Molecular aspects of the synthesis and deposition of hens' egg yolk with special reference to low density lipoprotein.

Avian egg yolk contains three main macromolecular constituents: 1) the yolk granules; insoluble particles consisting largely of lipovitellin and phosvitin. They are synthesized in response to hormones in the liver as a precursor protein, vitellogenin, which is soluble in blood. It passes through the oolemma by receptor-mediated endocytosis and is split up enzymically to give fragments that precipitate as granules in the yolk. 2) The livetins; essentially blood serum proteins. The mechanism for their transfer to yolk is not clear. 3) The yolk low density lipoprotein; the major part of yolk (60% of the dry weight). It is synthesized and assembled in the liver as a modified blood very low density lipoprotein, whose main apoprotein is apo B. As with vitellogenin, it enters yolk by endocytosis. Furthermore it is split enzymically to give most of the yolk-lipoprotein apoproteins (apovitellenins III to VI). New evidence for the relationship between yolk apoproteins and apo B has been derived from a comparison of the N-terminal amino acid sequences of apovitellenin IV and apo B.