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-D elta(9)-desaturase in carp liver. We hare determined thresholds for two und erlying mechanisms; namely, the activation of latent enzyme and the induced synthesis of new desaturase, Carp were progressively cooled from 30 degree s C to 23, 17 and 10 degrees C, where they were held for up to 5 days, Endo plasmic reticulum phospholipids showed substantial changes in fatty acid co mposition, with linear decreases in the proportion of saturates with temper ature 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 increa se in Delta(9)-desaturase activity without any change in the amount of desa turase protein or transcript. Further cooling to 17 degrees C induced a gre ater and more sustained increase in desaturase activity, reaching sevenfold on day 5, with a 10- to 20-fold increase in the amount of desaturase trans cript. 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 tha t observed at 17 degrees C, These results distinguish at least three mechanisms involved in cold-induce d lipid restructuring; the activation of latent desaturase observed with ge ntle 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 involv es changes in the activity of other biosynthetic enzymes, including elongas e and positional-and phospholipid-specific acyltransferases.