Journal of Lipid Research, Vol 36, 2344-2354, Copyright © 1995 by Lipid Research, Inc.
Role of lecithin:cholesterol acyltransferase and apolipoprotein A-I in cholesterol esterification in lipoprotein-X in vitro
K O and J Frohlich
Department of Pathology, University of British Columbia, Vancouver, Canada.
Lipoprotein-X (Lp-X) is an abnormal particle present in the plasma of
patients with familial lecithin:cholesterol acyltransferase (LCAT)
deficiency syndromes or cholestatic liver disease. Compared to other
lipoproteins, Lp-X contains a high content of unesterified cholesterol
(30%, w/w) to phosphatidylcholine (60%, w/w). The objective of this study
was to evaluate the role of LCAT and apolipoprotein A-I (apoA-I) in Lp-X
metabolism in vitro and to elucidate the regulation of cholesterol
esterification in this unique lipoprotein. Lp-X isolated from sera of
patients with obstructive jaundice had a high content of unesterified
cholesterol and phosphatidylcholine and contained apolipoprotein E, apoCs,
and albumin. Although human recombinant LCAT used as an enzyme source did
bind to isolated Lp-X, no cholesterol esterification was detected. However,
addition of human apoA-I in the presence of albumin resulted in significant
cholesterol esterification in Lp-X (Vmax 0.25 +/- 0.04 nmol/h per microgram
LCAT protein). Exogenous apoA-I did not change the size of Lp-X particle as
determined by quasi-elastic light scattering analysis. A reduction in Lp-X
size was observed when both apoA-I and LCAT were included in the reaction
mixture (from 47 nm to 42 nm). Furthermore, addition of apoA-I (but not
HDL) dramatically changed the electrophoretic mobility of Lp-X from
cathodic to anodic migration. Such changes are not due to displacement of
apoC or apoE proteins from Lp-X by apoA-I. While increasing apoA-I
concentration (up to 35 micrograms/ml) in the reaction mixture stimulated
cholesterol esterification in Lp-X, addition of apoA-I at the concentration
of 8 micrograms/ml inhibited cholesterol esterification in VLDL, LDL, and
HDL by more than 50%. Albumin was required for the LCAT reaction to Lp-X.
Our results suggest that while LCAT binds to isolated Lp-X, apoA-I is
needed for the LCAT reaction to proceed. The presence of apoA-I does not
result in the displacement of apoCs and apoE from Lp-X and addition of
apoA-I changes the electrophoretic mobility but not the size of Lp-X.
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