Fatty acid composition of the ingested food only slightly affects physicochemical properties of liver total phospholipids and plasma membranes in cold-adapted freshwater fish

The fatty acid content, fatty acid molecular species composition, and fluid ity of phospholipid vesicles and plasma membranes of livers of fish feeding on phytoplankton (silver carp. Hypophthalmichtys molitrix), zooplankton, ( big head carp, Hypophtalmitrix nobilis), higher plants, (grass carp, Ctenop haryngodon idella), and omnivorous fish, (common carp, Cyprinus carpio), ad apted to reduced temperatures, was determined. Levels of long chain polyuns aturated fatty acids (PUFAs) were highest in silver and big head carp and l owest in common carp. Differences in lone chain PUFAs were poorly reflected in fluidity, determined either by fluorescence anizotropy of 1,6-diphenyl 1,3,5-hexatriene or rotational correlation time of 16-doxyl- stearic acid, of phospholipid vesicles. Phospholipid vesicles of C. caripo and C. idella proved to be slightly more rigid than of H. molitrix and H. nobilis below c a. 15 degrees C, These differences were not seen with isolated plasma membr anes assessed with 1,6-diphenyl 1,3,5-hexatriene. Using electron spin reson ance spectroscopy plasma membranes from H. molitrix proved to be more fluid than those from C, carpio below ca, 15 degrees C, Phosphatidylethanolamine s were abundant in I-monounsaturated, 2-polyunsaturated(18:1/22:6, 18:1/20: 4) species with H. molitrix and H. nobilis more rich in these species than C, carpio and C. idella. It is suggested that differences in fluidities of phospholipid vesicles below 15 degrees C is due to differences in levels of the above molecular species. In native membranes these differences may be masked by proteins and other membrane components. It is also suggested that it is not the fatty acid composition of the ingested food but the specific response of the fish to temperature that is the most important factor cont rolling the physicochemical properties of membranes during thermal adaptati on.