7-Dehydrocholesterol: Difference between revisions

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'''7-Dehydrocholesterol''' ('''7-DHC''') is a [[zoosterol]] that functions in the [[blood plasma|serum]] as a [[cholesterol]] precursor, and is [[photochemically]] converted to [[Cholecalciferol|vitamin D₃]] in the [[skin]], therefore functioning as [[provitamin]]-D₃. The presence of this compound in human skin enables humans to manufacture vitamin D₃ ([[cholecalciferol]]).  Upon exposure to [[ultraviolet]] UV-B rays in the sun light, 7-DHC is converted into vitamin D₃ via [[Previtamin D3|previtamin D₃]] as an intermediate [[isomer]]. It is also found in the [[milk]] of several mammalian species. [[Lanolin]], a waxy substance that is naturally secreted by wool-bearing mammals, contains 7-DHC which is converted into vitamin D by sunlight and then ingested during grooming as a nutrient. In insects 7-dehydrocholesterol is a precursor for the hormone [[ecdysone]], required for reaching adulthood. It was discovered by Nobel-laureate organic chemist [[Adolf Windaus]].

==Biosynthesis==

It is synthesized from [[lathosterol]] by the enzyme [[lathosterol oxidase]] (lathosterol 5-desaturase).  This is the next-to-last step of cholesterol biosynthesis. Defective synthesis results in the human inherited disorder [[lathosterolosis]] resembling [[Smith–Lemli–Opitz syndrome]]. Mice where this gene has been deleted lose the ability to increase vitamin D₃ in the blood following UV exposure of the skin.

==Location ==

The skin consists of two primary layers: an inner layer, the [[dermis]], comprising largely [[connective tissue]], and an outer, thinner [[Epidermis (skin)|epidermis]]. The thickness of the epidermis ranges from 0.08&nbsp;mm to greater than 0.6&nbsp;mm (from 0.003 to 0.024 inches). The epidermis comprises five ''strata''; from outer to inner, they are the [[stratum corneum]], [[stratum lucidum]], [[stratum granulosum]], [[stratum spinosum]], and [[stratum basale]]. The highest concentrations of 7-dehydrocholesterol are found in the epidermal layer of skin—specifically in the stratum basale and stratum spinosum. The production of pre-vitamin D₃ is, therefore, greatest in these two layers.

==Radiation==

Synthesis of pre-vitamin D₃ in the skin involves [[UVB radiation]], which effectively penetrates only the epidermal layers of skin. 7-Dehydrocholesterol absorbs UV light most effectively at [[wavelength]]s between 295 and 300 [[nanometer|nm]] and, thus, the production of vitamin D₃ will occur primarily at those wavelengths. The two most important factors that govern the generation of pre-vitamin D₃ are the quantity (intensity) and quality (appropriate wavelength) of the UVB irradiation reaching the 7-dehydrocholesterol deep in the stratum basale and stratum spinosum. Light-emitting diodes ([[LED]]s) can be used to produce the radiation.

Another important consideration is the quantity of 7-dehydrocholesterol present in the skin. Under normal circumstances, ample quantities of 7-dehydrocholesterol (about 25&ndash;50 [[microgram|μg]]/cm² of skin) are available in the [[stratum spinosum]] and [[stratum basale]] of human skin to meet the body's vitamin D requirements. 7-DHC insufficiency has been proposed as an alternate cause for Vitamin D deficiency.

== Sources ==

7-DHC can be produced by animals and plants via different pathways. It is not produced by fungi in significant amounts. It is made by some [[algae]], but the pathway is poorly understood. It can also be produced by some bacteria.

Industrially, 7-DHC generally comes from [[lanolin]], and is used to produce vitamin D3 by UV exposure. [[Lichen]] ([[Cladonia rangiferina]]) is used to produce [[vegan]] D3.

7-DHC is used for vitamin D3 synthesis via [[lanosterol]] in land animals, via [[cycloartenol]] in plants, and in algae together with another provitamin D [[ergosterol]] for D2. In fungi solely ergosterol is used for synthesis of D2 via lanosterol.

==Interactive pathway map==

{{VitaminDSynthesis_WP1531|highlight=7-Dehydrocholesterol}}

== See also ==

* [[Vitamin D]]

* [[7-Dehydrocholesterol reductase]]

{{Vitamin}}

{{Cholesterol metabolism intermediates}}

{{Vitamin D receptor modulators}}

{{二次利用|date=9 December 2023}}

{{DEFAULTSORT:Dehydrocholesterol, 7-}}