The compositions and physical states of the Iver phospholipids of marine and freshwater fish adapted to relatively constant but radically different temperatures were investigated. Fish adapted to low temperature (5-10°C) accumulated more unsaturated fatty acids than those in a warm (25-27°C) environment. There were no measurable differences in the gross fatty acid compositions of the total liver phospholipids fom Ident thermal environments. Docosahexaenoic acid (22:6) Poikilotherms are often subjected to temperature stress or seasonal variation of temperature. These organisms exploit diversity in lipid structures to fashion membranes to prevailing ambient temperatures in such a manner that they become more fluid in a cold-acclimatized state and less fluid in a warmth-acclimatized state (1, 2). Sinensky (3) has termed this response to temperature "homeoviscous adaptation". The extent ofhomeoviscous efficacy with which cells compensate membrane fluidity in response to changes in ambient temperature is only partial (4,5). However, this response is rather rapid in the carp liver endoplasmic reticulum (6) and erythrocytes (7). Restructuring of the polar headgroup composition of phospholipid classes, together with modification of the unsaturation of their fatty acyl chains, may ensure that the components present are best suited to function within the constraints imposed by a prevailing thermal environment. One rational explanation of these responses is that the melting points of the fatty acids decrease with an increasing number of double bonds in the molecule, resulting in a more fluid structure in membranes rich in polyunsaturated fatty acids. Accepting this hypothesis, one would expect a marked difference between fish species inhabiting regions of ex-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.tremely different but relatively constant temperatures. Phospholipids from specific membranes of an Arctic marine fish proved more unsaturated than those of subtropical freshwater fish, and these differences were reflected in the microviscosity ofthese structures (8). The present article compares the liver phospholipid compositions of a number of marine and freshwater fish, and a feasible mechanism is proposed to explain the adaptational phenomena at a membrane structural level.MATERIALS AND METHODS Fish. The following fish were involved in this study. Warm-adapted marine fish (WAMF) included Carnax calla, Epinephelus bleekeri, Lutaganus sebeae, Mugil passia, Nemipterus hexodon, Nemipterusjavonicus, Pomadysys hasta, Sardinella longiceps, and Tachisurus yella from the South China Sea and the southwest coast of India (20-27C). Cold-adapted marine fish (CAMF) included Clupea harengus, Scorpaenichthys marmoratus, Hexaganus stelleri, Hipoglossus hipoglossus, Onchorhyncus kisutch, Onchorhyncus tshawsytscha, Ophioidon melanus, and Sebastosomus melanus from the...

Enzymes:Acid phosphatase, orthophosphoric-monoester phosphohydrolase (acid optimum) (EC 3.1.3.2); TV-Acetyl-ß-glucosaminidase, ./V-acetyl-ß-D-glucosaminide ./V-acetylglucosaminohydrolase (EC 3.2.1.30); Glucose-6-phosphatase, D-glucose-6-phosphate phosphohydrolase (EC 3.1.3.9); 5'-Nucleotidase, 5'-ribonucleotide phosphohydrolase (EC 3.1.3.5). -Abbreviations:Apo, apolipoprotein; 125 I-TC-LDL, 125 I-tyramine cellobiose-labelled low density lipoprotein; PVP, poly(vinylpyrrolidone); CURL, compartment for uncoupling of receptor and ligand; MVB, multivesicular body; Hepes, 4-(2-hydroxyethyl)-l-piperazineethanesulfonic acid.