Description | Ercalcitriol is the active circulating metabolite of vitamin D2. Vitamin D2 is modified by 25-hydroxylase in the liver and 25-hydroxyvitamin D *-hydroxylase in the kidney to form the active metabolite, ercalcitriol, which is then metabolized by 25-hydroxyvitamin D-24-hydroxylase (24-OHase, CYP24A1, EC 1.14.13.13). The binding of ercalcitriol or their analogs to Vitamin D receptor (VDR), a nuclear receptor, activates VDR to interact with retinoid X receptor (RXR) and forms the VDR/RXR/cofactor complex, which binds to Vitamin D response elements in the promoter region of target genes to regulate gene transcription. The kidney is the major site of 25-hydroxyvitamin D*-hydroxylase (CYP27B1, EC 1.14.13.13), which is responsible for the activation of 25-hydroxyvitamin D. Of all the steroid hormones, ercalcitriol represents the most difficult challenge to the analytical biochemist with respect to quantization. Ercalcitriol circulates at pmol concentrations, is highly lipophilic and its precursor, 25-hydroxyvitamin D2, circulates at nmol levels. (PMID: 16242929, 17867378, 17197173) |