Dioxin alters the human low-density and very low-density lipoprotein structure with evidence for specific quenching of Trp-48 in apolipoprotein C-II

The intercellular transport of cholesterol and triglycerides via lipoproteins interacting with their receptors is a critical component in human lipid metabolism. The delivery of cholesterol to cells is accomplished primarily through low-density lipoproteins (LDLs), while the transport of fatty acids to adipose and muscle tissue is accomplished primarily through the actions of very low-density lipoproteins (VLDLs). Disruption of lipoprotein structure leading to impaired binding between these lipoproteins and their obligate receptors is a known risk factor for cardiovascular disease. Because of recent investigations linking 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in humans with coronary artery disease, investigations have been carried out by fluorescence and circular dichroism to evaluate conformational changes in LDL and VLDL structure upon binding of TCDD. These studies demonstrate that, at a molar ratio of three TCDD molecules to one lipoprotein molecule, TCDD binds and disrupts the secondary and tertiary lipoprotein structure. Circular dichroism studies show that residues within the inner core of apoC-II, which compose a four-alpha-helix bundle when this apolipoprotein is associated with VLDL, are directly affected upon binding TCDD. Fluorescence also indicates the specific interaction of Trp-48 within apoC-II upon TCDD binding. We found that the TCDD/apoC-II complex suffers a 5-fold reduction in its ability to bind lipoprotein lipase compared to untreated apoC-II. The interaction of TCDD with LDL markedly altered the secondary structure of apoB reducing its alpha-helical content. These cumulative responses in lipoprotein structure may impair the LDL and VLDL cellular uptake leading to a buildup of serum lipoproteins and fats thus hastening the development of coronary artery disease.