The structure and morphology of the abnormal serum lipoprotein-X

The structure and morphology of an abnormal lipoprotein particle present in the serum of patients with obstructive jaundice has been investigated by gel filtration, electron microscopy and NMR spectroscopy. Lipoprotein-X is a spherical lipoprotein particle with an average Stokes diameter of approximately 40 nm and a wide size distribution ranging from 20 to 70 nm. Different from all lipoprotein structures known so far lipoprotein-X is a hollow particle (= vesicle) with a water-filled internal cavity surrounded by a continuous, single bilayer which is impermeable to cations and K3Fe(CN)6. The packing of the bilayer is tighter and the segmental motion of both the polar group and the hydrocarbon chains are significantly reduced relative to typical phosphatidylcholine bilayers. In terms of segmental motion and anisotropy of packing the lipoprotein-X bilayer closely resembles a model bilayer system consisting of phosphatidylcholine, lysophosphatidylcholine, sphingomyelin and cholesterol mixed in the same molar ratio as in lipoprotein-X. In that model system the phospholipid distribution between the two layers of the bilayer is asymmetric with (sphingomyelin + lysophosphatidylcholine) being preferentially located on the inner layer and phosphatidylcholine preferentially on the outer layer of the bilayer. By analogy with the model system the phospholipid distribution in lipoprotein-X bilayers is proposed to be also asymmetric.