Vitamin D is a prohormone, meaning that it has no hormone activity itself, but is converted to the active hormone 1 through a tightly regulated synthesis mechanism. Production of vitamin D in nature always appears to require the presence of some UV light; even vitamin D in foodstuffs is ultimately derived from organisms, from mushrooms to animals, which are not able to synthesize it except through the action of sunlight at some point in the synthetic chain. For example, fish contain vitamin D only because they ultimately exist on calories from ocean algae which synthesize vitamin D in shallow waters from the action of solar UV.
Production in the skin
The skin consists of two primary layers: the inner layer called the dermis, composed largely of connective tissue, and the outer, thinner epidermis. The epidermis consists of five ''strata''; from outer to inner they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.
Cholecalciferol is produced photochemically in the skin from 7-dehydrocholesterol; 7-dehydrocholesterol is produced in relatively large quantities in the skin of most vertebrate animals, including humans. The few exceptions are some bat species, mole rats, cats, and dogs, In most animals the highest concentrations of 7-dehydrocholesterol are found in the epidermal layer of skin, specifically in the stratum basale and stratum spinosum. Thus, individuals with higher skin melanin content will simply require more time in sunlight to produce the same amount of vitamin D as individuals with lower melanin content. The amount of time an individual requires to produce a given amount of vitamin D may also depend upon the person's distance from the equator and on the season of the year.
In some animals, the presence of fur or feathers blocks the UV rays from reaching the skin. In birds and fur-bearing mammals, vitamin D is generated from the oily secretions of the skin deposited onto the fur and obtained orally during grooming.
In 1923, Harry Goldblatt and Katherine Soames established that when 7-dehydrocholesterol (a precursor of vitamin D in the skin) is irradiated with light, a form of a fat-soluble vitamin is produced. Alfred Fabian Hess and Mildred Weinstock further substantiated that "equals vitamin D". Adolf Windaus, at the University of Göttingen in Germany, received the Nobel Prize in Chemistry in 1928, for his work on the constitution of sterols and their connection with vitamins. In 1930s, he clarified further the chemical structures of the vitamins D.
Synthesis mechanism (form 3)
|7-dehydrocholesterol, a derivative of cholesterol, is photolyzed by ultraviolet light in 6-electron conrotatory electrocyclic reaction. The product is ''pre-vitamin D3''.|| |
|Pre-vitamin D3 then spontaneously isomerizes to Vitamin D3 in a antarafacial hydride. At room temperature the transformation of previtamin-D3 to vitamin D3 takes about 12 days to complete. |
The VDR is known to be involved in cell proliferation and differentiation. Vitamin D also affects the immune system, and VDRs are expressed in several white blood cells, including monocytes and activated T and B cells.
Apart from VDR activation, various alternative mechanisms of action are known. An important one of these is its role as a natural inhibitor of signal transduction by hedgehog (a hormone involved in morphogenesis).
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