Properties of two pig low density lipoproteins prepared by zonal ultracentrifugation

Abstract

Pig plasma lipoproteins were separated into four density classes (very low density, two low density and high density lipoproteins, VLDL, LDL 1, LDL 2 and HDL respectively) from 670 ml plasma by ultracentrifugation in a continuous density gradient using the Spinco Til15 zonal rotor. LDL 1 and LDL 2 were partly characterised. LDL 1 and LDL 2 are β-migrating lipoproteins of different size and hydrated density; they are similar to human LDL 2 and LDL 3 respectively. Pig plasma contains about twice as much LDL 1 as LDL 2. LDL 1 migrates at S f 4.9 (modal value), and has a mean diameter of 217 Å and a modal density of 1.035 g/ml (range 1.03 – 1.04 g/ml). LDL 2 migrates at S f 1.8 and has a mean diameter of 195 Å and a density of 1.050 g/ml. Both lipoproteins are precipitated by heparin and Mn ++ or by dextran sulphate and Ca ++. The apoproteins of LDL1 and LDL2 are both largely insoluble in 8 M urea solution. When dissolved in 1% sodium dodecyl sulphate solution and electrophoresed on polyacrylamide gel at pH 7.0, the apoproteins of LDL 1 and LDL 2 formed a pattern of multiple bands of high molecular weight similar to that obtained from the apoprotein of human LDL. Both LDL 1 and LDL 2 share a major antigen with each other and with VLDL; in this respect again they resemble human LDL. The amino acid compositions of LDL 1 and LDL 2 are very similar. We concluded that the apoprotein moieties of pig plasma LDL 1 and LDL 2 are probably identical, and similar to apoprotein B in human serum. Zonal ultracentifugation has proved to be a rapid and effective method for isolating large quantities of these two lipoprotein classes for further metabolic studies. This method allows rapid bulk preparation of lipoproteins, and provides a record of their distribution and quantity in a continuous density gradient.