Abstract
Amphipathic alpha-helices are the main structure and the major lipid binding motif of exchangeable apolipoproteins. To understand how these apolipoproteins behave at an hydrophobic lipoprotein interface, the interfacial properties of a consensus sequence peptide (CSP) derived from three exchangeable apolipoproteins (A-I, A-IV, and E) were studied using an oil drop tensiometer at air/water (A/W) and dodecane/water (DD/W) interfaces. CSP ((PLAEELRARLRAQLEELRERLG)2-NH2) contains two 22-amino acid tandem repeat sequences that form amphipathic alpha-helices. CSP, when added into the aqueous phase, lowered the interfacial tension (gamma) of A/W and DD/W in a concentration-dependent fashion. The gammaA/W was lowered approximately 24 mn/m, and gammaDD/W approximately 31 mn/m, indicating a greater affinity of CSP for DD/W. Using the Gibbs equation for surface, the surface area per CSP molecule was estimated at approximately 702 A2 ( approximately 16 A2/amino acid) on A/W and approximately 622 A2 on DD/W ( approximately 14 A2/amino acid) suggesting that adsorbed CSP lies flat with alpha-helices in the plane of both interfaces. At equilibrium gamma, CSP desorbed from the interface when compressed and re-adsorbed when expanded. The adsorption rate was concentration-dependent, but the desorption rate was not. Less CSP desorbed from DD/W than A/W indicating that CSP has higher affinity for DD/W. Dynamic analysis of elasticity shows that the faster the oscillation period (4, 8 s) and the lower the oscillation amplitude the more elastic the surfaces. CSP can be compressed 6-12% while remaining on the surface, but large increases in pressure eject it from the surface. We suggest that surface pressure-mediated desorption and readsorption of amphipathic alpha-helices provide lipoprotein stability during remodeling reactions in plasma.
Highlights
When the increase in surface pressure is plotted against the initial pressure ⌸i of the monolayer a straight line is generated that extrapolates to the pressure where the apolipoprotein can no longer penetrate into the monolayer, i.e. no increase in pressure is recorded
A hypothesis suggested by ⌸e studies is that if the interfacial pressure in the phospholipid monolayer on a lipoprotein surface is greater than ⌸e for a given apolipoprotein, the apoprotein cannot penetrate and bind to the lipoprotein
This hypothesis suggests that the surface pressure and the kinetics of adsorption/desorption from lipoprotein surfaces governs which soluble apolipoproteins will be present on the lipoprotein
Summary
Materials—The CSP peptide ((PLAEELRARLRAQLEELRERLG)2NH2), which contains two 22-amino acid tandem repeats derived from the consensus sequence of the tandem repeats of human apoA-I, apoAIV, and apoE [25], was synthesized at Quality Controlled Biochemical Inc. (Hopkinton, MA) using a Biosearch9050 Plus continuous flow synthesizer, and purified to 97ϩ%. Estimation of the Surface Area per Molecule of Peptide—From the interfacial tension measurement, the equilibrium interfacial tension ␥ was obtained for each concentration, c, of peptide in the aqueous phase. Compression and Expansion of the Interfaces—Once the interfacial tension curve approached equilibrium, the air bubble or oil drop was compressed by rapidly decreasing the volume by about 25, 12, or 6%. If molecules adsorb from the bulk phase to adhere to the newly formed extra surface, ␥ will drop back toward equilibrium. This is called the adsorption curve and results from the expansion of the interfacial area to produce new area to which molecules from the bulk can adsorb. The time constants t1 and t2 are related to the desorption and adsorption processes
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