Dimeric Nature of Apo-Low Density Lipoprotein Extracted with Triton X–100

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Abstract
Human low density lipoprotein (LDL) was dissolved in 0.3 to 2.0% Triton X-100 at pH 7.5 and apo-LDL (B protein) was extracted from LDL to form B protein-Triton complex. Sedimentation equilibrium study of this complex in a solvent nearly isopycnic to Triton X–100 showed that the molecular weight of the protein in the complex was 570, 000. The complex eluted almost at the void volume of a Sepharose 6B column, as would be expected for a complex with a total molecular weight of roughly 900, 000, on the assumption that 0.52 g of Triton was bound to 1 g of protein (Helenius, A. and Simons, K. (1972) J. Biol Chem. 247, 3656–3661). The sedimentation coefficient of the complex gave f/f;min =2.2, indicating that the complex was either as asymmetric as a fibrinogen molecule or not compact. These results show that B protein exists in its complex with Triton X–100 as an elongated or a loosely expanded dimer based on the molecular weight of monomeric B protein of 270, 000. B protein may also exist in LDL as a dimer.

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