FLUORESCENCE DEPOLARIZATION STUDIES OF FLUIDITY AND PHASE-TRANSITION IN HUMAN APOPROTEIN.PHOSPHOLIPID COMPLEXES

The microviscosity of unilamellar vesicles of dimyristoyl‐3‐sn‐phosphatidylcholine and that of phosphatidylcholine · apoprotein complexes was followed by fluorescence depolarization after labeling with 1,6‐diphenyl‐1,3,5‐hexatriene. The transition temperature from gel‐crystalline to liquid‐crystalline phase is 24°C for the dimyristoyl‐phosphatidylcholine vesicles and is shifted to around 30°C in the complexes between phosphatidylcholine and apoA‐I, apoA‐II, apoC‐I, apoC‐III proteins while the cooperativity of the transition is decreased. At temperatures below the transition of the phospholipid, the microviscosity of the complexes of phosphatidylcholine with apoA‐I, apoA‐II and apoC‐I proteins is lower than that of the phosphatidylcholine, while the opposite effect is observed above 30°C. The phosphatidylcholine · apoprotein complexes isolated on a Sepharose 6B column have a molecular weight around 100000 and a phosphatidylcholine/apoprotein ratio of 2–2.6 (w/w). The microviscosity measurements at 35°C performed after elution of the column enable the complex to be detected. The size and microviscosity of the apoprotein · phosphatidylcholine complex is compatible with a model where the vesicular structure has disappeared and the amino acid side chains present hydrophobic interaction with the phosphatidylcholine acyl chains. Copyright © 1979, Wiley Blackwell. All rights reserved