Proteins and calcium in egg yolk

Certain proteins of egg yolk—X₁, a phosphoprotein, and X₂, a lipoglycoprotein—were compared to their serum precursors with the aid of chemical, electrophoretic and ultracentrifugal techniques.The X₁-X₂ components were found only within the egg yolk granules. The X₁ phosphoprotein was composed largely of serine and bound approximately 75 per cent of the calcium of egg yolk, thus displaying a capacity for calcium which far exceeds that of casein in milk. The X₂ lipoglycoprotein was comprised of 20 per cent lipid, which is mainly phospholipid, and a lesser amount of polysaccharide. It did not seem to bind calcium to any appreciable extent. (Serum X₁ and X₂ displayed very similar properties to their yolk counterparts.) The extragranular or fluid compartment of the egg yolk contained a protein resembling ovalbumin and a lipoprotein floating at a constant of Sf 21.X₁ and X₂ proteins co-precipitated when diluted greatly with distilled water. The precipitate could be dissolved by the addition of 2 per cent NaCl, a lesser amount of CaCl₂, or by adjustment of the pH to approximately 9 or greater.Making the assumption that the axial ratios of these molecules were comparable to that of serum albumin, the molecular weight of X₁ phosphoprotein was calculated to be 77,000 (s 4.75) and that of X₂ lipoglycoprotein to be 200,000 (s 10). When the same axial ratio was accorded the serum precursors of these molecules, it became apparent that the serum X₁ and X₂ proteins may have been reduced to half their molecular weight before, during or after transfer from the plasma to the egg yolk.¹