Journal of Lipid Research, Vol 28, 982-992, Copyright © 1987 by Lipid Research, Inc.
Enzymatically induced alterations in the structure of rat serum lipoproteins
JB Swaney, MW Orishimo and A Girard
Department of Biological Chemistry, Hahnemann University, Philadelphia, PA 19102.
Incubation of freshly isolated rat serum induces a large number of changes
in the properties of the serum lipoproteins, especially the high density
lipoproteins (HDL). The particle diameter of the HDL increases from about
10.4 nm to 12.3 nm and the protein content appears to increase by about
60,000 Daltons. Reactions catalyzed by lecithin:cholesterol acyltransferase
(LCAT) lead to a marked decrease in cholesterol and phospholipid content,
and an even greater increase in cholesteryl ester content. Especially
noteworthy are the marked increases in apoE and apoA-IV which are found
associated with HDL as a result of this process. Data indicate that the
affinity of apoE and apoA-IV for the HDL particles may be influenced by the
proportion of surface to core lipid and by the presence of products of the
LCAT reaction. Changes in the apoprotein content of very low density
lipoproteins are also observed, with A-I and A-IV appearing in this density
interval. All of the above changes can be prevented by the inclusion of
5,5'dithiobis(2-nitrobenzoate) or p- chloromercuriphenylsulfonate during
the incubation, or by heat treatment of serum at 56 degrees C for 30 min;
these treatments are known to inhibit LCAT activity. It is concluded that
LCAT action is the major cause of the various changes in HDL structure that
are observed and that alterations in apoprotein content occur to correct
the resultant imbalance between core lipid and coverage of this core by
amphiphilic components. Increased apoE association with cholesteryl
ester-rich HDL may provide an efficient means for receptor-mediated removal
of cholesterol from the circulation.
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