The qualitative and quantitative lipid composition of nuclei and nuclear membranes from pig and rat liver were determined. These determinations were compared with the corresponding data obtained for microsomes from the same material after similar treatments. The results indicate that, at least, by far the major part of the nuclear lipids is located in the membranes of the nuclear envelope. The phospholipid pattern of the nuclear membranes and the endoplasmic reticulum (ER) membranes in general is widely identical in both species. As a striking difference in the lipid composition, however, a fourfold increase of esterified cholesterol in the nuclear membranes was found. In a quantitative approach the ratio of total surface area of the nuclear lipids to the total surface area of the nuclear envelope membranes was calculated as being 3.6, a value which fairly approximates the requirements of a bimolecular lipid leaflet model.

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