α-Cyclodextrin extracts diacylglycerol from insect high density lipoproteins

Abstract

α-Cyclodextrins are water-soluble cyclic hexamers of glucose units with hydrophobic cavities capable of solubilizing lipophiles. Incubating α-cyclodextrin with high density lipophorin from Manduca sexta or Bombyx mori resulted in a cloudy, turbid solution. Centrifugation separated a pale yellowish precipitate. Thin-layer chromatography analysis of the lipid extract of the precipitate showed that the major lipid was diacylglycerol, while KBr density gradient analysis of the supernatant demonstrated the presence of a lipid-depleted very high density lipophorin. Transfer of diacylglycerol from lipophorin to cyclodextrin was specific to α-cyclodextrin and was not observed with β- or γ-cyclodextrins. pH had no effect on diacylglycerol transfer to α-cyclodextrin. However, the transfer was strongly dependent on the concentration of α-cyclodextrin and temperature. Increasing the concentration of α-cyclodextrin in the incubation mixture was associated with the formation of increasingly higher density lipophorins. Thus, at 20, 30, and 40 mM α-cyclodextrin, the density of B. mori lipophorin increased from 1.107 g/ml to 1.123, 1.148, and 1.181 g/ml, respectively. At concentrations greater than 40 mM, α-cyclodextrin had no further effect on the density of lipophorin. α-Cyclodextrin removed at most 83–87% of the diacylglycerol present in lipophorin. Temperature played an important role in altering the amount of diacylglycerols transferred to α-cyclodextrin. At 30 mM α-cyclodextrin, the amount of diacylglycerol transferred at different temperatures was 50% at 4°C, 41% at 15°C, 20% at 28°C, and less than 3% at 37°C.We propose that diacylglycerol transfers to α-cyclodextrin via an aqueous diffusion pathway and that the driving force for the transfer is the formation of an insoluble α-cyclodextrin–diacylglycerol complex.