In humans,
apolipoprotein E (
apoE) has three major
isoforms, E2 (Cys112, Cys158), E3 (Cys112, Arg158), and E4 (Arg112, Arg158). While epsilon4 is a genetic risk factor for
Alzheimer's disease (AD), epsilon2 may protect against late-onset AD. Using native preparations of
apoE from conditioned tissue
culture media or plasma
lipoproteins, we have previously shown that when equivalent amounts of
apoE3 or E4 were incubated with
beta-amyloid (A beta),
apoE3 formed 20 times as much SDS-stable complex with the
peptide as
apoE4. This preferential binding of A beta to
apoE3 was abolished when
apoE was purified by a process which includes delipidation and denaturation. Here we expand these observations to include A beta binding to
lipoprotein-associated and purified
apoE2.
Lipoproteins isolated from the plasma of individuals homozygous for either epsilon2 or epsilon3 were incubated with A beta(1-40). SDS-stable complex formation was analyzed by a non-reducing gel shift assay, followed by immunoblotting with either A beta or
apoE antibodies.
ApoE2:A beta complex formation was comparable to
apoE3:A beta in both native and purified preparations of
apoE. In addition,
lipoprotein-associated rat
apoE (Arg112, Arg158), like human
apoE4, did not form complex with A beta, while
lipoprotein-associated rabbit
apoE (Cys112, Arg158) did bind the
peptide. These binding studies provide one possible explanation for protective effects of both
apoE2 and E3 against the development of
Alzheimer's disease.