Figure 1.

7-Dehydrocholesterol undergoes specific alterations in the skin, liver and kidney. As a result calcitriol is formed and enters the cell cytosol through specific channels together with vitamin D receptor (VDR). After heterodimerization, the complex enters the nucleus and as a result, a multi-component complex is formed (phosphorylated calcitriol-VDR complex, retinol X receptor, 9cRa transcription factor) which subsequently binds to DNA. In the presence of HDAC complexes and other transcription factors, CYP27B1 gene responsible for parathormone production is repressed. However, in the presence of PBA/SWI/SNF complex other compounds are added (regulators of interaction, transcriptional factor IIB and most important of all, RNA polymerase II). As a consequence, CYP27B1 transcription occurs. The protein encoded by this gene localizes to the inner mitochondrial membrane where it hydroxylates 25-hydroxyvitamin D3 at the 1α position. This reaction synthesizes 1α,25-dihydroxyvitamin D3, the active form of vitamin D3, which binds to the VDR.