Design Of A New Class Of Amphipathic Helical Peptides For The Plasma Apolipoproteins That Promote Cellular Cholesterol Efflux But Do Not Activate Lcat

Amphipathic helical peptides represent the lipid-binding units of the soluble
plasma apolipoproteins. Several synthetic peptide analogues have been designed to
mimic such structures and have been used to unravel some of the mechanisms
involved in the physiological function of the apolipoproteins, including lipid
binding, LCAT activation, and enhancement of cholesterol efflux from lipid-laden
cells. A series of novel synthetic peptides, named ID peptides, was modeled on
the basis of the structural properties common to the amphipathic helices of
apolipoprotein (apo) A-I. In these new peptides, however, the segregation between
hydrophobic and hydrophilic faces of the helices is more pronounced than in
apoA-I, so that the surface of the hydrophobic and hydrophilic faces of the
amphipathic helices is equal. Moreover, there are fewer negatively charged
residues in the center of the hydrophilic face of the helical peptides. Most
charged amino acids are located along the edge of the helix and are susceptible
to forming salt bridges with residues of an antiparallel helix, such as around a
discoidal phospholipid/peptide complex. The physicochemical characteristics of
these peptides and their complexes with phospholipids were compared with those of
the 18A peptide and its lipid/peptide complex. All ID peptides bind
dimyristoylphosphatidylcholine vesicles more rapidly than the 18A peptide to
yield discoidal peptide/phospholipid complexes of comparable size. The
alpha-helical content of the lipid-free ID peptides is close to that of the 18A
peptide and increases slightly on lipid binding. The stability of the ID and 18A
peptides and of the phospholipid/peptide complexes against guanidinium
hydrochloride denaturation is higher than that of lipid-free and lipid-bound
apoA-I. LCAT activation by the 18A/phospholipid/cholesterol complexes equals that
of apoA-I/ phospholipid/cholesterol complexes, whereas none of the ID peptides
tested is able to activate LCAT to a significant extent. Incubation of the
peptide/phospholipid complexes with lipid-laden macrophages induces cellular
cholesterol efflux and incorporation of cholesterol into the complexes. The
cholesterol efflux capacity of the peptide/phospholipid complexes is comparable
among the peptides and higher than that of apoprotein/phospholipid complexes. In
conclusion, although the amphipathicity of the new peptides is higher than that
of the 18A model peptide, the lack of LCAT activation by the ID peptides suggests
that an enhanced segregation of the hydrophobic and hydrophilic residues, equal
magnitude of hydrophobic and hydrophilic faces of the helix, and the absence of
negatively charged residues in the central part of the hydrophilic face might
account for the lack of LCAT activity of these peptides. These parameters do not
affect the capacity of the peptide/phospholipid complexes to promote cellular
cholesterol efflux.