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Journal of Lipid Research, Vol 28, 693-703, Copyright © 1987 by Lipid Research, Inc.
Effect of lecithin:cholesterol acyltransferase on distribution of apolipoprotein A-IV among lipoproteins of human plasma
CL Bisgaier, OP Sachdev, ES Lee, KJ Williams, CB Blum and RM Glickman
The effect of cholesterol esterification on the distribution of apoA-IV in
human plasma was investigated. Human plasma was incubated in the presence
or absence of the lecithin:cholesterol acyltransferase (LCAT) inhibitor
5,5-dithiobis(2-nitrobenzoic acid) (DTNB) and immediately fractionated by
6% agarose column chromatography. Fractions were monitored for apoA-IV,
apoE, and apoA-I by radioimmunoassay (RIA). Incubation resulted in an
elevated plasma concentration of cholesteryl ester and in an altered
distribution of apoA-IV. After incubation apoA- IV eluted in the ordinarily
apoA-IV-poor fractions of plasma that contain small VLDL particles, LDL,
and HDL2. Inclusion of DTNB during the incubation resulted in some
enlargement of HDL; however, both cholesterol esterification and
lipoprotein binding of apoA-IV were inhibited. Addition of DTNB to plasma
after incubation and prior to gel filtration had no effect on the apoA-IV
distribution when the lipoproteins were immediately fractionated. Fasting
plasma apoE was distributed in two or three peaks; in some plasmas there
was a small peak that eluted with the column void volume, and, in all
plasmas, there were larger peaks that eluted with the VLDL-LDL region and
HDL2. Incubation resulted in displacement of HDL apoE to larger
lipoproteins and this effect was observed in the presence or absence of
DTNB. ApoA-I was distributed in a single broad peak that eluted in the
region of HDL and the gel-filtered distribution was unaffected by
incubation either in the presence or absence of DTNB. Incubation of plasma
that was previously heated to 56 degrees C to inactivate LCAT resulted in
no additional movement of apoA-IV onto lipoproteins, unless purified LCAT
was present during incubation. The addition of heat-inactivated LCAT to the
incubation, had no effect on movement of apoA-IV. These data suggest that
human apoA-IV redistribution from the lipoprotein-free fraction to
lipoprotein particles appears to be dependent on LCAT action. The mechanism
responsible for the increased binding of apoA-IV to the surface of
lipoproteins when LCAT acts may involve the generation of "gaps" in the
lipoprotein surface due to the consumption of substrate from the surface
and additional enlargement of the core. ApoA-IV may bind to these "gaps,"
where the packing density of the phospholipid head groups is reduced.

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Copyright © 1987 by the American Society for Biochemistry and Molecular Biology.
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