Effects of cholesterol on the properties of the membranes of isolated sheep liver nuclei and nuclear envelopes

Abstract

The exchangeability of cholesterol between sheep liver nuclear membranes and liposomes, and the effect of cholesterol on the fluidity of the membrane lipid were studied. In intact nuclei, the cholesterol/phospholipid ratio increased from 0.102 to 0.145 mol/mol on incubation with cholesterol-rich liposomes, with a time for half-maximal uptake of 4.2 h. In isolated envelopes under the same conditions, the ratio increased from 0.110 to 0.266 mol/mol with a time for half-maximal uptake of about 1.9 h. Moreover, the approximate order parameter of the spin label 5-(N- oxyl-4′,4′- dimethyloxazolidino)-stearic acid was 0.677 in intact nuclei and 0.723 in isolated envelopes prior to exchange; after exchange, these values increased to 0.717 and 0.756, respectively. These differences between the preparations could not be attributed to differences in the capacity for cholesterol uptake between the two nuclear membranes, or to a slow rate of exchange between them; the presence of an intact nuclear matrix appeared both to disorder the lipid partially and to inhibit cholesterol uptake. The differences indicate that conclusions based on physical studies of the membrane lipid in isolated envelopes are not necessarily applicable to the intact nucleus.