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J Biol Chem, Vol. 274, Issue 51, 36409-36414, December 17, 1999
Novel Physiological Function of Sphingomyelin in Plasma
INHIBITION OF LIPID PEROXIDATION IN LOW DENSITY
LIPOPROTEINS*
Papasani V.
Subbaiah ,
Veedamali S.
Subramanian, and
Kewei
Wang
From the Departments of Medicine and Biochemistry, Rush
Medical College, Chicago, Illinois 60612
Although sphingomyelin (SPH) is a major
constituent of all lipoproteins, its physiological function in plasma
is not known. In this study, we tested the hypothesis that SPH inhibits
lipid peroxidation in low density lipoproteins (LDL) because of its effects on surface fluidity and packing density and that the relative resistance of the buoyant LDL to oxidation, compared with the dense
LDL, is partly due to their higher SPH content. Depletion of SPH by
treatment with SPHase resulted in shortened lag times and increased
rates of oxidation in both LDL subfractions, as measured by the
conjugated diene formation in the presence of Cu2+.
Oxidation of LDL by soybean lipoxygenase was similarly stimulated by
the degradation of SPH. Oxidation-induced fluorescence decay of
diphenylhexatriene-labeled phosphatidylcholine (PC), equilibrated with
LDL-PC, was accelerated significantly by the enzymatic depletion of SPH
from the lipoprotein. Oxidation of 16:0-18:2 PC in the proteoliposomes
was inhibited progressively by the incorporation of increasing amounts
of egg SPH into the liposomes. Treatment of SPH-containing
proteoliposomes with SPHase reversed the effect of SPH, showing that
the presence of intact SPH is necessary for the inhibition of
oxidation. Although the incorporation of SPH into the same liposome as
the PC (intrinsic SPH) protected the PC against oxidation, the addition
of SPH liposomes to PC liposomes (extrinsic SPH) was not effective.
Oxidation of 16:0-18:2 PC in liposomes was also inhibited by the
incorporation of dipalmitoyl-PC, but not by free cholesterol. These
results suggest that SPH acts as a physiological inhibitor of
lipoprotein oxidation, possibly by modifying the fluidity of the
phospholipid monolayer and thereby inhibiting the lateral propagation
of the lipid peroxy radicals.
*
This research was supported by National Institutes of Health
Grant HL-52597 and by funds from the Max Baer Heart Fund of the Fraternal Order of Eagles.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Section of
Endocrinology and Metabolism, Rush Medical College, 1653 W. Congress Parkway, Chicago, IL 60612. Tel.: 312-455-2439; Fax: 312-455-9814; E-mail: psubbaia@rush.edu.
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.

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Copyright © 1999 by the American Society for Biochemistry and Molecular Biology.
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