Effect of diacylglycerol content of some physicochemical properties of the insect lipoprotein, lipophorin, Correlation with the binding of apolipophorin-III

Diacylglycerol (DG) is the main lipid component of the insect lipoprotein lipophorin. In order to study the effect of DG content on the structure and properties of lipophorin and to analyze the role of DG in the binding of apolipophorin-III (apoLp-III), an exchangeable apolipoprotein,we developed a method that allows the modification of the DG content of lipoproteins. This method employs sn-1,2-dioctanoyl glycerol (diC8-DG). The degree of incorporation of diC8-DG was determined by including [14C]-diC8DG in the incubation and subsequent purification of the diC8-DG-loaded lipophorins in a KBr gradient. The efficiency of diC8-DG loading of lipophorin is time dependent, but high levels of loading are obtained in relatively short periods of time (100% in 3-4 h). For DG loading up to about 15% (w/w), the efficiency and rate of diC8-DG loading are independent of the presence of apoLp-III. DG loading above 15% (w/w) in the absence of apoLp-III resulted in aggregation of the particles. Lipophorin particles enriched up to about 30% with diC8-DG were obtained by this procedure. When lipophorin particles were loaded with diC8-DG in the presence of apoLp-III, it was observed that binding of apoLp-III was proportional to the amount of diC8-DG incorporated into the lipophorin particle, indicating that the only requirement for apoLp-III binding to lipophorin is an increased DG content. A decrease in the degree of order of the lipid phase of the lipoproteins was observed by anisotropy of fluorescence of diphenylhexatriene as the content of diC8-DG was increased. A comparison of the lipid order of artificially loaded lipophorins, in the presence and absence of apoLp-III, with the lipid order of natural lipophorins, which differ in their DG content, and mammalian lipoproteins clearly showed that DG has a extraordinary perturbing effect on the lipid order of lipoproteins. It is suggested that this lipid-disordering effect of DG plays a prominent role in reorganizing the lipoprotein surface in such a manner as to permit apoLp-III to bind.