Fatty Acid Desaturation and Stearoyl-CoA Desaturase Expression in the Hepatic Microsomes of Milkfish, Chanos chanos, under Cold Shock

Milkfish, a commercially important culture teleost in Taiwan, are known to be less tolerant to the cold, and the culture industry in this country has often suffered from the mass mortality of winterkills. The membrane lipid desaturation and stearoyl-CoA desaturase expression in milkfish under cold shock were investigated. Milkfish were initially acclimated at 25℃ and then exposed to cold shock of 15℃. The proportions of saturated fatty acids (16: 0 and 18: 0) in the hepatic microsomes of milkfish at 15℃ were lower, while those of monounsaturated fatty acids (16: 1 and 18: 1) were significantly higher than the control fish at 25℃. Δ^9 desaturation index gradually increased in the early period of cold shock and peaked on day 5, followed by slight decrease thereafter. Concurrent changes in Δ^9 desaturation index (16: 1/16: 0 and 18: 1/18: 0) and the proportion of fatty acids in the hepatic microsomes of milkfish were further demonstrated. The trend of stearoyl-CoA desaturase activity changes was found parallel to those of unsaturated fatty acids composition and Δ^9 desaturation index, though 1-day time lag was evident. These parameters are significantly correlated among them, and Δ^9 desaturation index can accordingly be employed to represent the degree of fatty acid desaturation, stearoyl-CoA desaturase activities and membrane fluidity as well. The expression of stearoyl-CoA desaturase mRNA in milkfish is specifically induced by a low temperature, and those in stearoyl-CoA desaturase activities were parallel to the changes in desaturase mRNA transcript. The findings suggested that the increases in the membrane lipid desaturation and stearoyl-CoA desaturase activities in milkfish under cold shock were in accordance with other teleosts being reported elsewhere, and the quantitative compensatory mechanisms are predominantly involved in the acclimation process of cold shock in this species.