Journal of Lipid Research, Vol 32, 1789-1794, Copyright © 1991 by Lipid Research, Inc.
In vivo and in vitro loading of lipid by artificially lipid-depleted lipophorins: evidence for the role of lipophorin as a reusable lipid shuttle
MC van Heusden, DJ van der Horst, JK Kawooya and JH Law
Department of Biochemistry, University of Arizona, Tucson 85721.
Lipid transport in the hemolymph of Manduca sexta is facilitated by a high
density lipophorin in the resting adult insect (HDLp-A, d approximately
1.109 g/ml) and by a low density lipophorin during flight (LDLp, d
approximately 1.060 g/ml). Lipophorin presumably shuttles different lipids
between sites of uptake or storage, and sites of utilization. In order to
shuttle lipid, a lipid-depleted lipophorin should be able to reload with
lipid. To test this hypothesis, we used HDLp-A particles that were
artificially depleted of either phospholipid (d approximately 1.118 g/ml)
or diacylglycerol (d approximately 1.187 g/ml) and subsequently
radiolabeled in their protein moiety. Upon injection into adult moths, both
particles shifted their density to that of native HDLp-A, indicating lipid
loading. Also, upon subsequent injection of adipokinetic hormone, both
particles shifted to a lower density (d approximately 1.060 g/ml)
indicating diacylglycerol loading and conversion to LDLp. Both phospholipid
and diacylglycerol loading were also studied using an in vitro system. The
lipid-depleted particles were incubated with fat body that had been
radiolabeled in either the phospholipid or the triacylglycerol fraction.
Transfer of radiolabeled phospholipid and diacylglycerol from fat body to
lipophorin was observed. During diacylglycerol loading, apoLp-III
associated with lipophorin, whereas phospholipid loading occurred in the
absence of apoLp-III. The results show the ability of lipid- depleted
lipophorins to reload with lipid and therefore reaffirm the role of
lipophorin as a reusable lipid shuttle.
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