Studies on vitamin D metabolism: catabolism of 1 alpha, 25-dihydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3 to a metabolite inactive on bone mineral mobilization.

A heretofore unknown metabolite of vitamin D3 was isolated from the 1 alpha, 24, 25-trihydroxyvitamin D3 fraction of lipid extracts obtained from plasma of rats which were given intravenous or oral doses of 100 pmol/100 g of either 1 alpha-hydroxyvitamin D3 or 1 alpha, 23-dihydroxyvitamin D3. Doses of 25-250 pmoles of the new metabolite when given to a vitamin D deficient rat were completely inactive in terms of stimulating the classic vitamin D response of bone calcium mobilization. The nature of the metabolism of 1 alpha-hydroxyvitamin D3 or 1 alpha, 25-dihydroxyvitamin D3 to the metabolite is not clear at the present time, but it is probable that neither of these steroids undergo side-chain cleavage to yield the new metabolite.