d Department of Ecology and Natural Resource Management, Agricultural University of Norway, N-1432 Aas, Norway Received 14 March 2005; received in revised form 18 July 2005; accepted 19 July 2005 Abstract The mRNA levels of three antioxidant genes, Cu/Zn superoxide dismutase (SOD), catalase (CAT) and phospholipid hydroperoxide glutathione peroxidase (GSH-Px), were quantified with real-time qRT-PCR in liver of Atlantic salmon Salmo salar exposed to 80% (normoxia), 105% and 130% O 2 saturation for 54 days. The salmon were then translocated and exposed to 90% and 130% O 2 saturation for additional 72 days during smoltification. TBARS and vitamin E levels in liver and the levels of oxidized glutathione (GSSG), total glutathione (GSH) and the resulting oxidative stress index (OSI) in blood were quantified as traditional oxidative stress markers. No significant mean normalized expression (MNE) differences of SOD, CAT or GSH-Px were found in liver after hyperoxia exposure at the two sampling times. Significantly decreased OSI was found in smolt exposed to 130% O 2 saturation after 126 days (n = 18, P < 0.0001), indicating hyperoxia-induced oxidative stress. No effects were seen on growth, or on the levels of thiobarbituric reactive substances (TBARS) and vitamin E in liver after the exposure experiment. Overall, the mRNA expression of SOD, CAT and GSH-Px in liver related poorly with the hyperoxic exposure regimes, and more knowledge are needed before the expressed levels of these antioxidant genes can be applied as biomarkers of hyperoxia in Atlantic salmon. D 2005 Elsevier Inc. All rights reserved. Keywords: Hyperoxia; Oxidative stress; Atlantic salmon; Real-time qRT-PCR; Gene expression; mRNA; Vitamin E; Oxidative index; Glutathione 1. Introduction Hyperoxic condition is commonly used in intensive aquaculture using water recirculation or to increase biomass and production. Oxygen is added to the inlet water, resulting in oxygen saturation in rearing tanks that may reach levels well above 100%. In fish, exposure to hyperoxia induces a reduction in gill ventilation and elevates the partial pressure of CO 2 in the blood, resulting in a respiratory acidosis (Heisler, 1993). The respiratory acidosis is compensated within days, but short-term exposure to hyperoxia may cause gill oxidative cell damage (Brauner et al., 2000). Long-term effects of exposure to hyperoxia are less known, but reducing p O 2 is the simplest and most efficient way to limit production of reactive oxygen species (ROS) (Massa- buau, 2001). Directly or indirectly p O 2 -induced stress is likely to be seen in all metabolically active tissues including liver cells, as the liver is one of the most important detoxifying organs in fishes (Di Giulio et al., 1995). Recent studies have documented reduced growth in smoltifying Atlantic salmon exposed to 130% oxygen levels (G. Baeverfjord, unpublished results). Salmonid tissues are 1532-0456/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpc.2005.07.009 * Corresponding author. Tel.: +47 55905154; fax: +47 55905299. E-mail address: pal.olsvik@nifes.no (P.A. Olsvik). Comparative Biochemistry and Physiology, Part C 141 (2005) 314 – 323 www.elsevier.com/locate/cbpc