Methods recently developed for the preparation of fractions greatly enriched in the surface membrane of liver cells now allow a comparison of the lipid composition of surface and cytoplasmic membranes. Several laboratories a-5) have reported a relatively high content of cholesterol in surface membrane preparations, but the saturation characteristics of the fatty acids have been studied much less extensively. One report 4) has indicated that the surface membrane possessed relatively less polyunsaturated and relatively more saturated fatty acid than did liver homogenate.
This present study involves a complete fractionation of liver. It allows a comparison of fatty acid saturation and cholesterol content between individual membrane types, and furthermore, the use of perfused and centrifuged material avoids contributions from "free fat", erythrocytes and plasma.
The rats used in these experiments were aged between 6-12 weeks and of either sex. Those used in the preparation of nuclei were mainly from the upper part of the age range. Each experimental value represents measurements on lipid extracts from preparations derived from 3-6 animals. All had been fed upon the same diet. Purified surface membrane was isolated from previously perfused livers using 0.3 M sucrose2). Mitochondria and microsomes were isolated from supernatant fraction of the same procedure2,6). Nuclei were isolated by a method using 2.3 M sucroseT). Preparations were monitored for purity and content by enzymatic markers and by electron microscopy 2). The subcellular fractions were stored at -20 Β° C until required for analysis. Before lipid extraction, the fractions were thawed, centrifuged (6 x 106 g/rain) and the pellet resuspended in 20 volumes of water, this was then centrifuged (6x 106 g/min) and the resultant pellet suspended in a convenient volume of water. The sucrose content of the extracted suspensions was therefore no greater than 0.005 M.