The role of apolipoprotein VLDL-II in regulating the diameter of VLDLy

Abstract

The physiological function of each class of lipoproteins is dictated in part by the structural biology of the lipoprotein particle. Proteins embedded in the superficial phospholipid monolayer (apolipoproteins) facilitate the receptor mediated binding of lipoproteins, serve as activators of hydrolytic enzymes, and possess direct lipid transfer activities that allow for the lipid exchange of lipids. Mature hens assemble small diameter (range 25-35 nm) apolipoprotein-B (Apo-B) containing triacylglycerol-rich very low density lipoproteins (VLDLy) for yolk formation in response to estrogen. Assembly coincides with the expression of a critical protein, apolipoprotein VLDL-II (Apo-VLDL-II), which is aassociated with the ability of VLDLy responsible for its to resistance to intravascular hydrolipolysis, apolipoprotein VLDL-II (Apo-VLDL-II). Hens with non-optimally sized VLDL diameter distributions stop laying eggs indicating, a strong structure/function relationship within this class of triacylglycerol-rich lipoproteins. It has been hypothesized, but not proven, that Apo-VLDL-II, unique to avian species, is also responsible for regulating the size of the yolk-targeted lipoprotein particles. Two complementary techniques were employed in these studies, size exclusion chromatography and dynamic laser light scattering, were used to first separate lipoproteins VLDL into subfractions of different diameters and dynamic laser light scattering (DLLS) to then measure the actual diameters of those subfractionsseparated lipoproteins. The apoprotein content of the subfractions was determined using electrophoretic and immuno-chemical techniques in order to calculate the molar Apo-VLDL-II:Apo-B and determine its relationship to VLDLy diameter. Results showed that in normal laying hens aApo-VLDL-II:aApo-B did not differ significantly among size separated VLDLy whose diameters varied two-fold (48 to 20 nm). Hens fed diets supplemented withcollected after this study showed feeding corn oil and with cholesterol experienced increased plasmathe concentration, cholesterol content, and VLDL diameter of VLDL. Despite this increase, VLDLThe diameter distributions remained of lipoprotein populations within the range associated with VLDLy did, however, remain within the range positively associated with egg production. However, despite maintaining a physiologically optimal diameter distribution for egg production, cholesterolIn addition, feeding with cholesterol altered the relationship between the apolipoproteins associated with VLDLy as the Apo-VLDL-II:molar concentration of Apo-B increased disproportionately to Apo-VLDL-IIsignificantly decreased due to increased Apo-B content of VLDL. In cholesterol supplemented hens, Apo-VLDL-II:Apo-B was greatest The highest molar ratio of Apo-VLDL-II to Apo-B was observed in particles with a diameter within the range most closely associated with continued egg production, i.e. those fitting the functional definition of VLDLy. If Apo-VLDL-II controls VLDLy diameter, its degree of control is not stringent. There was a significant difference in the Apo-VLDL-II:Apo-B ratio in particles separated from hens fed a standard and lipid supplemented diet.