Properties of human apolipoprotein A-I at the air-water inferface

Abstract

In order to study the properties and structure of human apolipoprotein A-I (apo-A-I) at the air-water interface, we formed monolayers of apo-A-I by either spreading or spontaneous adsorption from the subphase. The rate of monolayer formation, force-area (II-A) curves, and surface potentials were analyzed quantitatively. We constructed force-area curves by keeping constant either (a) the total area or (b) the total number of molecules. The results obtained by method a were reproducible and superimposable with those obtained from adsorbed monolayers. In turn, the results obtained by method b indicated aging effects by both pressure and potential measurements, suggesting irreversible denaturation of apo-A-I at areas larger than 50 A2/amino acid. At pressures lower than 0.5 dyn/cm, apo-A-I monolayers followed the two-dimensional ideal gas law; the molecular weight of the apoprotein calculated from the intercept on the ordinate of the IL4 vs. plot had an average value of 25 000. At higher pressures, the monolayer deviated from ideality, and the Recent physical and chemical studies on both high-density lipoproteins (HDL) and low-density lipoproteins (LDL) have suggested that apoproteins, cholesterol, and phospholipids form a monolayer at the outer surface of these lipoproteins (Morrisett et al., 1975). The enzymatic proteolysis of the intact lipoproteins (Aggerbeck et Camejo, 1969;Pattnaik et al., 1976) results in aggregation, indicating that the apoproteins play an important role in maintaining the lipoprotein structure. Such stabilization should be intimately related to the surface properties of the apoproteins. In order to explore the behavior of apolipoproteins at amphiphilic interfaces and their relation to the surface organization of human serum HDL3, we turned our attention to apolipoprotein A-I (apo-A-I), which is the major protein component of plasma HDL.

HDL: bridging past and present with a look at the future

“…From predictions of its secondary structure based on the knowledge of the amino acid sequence (13, 14), apo A‐I consists of amphipathic helices of repeating 11 or 22 amino acids separated by proline residues (15). Experimental data at the air‐water interface (16–19) have shown unambiguously the high surface activity of apoA‐I and its dependence on its amphipathic properties (20) also documented by spectroscopic studies (21). Denaturation and ultracentrifugal studies indicated that apoA‐I is asymmetric (22) and flexible with a tendency to self‐associate into dimers, tetramers, and octamers (23).…”

Interfacial Properties of an Amphipathic α-Helix Consensus Peptide of Exchangeable Apolipoproteins at Air/Water and Oil/Water Interfaces

“…Weinberg et al (21) studied the adsorption of 10 g/ml solutions of apoA-I and A-IV to triolein droplets using a drop volume technique. By expressing the droplets at different rates they were able to show that apoA-I approached an equilibrium interfacial value of about 16 interfacial tension to about 20 mN/m. Whereas these studies show that apoA-I and apoA-IV adsorb to a triolein/water interface and lower interfacial tension they do not give information on the effects of protein concentration, the rates of adsorption or desorption, or the viscoelastic behavior of these proteins at an oil/water interface.…”

Dynamic interfacial properties of human apolipoproteins A-IV and B-17 at the air/water and oil/water interface

“…6) and yielded a calculated interfacial molecular weight of 45,316 with a limiting area of 24.4 Å/residue. These data indicate that despite its high affinity self-association in solution (51), like other apolipoproteins (48,52), apoA-IV is monomeric at the interface.…”

“…A-⌸ isotherms were calculated from the barrier displacement and the mass of protein spread on the sur face; inflection points were identified from second derivatives of the curves. The interfacial molecular weight of apoA-IV was calculated by analyzing the low pressure region of it's a-⌸ isotherm with the ideal gas equation, ⌸ A ϭ ⌸ A 0 ϩ nRT, where A is total surface area and A 0 is the limiting area occupied by n moles of protein (48). Thus the intercept of a plot of ⌸иA versus ⌸ yields the interfacial molecular weight, and the slope yields the limiting residue area.…”

“…Analysis of the low pressure portion (Ͻ0.5 mN/m) of apoA-IV compression isotherm on buffer using the ideal gas equation. The intercept of the plot of ⌸иA versus ⌸ yields the interfacial molecular weight; the slope yields the limiting residue area(48).…”