An isothermal induction of delta 9-desaturase in cultured carp hepatocytes

Cold exposure of carp leads to the induced activity of the hepatic delta 9-desaturase (Schünke, M. and Wodtke, E. (1983) Biochim. Biophys. Acta 734, 70-75). We have investigated the controlled expression of this enzyme using isolated carp hepatocytes. Culture at 30 degrees C, of cells isolated from 30 degrees C-acclimated carp. resulted in an 8-13-fold increase in desaturase-specific activity over 4 days, whilst another enzyme of intermediary metabolism, glucose-6-phosphatase, decreased by more than 60%. This desaturase induction was associated with a loss of intracellular lipid vesicles and with increases in the levels of oleic acid of membrane phosphoglycerides and corresponding decreases in 22:6(n - 3). Supplementation of cultures with oleic acid and with polyunsaturated fatty acids did not cause any reduction in the desaturase induction. The level of immunodetectable desaturase protein increased during culture at 30 degrees C and a desaturase mRNA was detected after 2 days of culture by Northern analysis. These results suggest that in vitro culture leads to an increased synthesis of desaturase protein by means of activated gene transcription. Significantly, transfer of cultures of 30 degrees C-acclimated hepatocytes to 10 degrees C resulted in a smaller induction of desaturase activity; thus cold transfer of cells in itself did not induce hepatocyte desaturase activity as does whole animal cooling. This suggests either that cold induction of desaturase activity in vivo involves systemic control or that the conditions imposed by culture prevent cold induction.