A method is described for the chromatographic separation of mixtures of egg-yolk proteins of low solubility by using a hydrophobic column (phenyl-Sepharose) and eluting with increasing concentrations of aqueous urea at low pH. The resolving power of the method was established by tests on proteins and protein fragments of known sequence. The theoretical basis for the method remains, however, unclear. Factors such as the aggregation of the protein often appeared to be more important than its hydrophobicity in determining the urea concentration needed for elution. The method was applied to the mixture of apoproteins from the low-density lipoprotein (density about 0.95 g/ml) of avian egg yolk. For the previously studied apoproteins from egg yolk of the hen (Gallus domesticus), hydrophobic chromatography provided a new and convenient method for isolating the main apoproteins (hen apovitellenins I-VI). For the hitherto unexplored apoproteins from egg yolk of the duck (Anas platyrhynchos) the method has now been used to isolate three new proteins, two of which were not readily separated by methods based on molecular size. The elution pattern obtained with duck egg-yolk apoproteins is not the same as that of the hen egg-yolk apoproteins, although we suggest a relationship for the three new apoproteins based on their amino acid compositions and other properties. Possible roles for the apoproteins in avian egg yolk are described.

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