title: Vitamin D in the avian egg. Its molecular identity and mechanism of incorporation into yolk.
abstract: The chemical identity of vitamin D in the egg of the domestic fowl was studied by analysing radioactivity in eggs from hens injected with [3H]cholecalciferol. Labelled 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 chloroform-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. It is postulated that cholecalciferol forms 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.
mesh_terms: D000818:Animals; D001798:Blood Proteins; D002645:Chickens; D002762:Cholecalciferol / Q000378:metabolism; D004530:Egg Yolk / Q000032:analysis; D004531:Eggs / Q000032:analysis*; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D006887:Hydroxycholecalciferols / Q000378:metabolism; D010047:Ovalbumin / Q000378:metabolism; D010063:Ovum / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects; D014807:Vitamin D / Q000378:metabolism*
chemical_list: D001798:Blood Proteins; D006887:Hydroxycholecalciferols; D014807:Vitamin D; D002762:Cholecalciferol; D004958:Estradiol; D010047:Ovalbumin