Fig. 2
From: Impact of vitamin D metabolism on clinical epigenetics
Synthesis and catabolism of calcitriol. In a multistep process vitamin D3 (cholecalciferol) is hydroxylated by liver mitochondrial and microsomal 25-hydroxylase (25-OHase) CYP27A1. The resultant pre-hormone 25-hydroxycholecalciferol, named in this figure as calcidiol is 1α hydroxylated in the kidney by CYP27B1 (a mitochondrial 1α-hydroxylase). This yields the hormonally active secosteroid 1α 25(OH)2D3 (calcitriol). 24-hydroxylation of 25(OH)D3 and 1α 25(OH)2D3 by the cytochrome P450 enzyme25-hydroxyvitamin D24-hydroxylase CYP24A1 is the rate-limiting step for calcitriol catabolism