Vitamin D
4
basale. Vitamin D is produced in the two innermost strata, the stratum basale and stratum spinosum. 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 naked mole rat appears to be naturally cholecalciferol deficient as serum 25-OH vitamin D levels are undetectable. Interestingly, the naked mole rat is resistant to aging, maintains healthy vascular function and is the longest lived of all rodents. 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, it was established that when 7-dehydrocholesterol is irradiated with light, a form of a fat-soluble vitamin is produced. Alfred Fabian Hess showed that "light 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 the 1930s he clarified further the chemical structure of vitamin D. In 1923, Harry Steenbock at the University of Wisconsin demonstrated that irradiation by ultraviolet light increased the vitamin D content of foods and other organic materials. After irradiating rodent food, Steenbock discovered that the rodents were cured of rickets. It is now known that vitamin D deficiency is a cause of rickets. Using $300 of his own money, Steenbock patented his invention. Steenbock's irradiation technique was used for foodstuffs, most memorably for milk. By the expiration of his patent in 1945, rickets had all but been eliminated in the US. Following the final converting step in the kidney, calcitriol (the physiologically active form of vitamin D) is released into the circulation. By binding to vitamin D-binding protein (VDBP), a carrier protein in the plasma, calcitriol is transported to various target organs.
Synthesis mechanism (form 3)
Mechanism of action