Effects of Lipophilin on Dimyristoylphospha-tidycholine Bilayers

Abstract

<p>In this study the technique of scanning densimetry was used to investigate the effects of lipophilin, an integral myelin protein, on the packing and phase transitional properties of dimyristoylphosphatidylcholine bilayers. The technique was shown to give both precise and accurate measurement of the volume properties of the system.</p> <p>The phase transition of phospholipids from gel to liquid crystalline state is accompanied by a 3% increase in volume. Lipophilin does not significantly alter the phase transition temperature of 24°C of the pure lipid. However, the phase transition is broadened when the lipophilin concentration is greater than 10% by weight. This can be explained by a decrease in the cooperativity of the lipid transition induce by the protein.</p> <p>As the percent lipophilin incorporated into the lipid is increased, there is a reduction in the magnitude of the change in partial specific volume which occurs at phase transition temperature. From this data, the number of lipid molecules surrounding each lipophilin molecule was calculated. The calculated value of 19 lipid molecules per molecule of protein agrees well with estimates based on calorimetric results.</p> <p>The partial specific volume of lipophilin has never been reported. In this study it was determined in a number of solvents at 20°C. The partial specific volume of lipophilin in water was 0.715 ± 0.004 ml/g while the partial specific volume in the buffer system employed was 0.732 ± 0.003 ml/g. These results are explained in terms of differences in the solvation of the protein.</p> <p>Assuming no large change in the partial specific volume of the protein upon incorporation into lipid, the results demonstrate that lipophilin causes a reduction in the volume of dimyristoylphosphatidyl-choline bilayers below, above and near the phase transition temperature. This may be a fundamental property of boundary lipid that surrounds protein.</p>