"Activation" of vitamin D by the liver.

Isolation of the liver from the circulation of rats eliminates almost completely their ability to convert [1,2]-(3)H vitamin D(3) into its biologically active metabolite, 25-hydroxycholecalciferol, as well as certain other metabolites. It is concluded that the liver is the major if not the only physiologic site of hydroxylation of vitamin D(3) (cholecalciferol) into 25-hydroxycholecalciferol. The osteodystrophy and the higher requirements for vitamin D observed in hepatic insufficiencies may be due to an inability of the liver to transform vitamin D into its metabolically active form.