Lipid-Apolipoprotein Interactions in Surfactant Studied by Reassembly

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Abstract
Recent evidence suggests that the structure of tubular myelin, an extracellular form of pulmonary surfactant, is dependent on the interaction of lipids with certain proteins specific for this material and with calcium ions. In order to investigate how protein and calcium may affect the surfactant complex, we studied the composition and properties of reassembly materials formed with a major surfactant apolipoprotein (35,000-38,000 molecular weight) and the principal lipids found in the natural material. We were interested in three questions: 1) Does this apolipoprotein preferentially associate with certain of the lipids in surfactant? 2) What forces are involved in the binding? 3) Does the interaction result in changes in the physical state of the lipid? We found that this apolipoprotein binds phosphatidylcholines that are in a gel phase with much greater affinity than it does phosphatidylcholines that are liquid-crystalline. However, maximum binding does not occur with the pure phosphatidylcholines but rather with mixtures of phosphatidylcholines and 15% phosphatidylglycerol. Calcium ions have no effect on the amount of apolipoprotein that is bound, but they do modify the physical state of the reassembly lipoprotein and the stoichiometry of lipid to protein. These results indicate that the binding of the apolipoprotein to the lipid does not primarily involve ionic bonds. However, apolar interactions, which are influenced by the state of the lamellar phospholipid, appear to be important. Small amounts of phosphatidylglycerol and other glycolipids, which probably disrupt the regularity of a gel-phase lamellar structure when mixed with saturated phosphatidylcholines, may provide binding sites favoring the interaction. Indirect evidence, based on thermodynamic analyses, suggests that phosphatidylcholines may be partially immobilized about the protein in the formation of the complex. This conclusions is reinforced by the preliminary findings obtained from the differential scanning calorimetry of the reassembly materials.