7-Dehydrocholesterol (7-DHC) is a zoosterol that functions in the serum as a cholesterol precursor, and is photochemically converted to 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 as an intermediate isomer. It is also found in the milk of several mammalian species.[1][2]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.[3] 7-DHC 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.[4] Defective synthesis results in the human inherited disorder lathosterolosis resembling Smith–Lemli–Opitz syndrome.[4] Mice where this gene has been deleted lose the ability to increase vitamin D3 in the blood following UV exposure of the skin.[5]
Location
The skin consists of two primary layers: an inner layer, the dermis, comprising largely connective tissue, and an outer, thinner epidermis. The thickness of the epidermis ranges from 0.08 mm to greater than 0.6 mm (from 0.003 to 0.024 inches).[6] 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.[7] 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 wavelengths between 295 and 300 nm and, thus, the production of vitamin D3 will occur primarily at those wavelengths.[8] 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.[7] Light-emitting diodes (LEDs) can be used to produce the radiation.[9]
Another important consideration is the quantity of 7-dehydrocholesterol present in the skin. Under normal circumstances, ample quantities of 7-dehydrocholesterol (about 25–50 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.[10]
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.[11]
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.[15]
Interactive pathway map
Click on genes, proteins and metabolites below to link to respective articles.[§ 1]
^MacLaughlin JA, Anderson RR, Holick MF (May 1982). "Spectral character of sunlight modulates photosynthesis of previtamin D3 and its photoisomers in human skin". Science. 216 (4549): 1001–3. doi:10.1126/science.6281884. PMID6281884.
^Holick MF (November 2005). "The vitamin D epidemic and its health consequences". The Journal of Nutrition. 135 (11): 2739S–2748S. doi:10.1093/jn/135.11.2739S. PMID16251641. [Vitamin D3] is produced commercially by extracting 7-dehydrocholesterol from wool fat, followed by UVB irradiation and purification [...] [Vitamin D2] is commercially made by irradiating and then purifying the ergosterol extracted from yeast
