Thermal thresholds of lipid restructuring and delta(9)-desaturase expression in the liver of carp (Cyprinus carpio L.).

Cold acclimation induces a transient enzymatic activation of the acyl CoA-(&Dgr;)(9)-desaturase in carp liver. We have determined thresholds for two underlying mechanisms; namely, the activation of latent enzyme and the induced synthesis of new desaturase. Carp were progressively cooled from 30 degrees C to 23, 17 and 10 degrees C, where they were held for up to 5 days. Endoplasmic reticulum phospholipids showed substantial changes in fatty acid composition, with linear decreases in the proportion of saturates with temperature over the full range of cooling (11.3 % in phosphatidylcholine and 15.8 % in phosphatidylethanolamine). In the phosphatidyl-ethanolamine fraction, this was linked to increased proportions of monoenes, particularly 20:1(n-9). Modest cooling to 23 degrees C on day 1 induced a 2.5-fold transient increase in delta(9)-desaturase activity without any change in the amount of desaturase protein or transcript. Further cooling to 17 degrees C induced a greater and more sustained increase in desaturase activity, reaching sevenfold on day 5, with a 10- to 20-fold increase in the amount of desaturase transcript. Extreme cooling to 10 degrees C led to a very large, but transient, 40- to 50-fold increase in desaturase transcript amounts, a modest 40-50 % increase in desaturase protein but no further increase in activity over that observed at 17 degrees C. These results distinguish at least three mechanisms involved in cold-induced lipid restructuring; the activation of latent desaturase observed with gentle cooling, the induction of desaturase gene transcription and, finally, a third unidentified lipid compensatory mechanism that occurs with extreme cooling. The complex nature of cold-induced lipid restructuring also involves changes in the activity of other biosynthetic enzymes, including elongase and positional- and phospholipid-specific acyltransferases.