| Abstract: | Liposomes containing 79 mM Tris-acetate and 50 mM KCl were preparedfrom the total lipid extracts of smooth microsomal membranesisolated from 2, 4, 7 and 9 d old bean cotyledons. Permeabilityto glycerol was determined by spectrophotometric measurementsof osmotic swelling when the liposomes were placed in eitherisotonic or slightly hypotonic glycerol. For liposomes from2 and 4 d old membrane there was a time-dependent decrease inabsorbance at 450 nm from which initial swelling rates reflectingthe influx of glycerol and water were calculated. At 25 °Cthese rates were not significantly different For liposomes from7 and 9 d old membrane there was no change in absorbance withtime at 450 nm signifying that these older liposomes were equallypermeable and non-electrolytes and non-electrolytes, and thereforeincapable of swelling. Permeability to glucose was determinedby preparing the liposomes in a solution of the sugar, passingthem through a Sephadex column to eliminate unsequestered glucose,and quantifying sugar leaked from the liposomes over time bymeasuring NADPH formation through the tandem actions of hexokinaseand glucose-6-phosphate dehydrogenase. The rate constants forglucose leakage from 2 and 7 d old liposomes were 0.55 and 3.94respectively, again indicating a dramatic increase in permeabilitywith advancing age. These changes in permeability correlatetemporally with the appearance of gel phase lipid in both liposomesand the membranes from which they were derived, suggesting thatthe coexistence of discrete liquid-crystalline and gel phaselipid domains renders membranes leaky and contributes to lossof intracellular compartmentation in senescing tissue. |
