Structural studies on apolipoprotein B: controllable heterogeneity of the complex formed with the surfactant, Triton...

Apolipoprotein B complexed with Triton X-100 (T-ApoB) has been isolated from human low density lipoprotein (LDL). Preparations are heterogeneous when analyzed by sedimentation velocity, with a major 12 S species and minor 17 S species present. The 12 S T-ApoB complex possesses a molecular weight of 880,000 containing 400,000 daltons of protein. Hydrodynamic measurements on this complex are consistent with a prolate ellipsoid model having an axial ratio of 13:1 and 0.22 g/g of bound water. Heterogeneity results from the irreversible aggregation of 12 S complexes into discrete 17 S and faster sedimenting components. A significant finding is that three determinants of this T-apoB heterogeneity could be elucidated and controlled. First, the initial state of aggregation is mainly influenced by the technique by which Triton and LDL are mixed. Second, once isolated, T-ApoB complexes slowly but spontaneously undergo further aggregation at 4 degrees C; the rate and extent of aggregation is enhanced remarkably with increasing temperature. Finally, reagents that unfold and expose protein structure (perchlorate, thiocyanate, and reducing reagents) lead to increased aggregation. The ability to control heterogeneity carries important implications for other studies concerning interactions of apoB with surfactants and lipids.