Structural studies on apolipoprotein B : controllable heterogeneity of the complex formed with the surfactant , Triton X 4 0 0 Bruce

Apolipoprotein B complexed with Triton X-100 (TApoB) 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 4OC; 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.-Pattcreon, B. W., L. L. Kilgore, P. W. Chun, and W. R. Fisher. Structural studies on apolipoprotein B: controllable heterogeneity of the complex formed with the surfactant, Triton X-100. J. Lipid Res. 1984. 2 5 763-769. Supplementary key woda low density lipoproteins Apolipoprotein B (apoB), the structural protein of plasma low density lipoprotein, plays a crucial biological role in intercellular transport and elivery of cholesterol and in the pathogenesis of atherosclerosis (1). A detailed knowledge of apoB/surfactant interactions is a requisite for many studies involving immunologic structural probes (2), proteolytic digestion (3), and reconstitution with natural lipids (4-6). Tanford et al. (7) and Reynolds and Tanford (8) have stressed the importance of characterizing protein/detergent complexes, for proteins which natively associate strongly with lipids, in order to determine protein molecular weight and native states of aggregation. A number of laboratories have examined the interaction of apoB with a variety of synthetic and natural surfactants including sodium dodecyl sulfate (SDS) (9ll),TritonX-100(10,12-14),Tween80(15),ndodecyl octaethylene glycol monoether (C12EB) (16), sodium deoxycholate (10, 12), and cetyltrimethylammonium bromide (1 0). In this report, we describe conditions that promote the aggregation of Triton/apoB (T-ApoB) complexes. The ability to reduce or eliminate such undesirable aggregation is crucial when studying the physical, biologic, or immunologic properties of apoB/surfactant or apoB/