In vitro inhibitor studies of vitamin D 25-hydroxylase in rat liver microsomes.

The ability of four vitamin D analogs to inhibit the liver microsomal vitamin D-25-hydroxylase was determined. 19-Hydroxy-10(S),19-dihydrovitamin D3,25-fluorovitamin D3, 3 beta-hydroxy-9,10-seco-5,7,10(19)-choletrien-24-oic acid dimethylamide and 25-aza-vitamin D3 were competitive inhibitors with apparent KI values of 44, 137, and 870 nM, and 6.4 microM, respectively. The values for the 19-hydroxy-10(S), 19-dihydrovitamin D3, 25-fluorovitamin D3, and 25-aza-vitamin D3 correspond well to other literature reports with respect to their relative in vivo inhibitory properties. 24-Oxovitamin D3 oxime also proved to be a potent inhibitor but a detailed analysis was prohibited by the lack of material. The 3 beta-hydroxy-9,10-seco-5,7,10(19)-choletrien-24-oic acid dimethylamide was also tested in vivo but had no antagonistic activity when provided at a 2000-fold excess over vitamin D3.