Comparison of lipid and protein content of serum very low density lipoproteins isolated by ultrascentrifugation in uniform density and in density gradient.

Very low density lipoproteins (VLDL) were isolated from 21 fasting sera with serum triglyceride concentrations ranging from about 1 to 60 mmol/1 (90–5300 mg/dl) by two different preparative ultracentrifugation procedures. One was centrifugation in a density gradient under conditions calculated to float all VLDL. The other was in a fixed angle rotor at a density of 1.006 g/cm³ (routine centrifugation). Lipids were extracted with chloroform/methanol for the gradient fractions and with isopropanol for the routine fractions.The triglyceride content was the same for VLDL obtained by the two methods while the gradient method yielded VLDL with a higher cholesterol content. The ratio of cholesterol to triglycerides was higher for gradient than for routine VLDL. This difference could only in part be due to the differences in techniques for lip id extractions. A strong linear correlation (r = 0.99) existed between the amount of lipids found in VLDL prepared by the two methods.The amounts of total protein as well as of protein soluble in tetramethylurea and in isopropanol were more than 30% higher in VLDL prepared by the routine preparation than with the gradient method. Isopropanol extracted about 20% more protein from both VLDL preparations than tetramethylurea.The calculated values for insoluble protein, assumed to represent apolipo-protein B, showed a very good agreement between VLDL obtained by gradient and routine centrifugations except for two sera with very high levels of insoluble protein.