Differential scanning calorimetry and fluorescence probe investigations of very low density lipoprotein from the isolated perfused rat liver.

Isolated rat livers were perfused at 37 degrees C with blood-free, defined medium containing delipidized bovine serum albumin (BSA), BSA-oleate, or BSA-palmitate. Very low density lipoproteins (VLDL) were isolated from the perfusate at 12 degrees C and lipid components were extracted and purified. Differential scanning calorimetry indicated multiple phase alterations in intact VLDL. The extracted triglycerides exhibited phase alterations at similar temperatures as the intact VLDL. The small quantities of cholesteryl esters present in the VLDL generally did not greatly affect the VLDL triglyceride transitions. The extracted phospholipids showed detectable transitions that were abolished by cholesterol at mole ratios found in the respective VLDL. The phase behavior of VLDL and its component triglycerides was associated with the degree of unsaturation of the infusate fatty acid, and by variation of fatty acid chain length. The structural differences between VLDL lipid fractions noted by DSC were also monitored by fluorescence probes. The data indicated that in the intact VLDL the physical properties of the 'interior core' lipids can affect the properties of the 'surface monolayer'. In addition, Arrhenius plots of corrected fluorescence indicated that trans-parinarate and diphenyl-hexatriene detected different characteristic breakpoint temperatures in the phospholipids. The breakpoints of triglycerides were highly dependent on the type of fatty acid in the infusate, but were similar to those noted in intact VLDL. Finally, the breakpoints in Arrhenius plots of fluorescence probe parameters did not necessarily coincide with onset or end temperatures of DSC transitions.