Rat kidney microsomes convert 25-hydroxyvitamin D3 to an unidentified metabolite

Kidney microsomes from vitamin D-deficient rats and from thyroparathyroidectomized rats converted 25-hydroxyvitamin D3 to an unidentified metabolite. The addition of the cytosolic fraction enhanced microsomal synthesis of this metabolite two-fold. The kinetics of the conversion in the presence of the cytosolic fraction was allosteric, suggesting that the enzyme responsible for synthesis of this metabolite might serve some role in the regulation of vitamin D metabolism. Microsomes from vitamin D-fed thyroparathyroidectomized rats also produced a second metabolite, tentatively identified as 25,26-dihydroxyvitamin D3 because of its comigration with 25,26-dihydroxyvitamin D3 in three different chromatographic systems.