Effect of particle size and temperature on the conformation and physiological behavior of apolipoprotein E bound to model lipoprotein particles

Abstract

The effect of particle size and structural order/disorder of the lipid domains on the conformation and physiological behavior of lipid-associated apolipoprotein E (apoE) was evaluated. Circular dichroic (CD) spectra of apoE bound to large (LME) and small (SME) microemulsion particles, composed of dimyristoylphosphatidylcholine (DMPC) and cholesteryl oleate (CO), and to DMPC disks revealed that at 4.degree.C, where all of the lipid constituents were in an ordered state, apoE bound to LME displayed .apprx. 60% .alpha.-helicity, while apoE bound to SME and DMPC disks displayed 73% and 95% helicity, respectively. Over the temperature range 4-50.degree.C, encompassing the lipid thermal transitions, only apoE bound to LME demostrated an abrupt change in its CD spectrum (decrease in .alpha.-helicity) in response to temperature. To determine the sourceof the abrupt CD change, the constants for dissociation (Kd) of apoE from the surface of the large and small microemulsion particles were determined at 4, 25, and 37.degree.C. These results demonstrated that at 4.degree.C, the Kds for binding of apoE to the LME and SME were approximately equal; however, between 4 and 25.degree.C, there was a 5-fold increase in the Kd for binding of apoE to the LME, whereas the Kd for binding to the SME remained constant. The physiological effects of these differences in apoE secondary structure and equilibrium binding were examined by measuring the capacity of each apoE-containing particle to compete with LDL for binding to human fibroblasts, and by measuring the capacity of the apoE-microemulsion particles to suppress HMG-CoA reductase activity. This measurement revealed that all three model lipoproteins competed with 125I-LDL for receptor binding. Microemulsions alone were incapable of suppressing reductase activity; however, in the presence of apoE, the microemulsions were bound and internalized, and their cholesterol component suppressed HMG-CoA reductase activity. These studies indicate the dependence of apoE conformation and lipid binding constant on the size and temperature of the lipid particle and suggest the value of the DMPC/CO/apoE system for introducing exogenous materials into cells via the B/E receptor.