The interfacial properties of ApoA-I and an amphipathic α-helix consensus peptide of exchangeable apolipoproteins at the triolein/water interface

Abstract

Apolipoprotein A-I (apoA-I) is the major protein in high density lipoprotein (HDL). During lipid metabolism, apoA-I moves among HDL and triacylglycerol-rich lipoproteins. The main structure and the major lipid binding motif of apoA-I is the amphipathic α-helix. To understand how apoA-I behaves at hydrophobic lipoprotein interfaces, the interfacial properties of apoA-I and an amphipathic α-helical consensus sequence peptide (CSP) were studied at the triolein/water (TO/W) interface. CSP ((PLAEELRARLRAQLEELRERLG)2-NH2) contains two 22-residue tandem repeat sequences that form amphipathic α-helices modeling the central part of apoA-I. ApoA-I or CSP added into the aqueous phase surrounding a triolein drop lowered the interfacial tension (γ) of TO/W in a concentration- and time-dependent fashion. The γTO/W was lowered ∼16 millinewtons (mN)/m by apoA-I at 1.4 × 10-6 M and ∼15 mN/m by CSP at 2.6 × 10-6 M. At equilibrium γ, both apoA-I and CSP desorbed from the interface when compressed and readsorbed when expanded. The maximum surface pressure CSP could withstand without being ejected (ΠMAX) was 16 mN/m. The ΠMAX of apoA-I was only 14.8 mN/m, but re-adsorption kinetics suggested that only part of the apoA-I desorbed at Π between 14.8 and 19 mN/m. However, above ∼19 mN/m (ΠOFF) the entire apoA-I molecule desorbed into the water. ApoA-I was more flexible at the TO/W interface than CSP and showed more elasticity at oscillation periods 4-128 s even at high compression, whereas CSP was elastic only at faster periods (4 and 8 s) and moderate compression. Flexibility and surface pressure-mediated desorption and re-adsorption of apoA-I probably provides lipoprotein stability during metabolic-remodeling reactions in plasma.