Metabolism of vitamin D. A new cholecalciferol metabolite, involving loss of hydrogen at C-1, in chick intestinal nuclei.

1. A comparison was made of the nature and intestinal intracellular distribution of the metabolites formed in vitamin D-deficient chicks from [4-(14)C]cholecalciferol and [1-(3)H]cholecalciferol. 2. The simultaneous administration of the two radioactive substances showed the presence in blood, liver, intestine, kidney and bone of cholecalciferol, its ester, 25-hydroxycholecalciferol and a further metabolite of cholecalciferol more polar than 25-hydroxycholecalciferol. The (3)H/(14)C ratios in these four radioactive components were the same as that of the dosed material (4.7:1) with the exception of the most polar material. The (3)H/(14)C ratio was lower in the fourth, most polar, metabolite (0.4:1-1.8:1) in all tissues examined, with the exception of blood. 3. In the chick intestine the polar metabolite accounted for almost 70% of the radioactivity in this tissue after a dose of 0.5mug. of [4-(14)C,1-(3)H]cholecalciferol. This polar metabolite from the intestine also had the lowest (3)H/(14)C ratio of all the tissues. It appears that in the chick intestine the polar metabolite reaches a maximum concentration of 1ng./g. of tissue, above which it cannot be increased irrespective of the dose of the vitamin. 4. The intestinal intracellular organelle with the highest concentration of (14)C radioactivity is the nucleus, and this radioactivity is almost entirely due to the polar metabolite with the lowered (3)H/(14)C ratio, in this case <0.2:1. It appears to be further localized in the chromatin of the nuclei. However, about half of the polar metabolite in the intestine is extranuclear. 5. Double-labelled 25-hydroxycholecalciferol was prepared and after its administration to vitamin D-deficient chicks the polar metabolite with the lowered (3)H/(14)C ratio was detected in liver, kidney, intestine, bone, muscle and heart. 6. None of the polar metabolite with the lowered (3)H/(14)C ratio was detected 16hr. after dosing with either the double-labelled vitamin or the double-labelled 25-hydroxycholecalciferol in blood and adipose tissue of vitamin D-deficient chicks, nor in the intestine, liver and kidney of supplemented birds. 7. The reasons for this loss of (3)H relative to (14)C are discussed in relation to possible chemical structures of this new polar metabolite.