Studies on calciferol metabolism

It is known that after birth of a vertebrate there is a requirement for the metabolism of Vitamin D 3 (cholecalciferol) to 1,25-(OH) 2 -Vitamin D 3 to produce the hormonally active form essential for calcium homeostasis. However it is not known whether the enzymatic capability to produce 1,25-(OH) 2 -D 3 only appears after birth or whether it is generated in the embryo. Presented in this paper are results of studies designed to measure the production and localization of 1,25-(OH) 2 -D 3 in the embryo. It was found that the renal enzyme, 25-OH-cholecalciferol-1-hydroxylase, which is capable of producing 1,25-(OH) 2 -D 3 , is present as early as day 9 of incubation (12 days before hatch) in White Leghorn chicks. Further, the enzyme activity increases 6-fold to a maximal level which occurs on the day of hatching. 1,25-(OH) 2 -D 3 was shown to be produced in vivo at day 17 and was found then in low levels in the embryonic intestine and kidney. Thus we have shown that 1,25-(OH) 2 -D 3 is made by embryonic chick kidneys and is found in low levels in embryonic chick intestine and kidney significantly before hatch.