[Vitamin D3].

With the application of biochemical technique came the discovery that vitamin D is metabolized sequentially to 25-hydroxyvitamin D in liver and then to 1.25-dihydroxyvitamin D (1.25D), the hormonal form of vitamin D, in kidney. The latter process is strictly controlled with the demand of calcium and phosphorus. The produced 1.25D binds its specific receptor probably located in the nuclei. With the application of modern tools of molecular biology came the discovery that vitamin D receptor (VDR), which belongs to steroid receptor family, acquires increased affinity for DNA when it binds to 1.25D and then VDR complexes bind to DNA with its two zinc-finger projections. VDR complexes recognize vitamin D responsive element on DNA, two tandemly repeated hexanucleotide sequences separated by three base pairs. Furthermore, to interact with the VDRE, VDR complexes require nuclear accessory factor, which might be retinoid X receptor (RXR). As above, molecular biology helps us to extend our knowledge in the field of vitamin D.