Lipid-apolipoprotein interactions in surfactant studied by reassembly

Area of Science:

• Biochemistry
• Pulmonary Medicine
• Extracellular Matrix Biology

Background:

• Tubular myelin, a form of pulmonary surfactant, is crucial for lung function.
• Its structure depends on interactions between lipids, specific proteins (apolipoproteins), and calcium ions.

Purpose of the Study:

• Investigate how apolipoproteins and calcium influence surfactant complex composition and properties.
• Determine if apolipoproteins preferentially associate with specific lipids.
• Identify the forces involved in apolipoprotein-lipid binding.
• Assess changes in lipid physical state upon interaction.

Main Methods:

• Reassembly of surfactant materials using a major apolipoprotein and principal lipids.
• Studying binding affinities and interactions under varying conditions.
• Thermodynamic analyses and differential scanning calorimetry.

Main Results:

• Apolipoprotein shows higher affinity for gel-phase phosphatidylcholines (PCs) than liquid-crystalline PCs.
• Maximum binding occurs with PC/phosphatidylglycerol (PG) mixtures (15% PG).
• Calcium ions do not affect apolipoprotein binding but alter reassembly complex physical state and stoichiometry.
• Binding appears to involve apolar interactions, influenced by phospholipid lamellar state.

Conclusions:

• Apolipoprotein-lipid binding is not primarily ionic; apolar interactions are significant.
• Phosphatidylglycerol and glycolipids may facilitate binding by disrupting lamellar structure.
• Phosphatidylcholines may become partially immobilized around the apolipoprotein in the complex.