Figure 5

Production of active vitamin D in renal and immune cells. Vitamin D3 is initially 25-hydroxylated to the main circulating form of vitamin D, 25-hydroxy vitamin D. This molecule is then 1alpha-hydroxylated to the active form of vitamin D, 1,25-dihydroxy vitamin D. This binds to the vitamin D receptor (VDR) in the nucleus and forms heterodimers with the retinoid X receptor (RXR). The complex acts as a transregulatory complex, controlling transcription of several genes. The enzyme 24-hydroxylase acts as a negative feedback control pathway by synthesizing inactive 1, 24, 25-trihydroxy-vitamin D.