VITAMIN-D IN AVIAN EGG - ITS MOLECULAR IDENTITY AND MECHANISM OF INCORPORATION INTO YOLK

The chemical identity of vitamin D in the egg of the domestic fowl [chicken] was studied by analysing radioactivity in eggs from hens injected with [3H]cholecalciferol. Labeled molecules were found throughout the egg, but the concentration of total radioactivity in albumin was only 5-7% of that in yolk. In lipid extracts of yolk, more than 90% of the radioactivity was as unchanged cholecalciferol and 5% as 25-hydroxycholecalciferol. Only about 3% of the radioactivity in albumin was choloroform-soluble, and of this, 40% was 25-hydroxycholecalciferol and 15% was cholecalciferol. Evidence is presented to support the idea that the specific transport of cholecalciferol into yolk is mediated by a cholecalciferol-binding protein in blood. This protein forms a complex with yolk proteins in transit from liver to ovary via the blood. A cholecalciferol-binding protein, chromatographically similar to that from blood, was found in egg yolk. Cholecalciferol may form part of a complex with its specific binding protein, Ca2+ and the yolk phosphoprotein, phosvitin. This complex is then incorporated into yolk by the thecal cells of the ovarian follicle.