ROLE OF PHOSPHOLIPID MOLECULAR-SPECIES IN MAINTAINING LIPID-MEMBRANE STRUCTURE IN RESPONSE TO TEMPERATURE

The compositions and physical states of the liver phospholipids of fish and phospholipids of shrimps adapted to relatively constant but radically different temperatures were investigated. There were no measurable differences in their gross fatty acid compositions of phospholipids from the species obtained from identical temperature. Saturated-to-unsaturated fatty acid ratio did not show any convincing difference. Docosahexaenoic acid (22:6) did not seem to participate in the process of adaptation. Cold adaptation was coincidental with oleic acid (18:1) accumulation, preferentially in the phosphatidylethanolamine. In the experiment with rats, it has been found that the fish oil fed rats showed a similarity in their liver phospholipid fatty acid composition like fish liver phospholipids. Determination of molecular species composition of phosphatidylcholine and phosphatidylethanolamoine revealed a 4- to 5-fold and 10-fold increase in the level of 18:1/22:6 and 18:1/20:5 species respectively in favor of cold adaptation. Phospholipids from cold-adapted species showed a more fluid structure than that of warm-adapted species near the C-2 segment of the bilayer, but not in the deeper regions. Phospholipids from the rat livers did not show any change irrespective of diet. Phosphatidylcholines from rat liver, fish liver or shrimps did not show any difference among them in their fluidity irrespective of their environmental conditions or diet. An appropriate combination (75:25) of phosphatidylcholine from rat liver with phosphatidylethanolamines from cold adapted species showed a drastic fluidization at the C-2 segment, in comparison with their phosphatidylcholines. In the reverse combination it showed a rigidification at the same segment. Role of saturated-to-unsaturated fatty acid ratio in lipid membrane adaptation has been argued and the role of specific molecular species has been proposed.