Spontaneous Phosphatidylcholine Exchange Between Vesicles and Micellar Lipid-Apolipoprotein Complexes
The spontaneous exchange of phosphatidylcholine (PC) between model lipoproteins and small unilamellar vesicles was investigated. The model lipoproteins were recombinants of bovine apolipoprotein A-I, egg PC, and cholesterol produced by cholate dialysis. Vesicles contained egg PC and cholesterol in varying molar ratios. Lipid exchange was followed by incubating radiolabeled vesicles (('3)H-PC, plus ('14)C-cholesterol oleate as a nonexchangeable marker) with unlabeled lipid-apolipoprotein complexes at a constant temperature of 37(DEGREES) C. Incubation mixtures were fractionated on Bio-Gel A-5 m columns at 5(DEGREES) C and the ('3)H cpm/('14)C cpm ratio under the vesicle peak was determined. Comparison of this ratio with that of vesicles before incubation was used to calculate the proportion of radiolabel transferred to complexes at various times. The results show that no net transfer of PC occurs under our experimental conditions; 70% of vesicle PC exchanges with complexes, indicating that only the outer monolayer of the vesicle is available for exchange; in contrast, all of the complex PC is exchangeable. Exchange is temperature dependent with an activation energy of 22.9 (+OR-) 2.0 kcal/mol. Under our experimental conditions the rate of PC exchange is linearly dependent upon both vesicle and complex PC concentrations with a rate constant of 6.9 (+OR-) 0.7 (mu)M('-1)h('-1); and the rate constant varies inversely with the cholesterol content of vesicles or complexes. These results imply that a complete kinetic model of spontaneous lipid exchange must account for the dependence of exchange rates not only on the concentration and composition of the donor particles, but on the concentration and chemical nature of the acceptors, as well.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign