Cholesterol Pools in Mycoplasma Membranes: Modifications in Phospholipid Composition Affect the Kinetics of Cholesterol Exchange with Lipid Vesicles
The exchange of cholesterol between membranes represents an important process by which cholesterol composition is varied. Mycoplasmas are well suited for cholesterol exchange studies. These organisms require cholesterol for growth.l As they are unable to synthesize cholesterol or to modify it, large amounts of exogenous cholesterol are being incorporated from the growth medium into the Mycopiasma cell membrane without any structural modification.2 The incorporated cholesterol undergoes translocation from the outer to the inner leaflet of the bilayer3 and can be exchanged with cholesterol from PC-cholesterol vesicles4 or with cholesterol from serum lipoproteins.5 The kinetics of the choletserol exchange exhibited a biphasic kinetic curve. Based on the assumption that the transbilayer cholesterol movement is relatively slow, the biphasic curve was interpreted to represent cholesterol pools asymmetrically located in the outer and inner leaflets of the lipid bilayer.4,6 The rapidly exchangeable pool was taken to represent the removal of cholesterol from the outer leaflet whereas the slow exchangeable pool was taken to represent less accessible cholesterol located in the inner leaflet. If this interpretation is valid one would expect to obtain a single kinetic exchange with membrane fragments having both leaflets exposed to the surrounding medium, yet a biphasic kinetic was obtained even with unsealed M. gallisepticum membrane fragments.4 Futhermore, rapid transbilayer movement of cholesterol was recently demonstrated in several biological membranes.7,8 In the present study we have attempted to further characterize the two kinetic pools of cholesterol and to assess the factors affecting their relative size and exchangeability. The fact that M. gallisepticum cells synthesize mainly one phospholipid, PG,9 and are capable of incorporating large amounts of chemically defined PC molecules10 make this organism an attractive system with which to study the role of induced modifications in phospholipid content and composition on the cholesterol exchange kinetics.
KeywordsExchange Rate Ultrasonic Irradiation Lipid Vesicle Membrane Cholesterol Cholesterol Pool
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- 7.J. M. Becker and E. A. Dawidowicz, Transmembrane movement of cholesterol in small unilamellar vesicles detected by cholesterol oxidase, J. Biol. Chem. 256: 586 (1981).Google Scholar
- 11.S. Razin and S. Rottem, Techniques for the manipulation of mycoplasma membranes, in: “Biochemical Analysis of Membranes,” A.H. Maddy, ed., Chapman and Hall, London.Google Scholar
- 20.S. Clejan and R. Bittman, Decreases in rates of lipid exchange between Myeop lasma gallisepticum cells and unilamellar vesicles by the incorporation of sphingomyelin, J. Biol. Chem. 259: 823 (1984).Google Scholar