The yolk granules from hen's egg represent on a dry basis 23% of the yolk solids, and they contain about 90% of the protein phosphorus, 95% of the iron, and nearly 70% of the calcium in yolk. Ultracentrifugal and other analyses on solutions of the granules show that they are 70% α- and β-lipovitellin in an approximate ratio of 1:1.8, 16% phosvitin, and 12% low-density lipoprotein. The properties and composition of the two lipovitellins isolated from the granules are the same as those isolated from solutions of whole yolk. Further purification reduces the protein phosphorus in α-lipovitellin to 0.50% and in β-lipovitellin to 0.27%, and this confirms that α-vitellin has a higher phosphorus content. Experiments at low temperature suggest that phosvitin exists in the granules as a high molecular weight complex.
Phosvitin was precipitated from egg yolk solution in 0.4 M magnesium sulphate by adding an equal volume of water. Purification was accomplished by repeatedly precipitating and dialyzing at pH 4.0 to remove lipovitellin. Phosvitin so prepared was free of lipid, contained 9.6% phosphorus, and had a molecular weight of 3.0 × 104 at pH 4.0 and a frictional ratio that indicates an elongated molecule. It is a polyelectrolyte with properties that are dependent on both the concentration of the solute and the composition of the solvent. In buffer solutions containing low concentrations (0.01 to 0.05 M) of magnesium sulphate, it forms complexes having a particle weight of 14 × 105 and with calcium salts a particle weight of 7.5 × 105.
Pig serum contained three major classes of lipoproteins resembling the very low density (VLDL), low density (LDL), and high density lipoproteins (HDL) found in human serum but in smaller amounts. The LDL class had two clearly resolved components (LDL1 and LDL2) that were separated by differential centrifugation at solvent densities of 1.060 and 1.090. Heterogeneity in density was evident in all fractions, especially in those of highest lipid content. Average molecular weights and lipid contents of the isolated fractions were: VLDL, 17 × 106 and 90%; LDL1, 3 × 106 and 79%; LDL2, 2 × 106 and 72%; and HDL, 0.25 × 106 and 51%. Apart from small differences in proportion, the lipoproteins in the pooled samples collected over an 18-month period were quite similar although more extensive measurements may reveal differences. The presence of these distinct and separable lipoprotein fractions suggest structural differences worthy of further study.
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