www.ccsenet.org/mas Modern Applied Science Vol. 4, No. 8; August 2010 ISSN 1913-1844 E-ISSN 1913-1852 88 Thermal Stress Induced Catalase Activity Level in Selected Bivoltine Breeds of Mulberry Silkworm Bombyx mori L. Pezhman Nabizadeh(Corresponding author) Department of studies in Sericulture Science, University of Mysore Manasagangotri, Mysore, 570 006, India Department of Biotechnology, Islamic Azad University of Shahrekord Branch, Shahrekord, Iran E-mail: pejman_nabizadeh@hotmail.com Tel: 91-99-8632-1678 T.S. Jagadeesh Kumar Department of studies in Sericulture Science, University of Mysore Manasagangotri, Mysore, 570 006, India Abstract In order to understanding the importance and level of changes in catalase activity in the three body tissues of silkworm Bombyx mori L. under high temperature conditions, the larvae of five bivoltine breeds were exposed to 35±1°C and 40±1°C, during fifth instar. Based on the results in the present investigation, Catalase, significantly (p<0.001) revealed the highest level of activity in the fat body tissue, compared to midgut and haemolymph. As well as, compared to CSR 4 , JROP, NB 4 D 2 and KA breeds, CSR 2 relatively showed the greater level of catalase activity under the control (28±1°C) and imposed thermal stresses. Keywords: Bivoltine breeds, Bombyx mori L., Catalase activity, High temperature 1. Introduction Temperature plays a major role on the growth and productivity of silkworms, as the silkworm is poikilothermic (Benchamin and Jolly, 1986). There is ample literature showing that good quality cocoons are produced within a temperature range of 22 - 27°C and levels above these makes the cocoon quality worse (Krisnaswami et al., 1973). The effect of higher temperatures on silkworm larvae was reported earlier by Takeuchi et al. (1964) and Ohi and Yamashita (1977). India being a tropical country, temperature shoots up in the day time. In summer it goes up to 35 to 40°C or even more. These fluctuations in temperature have an adverse effect on the survival and pupation of silkworm, especially the bivoltine breeds, incurring heavy loss to the industry. Many of the silkworm characters are not only controlled by genes but also influenced by environmental factors such as temperature (Watanabe, 1918, 1919, 1924, 1928 & Kogure, 1933). Earlier reports suggested that environmental stresses diminish in vivo antioxidant status and cause oxidative stress in living organism (Klasing, 1998 & Sahin et al., 2001). Oxidative stress is the result of an imbalance between pro-oxidant species and the levels of the defences resulting from the generation of reactive oxygen species (ROS) (Santoro & Thiele, 1997). Living organisms need mechanisms regulating reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion. Catalase (CAT) (H 2 O 2 :H 2 O 2 oxidoreductase, EC 1.11.1.6, CAT) is one of the antioxidant enzymes and catalyzes the degradation of H 2 O 2 to water and oxygen (Switala & Loewen, 2002). In chickens, oxidative stress was observed on exposure to acute heat stress (Mujahid et al., 2005 & Lin et al., 2006). In fact, ROS is harmful to living organisms because ROS tends to give oxidative damages to proteins, nucleic acids, and lipids (Hermes-Lima & Zenteno-Savı´n, 2002). In this context, ROS has been recognized to be related to aging and life span (Sohal et al., 1990, Orr and Sohal, 1994 and Parkes et al., 1999). On the other hand, ROS plays a helpful role in the innate immunity system of an insect (Hao et al., 2003 & Kumar et al., 2003). In insects, CAT is recognized as the key enzyme to be solely responsible for the scavenger of ROS (Felton & Summers, 1995). However, there is little information on insect CAT especially on its biochemical properties. High temperature affects nearly all biological processes including the rates of biochemical and physiological reactions (Hazel, 1995 & Willmer et al., 2004), and it eventually can affect on the quality or quantity of cocoon crops in the silkworm. Several reports (Ueda & Lizuka, 1962, Shirota, 1992 & Tazima & Ohuma, 1995) demonstrated that silkworms were more sensitive to high temperature during 4 th and 5 th stages which are