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

The compositions and physical states of the liver phospholipids of fis h and phospholipids of shrimps adapted to relatively constant but radi cally different temperatures were investigated. There were no measurab le differences in their gross fatty acid compositions of phospholipids from the species obtained from identical temperature. Saturated-to-un saturated fatty acid ratio did not show any convincing difference. Doc osahexaenoic acid (22:6) did not seem to participate in the process of adaptation. Cold adaptation was coincidental with oleic acid (18:1) a ccumulation, preferentially in the phosphatidylethanolamine. In the ex periment with rats, it has been found that the fish oil fed rats showe d a similarity in their liver phospholipid fatty acid composition like fish liver phospholipids. Determination of molecular species composit ion 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 de eper regions. Phospholipids from the rat livers did not show any chang e irrespective of diet. Phosphatidylcholines from rat liver, fish live r 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 phosph atidylethanolamines from cold adapted species showed a drastic fluidiz ation at the C-2 segment, in comparison with their phosphatidylcholine s. In the reverse combination it showed a rigidification at the same s egment. Role of saturated-to-unsaturated fatty acid ratio in lipid mem brane adaptation has been argued and the role of specific molecular sp ecies has been proposed.