MOLECULAR AND STRUCTURAL COMPOSITION OF PHOSPHOLIPID-MEMBRANES IN LIVERS OF MARINE AND FRESH-WATER FISH IN RELATION TO TEMPERATURE

The compositions and physical states of the liver phospholipids of mar ine and freshwater fish adapted to relatively constant but radically d ifferent temperatures were investigated. Fish adapted to low temperatu re (5-10-degrees-C) accumulated more unsaturated fatty adds than those in a warm (25-27-degrees-C) environment. There were no measurable dif ferences in the gross fatty acid compositions of the total liver phosp holipids from identical thermal environments. Docosahexaenoic acid (22 :6) did not seem to participate in the process of adaptation. Cold ada ptation was coincidental with oleic add (18:1) accumulation, preferent ially in the phosphatidylethanolamine. Determination of the molecular species composition of phosphatidylethanolamine revealed a 2- to 3-fol d and 10-fold increase in the level of 18:1/22:6 and 18:1/20:5 species , respectively. ESR spectroscopy revealed a 7-10% compensation in the ordering state of native phospholipids with temperature. Combination o f 16:0/22:6 phosphatidylcholine with phosphatidylethanolamines of cold -adapted marine fish showed a drastic fluidization near the C-2 segmen t of the bilayer, but not in the deeper regions. An appropriate combin ation (75:25) of phosphatidylcholines from warmth-adapted marine fish with phosphatidylethanolamines from cold-adapted marine fish mimicked a 100% adaptational efficacy in the C-2 segment as compared with the p hosphatidylethanolamines of warmth-adapted marine fish. A specific rol e of 18:1/22:6 phosphatidylethanolamine in controlling membrane struct ure and physical state with thermal adaptation is proposed.