Fluorescence depolarization studies and phase transition in human apoprotein . phospholipid complexes

Area of Science:

• Biochemistry
• Biophysics
• Lipid-protein interactions

Background:

• Unilamellar vesicles composed of dimyristoyl-3-sn-phosphatidylcholine (DMPC) are model systems for studying membrane properties.
• Apolipoproteins are key components of lipoproteins involved in lipid transport and metabolism.

Purpose of the Study:

• To investigate the effect of apolipoprotein complexation on the microviscosity and phase transition of DMPC vesicles.
• To characterize the biophysical properties of phosphatidylcholine-apoprotein complexes.

Main Methods:

• Fluorescence depolarization using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe.
• Differential scanning calorimetry to determine phase transition temperatures.
• Sepharose 6B gel filtration chromatography for complex isolation and molecular weight determination.

Main Results:

• Complexation of DMPC with apolipoproteins (apoA-I, apoA-II, apoC-I, apoC-III) shifted the gel-to-liquid crystalline phase transition temperature from 24°C to approximately 30°C, decreasing transition cooperativity.
• Below the transition temperature, microviscosity of DMPC-apolipoprotein complexes was lower than DMPC alone; above 30°C, it was higher.
• Isolated complexes had a molecular weight of ~100,000 Da and a DMPC/apolipoprotein ratio of 2-2.6 (w/w).

Conclusions:

• Apolipoprotein binding alters the physical properties of DMPC vesicles, including microviscosity and phase transition behavior.
• The results support a model where vesicular structure is lost upon complexation, with hydrophobic interactions between apolipoprotein side chains and DMPC acyl chains being significant.
• These findings provide insights into the structural organization of lipid-apolipoprotein complexes relevant to lipoprotein structure and function.