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Journal of Experimental Biology, Vol 203, Issue 3 641-650, Copyright © 2000 by Company of Biologists

JOURNAL ARTICLES

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

RJ Trueman, PE Tiku, MX Caddick and AR Cossins
Integrative Physiology Research Group, School of Biological Sciences, University of Liverpool, Liverpool L69 3BX, UK.

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.
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