ABSTRACT
The molecular structure of vitamin D is closely allied to that of classical steroid hormones. Technically, vitamin D is a seco-steroid. There is a family of vitamin D-related steroids that differ in the precise structure of the side chain attached to carbon-17. A formal definition of a vitamin is that it is a trace dietary constituent required to effect normal functioning of a physiological process. The plasma compartment contains a specific protein termed the vitamin D-binding protein, which is utilized to transport vitamin D seco-sterols. Identification of vitamin D-regulated transcription for specific gene products is typically supported by one or several observations. An animal model of vitamin D-dependent rickets-type II was engineered most recently by targeted disruption of DNA encoding the first and the second zinc finger of the DNA-binding domain of the vitamin D3 receptor, respectively, by two different groups independently.
