| Summary: | 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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