Organization of the core lipids of high density lipoproteins in the lactating bovine.

During lactation, cows develop greatly increased levels of plasma high density lipoproteins (HDL), including a population of particles of larger size than normal human plasma HDL. To study the relationship between lipoprotein size and organization of their core lipids, HDL from lactating Jersey and Holstein cows were fractionated by isopycnic density gradient ultracentrifugation, and individual fractions between densities 1.06 and 1.13 g/ml were examined by differential scanning calorimetry and negative stain electron microscopy. With increasing density, particles decreased in mean diameter from 16.0 to 9.0 nm. Scanning calorimetry of HDL of diameters 12.0 to 16.0 nm showed a reversible thermal transition between 20 and 35”C, resembling the liquid crystalline transitions of cholesterol esters in the lipid extract of bovine HDL. The enthalpy of this transition was markedly reduced compared to pure cholesterol esters or to the cholesterol ester transitions described previously in larger cholesterol ester-rich lipoproteins. Scanning calorimetry of bovine HDL of diameter less than 12.0 nm showed no thermal transitions between 0 and 60°C. However, the disappearance of the low temperature cholesterol ester transition was associated with the appearance at high temperatures (94 to 113°C) of a distinct second component of the lipoprotein denaturation endotherm. This component represented a constant entropy change of cholesterol ester (0.0032 cal/g/”C) which was similar to that of the smectic-liquid transition of pure cholesterol esters. Thus, as HDL particles decrease in diameter from 16.0 to 12.0 nm, their core probably becomes too small to allow melting of liquid crystalline cholesterol esters. The high temperature component of the thermal denaturation of small HDL particles may be due to disordering of cholesterol esters that becomes possible secondary to lipoprotein disruption.