Effects of temperature on the structure and metabolism of cell membranes in fish.

The metabolic adjustments responsible for the "homeoviscous adaptation" of membrane lipid composition in fish are examined with special reference to the rainbow trout, Salmo gairdneri. The percentage of fatty acid lipogenesis attributable to unsaturates was elevated after an acute drop in temperature but declined with continued cold exposure (i.e., cold acclimation). In contrast, selected desaturation reactions [particularly those involved in the production of polyunsaturated fatty acids (PUFA) of the n-3 and/or n-6 families] proceeded more rapidly in cold-than in warm-acclimated trout. Different time courses for the change in monoene and PUFA levels of hepatic microsomal membranes during thermal acclimation suggest that the various desaturase enzymes contribute to the acclimatory response at different times. Certain fatty acids, particularly the delta 5-desaturation products of the n-3 (20:5 delta 5,8,11,14,17) and n-6 (20:4 delta 5,8,11,14) series, were preferentially incorporated into phospholipids at cold temperatures and by cold-acclimated trout, due in part to the direct effect of temperature on the substrate preferences of the phospho- and acyltransferase enzymes of de novo phospholipid biosynthesis; however, chain length rather than degree of unsaturation per se may determine the temperature-dependent pattern of fatty acid incorporation. Both acute and chronic cold exposure elevated the incorporation of PUFA into phosphatidylserine (PS), suggesting that the conversion of PS to phosphatidylethanolamine (PE) may be activated at cold temperatures. The rate of homeoviscous adaptation appears to be limited by the rate of membrane lipid turnover, which although generally positively correlated with acclimation temperature, did vary depending on the phospholipid moiety and tissue considered. Finally the direct acylation of lysophospholipids formed during the process of membrane turnover may contribute to both rapid and acclimatory adjustments in membrane lipid composition.