Vitamin D metabolism

Abstract

Irradiation of human skin with ultraviolet B (280-320 nm) initiates the photochemical conversion of 7-dehydrocholesterol via previtamin D3 to vitamin D3. Vitamin D3 needs for its activation two hydroxylation steps in the liver and kidney. The final product, hormonally active 1alpha,25-dihydroxyvitamin D3 (calcitriol), arrives via the circulation to its target tissues and acts in a genomic or nongenomic manner. It has been found that human skin irradiated with ultraviolet B also is able to produce calcitriol in substantial amounts. This cutaneous vitamin D3 pathway is unique and, most likely, of considerable relevance for healthy and diseased skin. It is well known that topical application of calcitriol and its analogs can improve hyperproliferative skin diseases. Some studies have convincingly demonstrated that calcitriol and other vitamin D analogs may also be used for the treatment of immunological, inflammatory, and infectious skin diseases. More recently, it has been found that calcitriol or vitamin D analogs have photoprotective effects and can reduce UV-induced deoxyribonucleic acid damage.

A Dermatologist's Perspective on Vitamin D

“…Vitamin D 3 is then transported to the bloodstream. [1][2][3][4][5][6][7][8][9]11,12 The UV-B action spectrum of vitamin D biosynthesis is the same as that responsible for the sunburn response and photocarcinogenesis. The conversion of previtamin D 3 to the inactive photoproducts lumisterol and tachysterol balances the cutaneous biosynthesis of vitamin D 3 as a feedback loop.…”

“…[1][2][3][4][5][6][7][8][9]11,12 Megalin and cubilin are endocytic receptors in the proximal tubule cells of the kidney. They are involved in reabsorption of VBP, which, in the presence of 1-␣ hydroxylase, results in intracellular , and mobilizing calcium from the skeleton through resorption.…”

“…These functions may in turn have roles in cancers, immunity, and many organ systems. [1][2][3][4][5][6][7][8][9] The potential myriad effects of this vitamin in human health and disease have led to popular interest in vitamin D inadequacy and the best method to normalize suboptimal levels. To date, however, no definitive data on causal relationships are available for the role of vitamin D in areas other than calcium homeostasis and bone health.…”

“…This mechanism ensures that one cannot "overdose" on vitamin D 3 by photoexposure alone. After less than 1 minimal erythema dose (MED; ie, the amount of photoexposure required to produce faint pinkness in the skin at 24 hours after exposure), the concentration of previtamin D 3 reaches maximal levels and further UV radiation merely results in the production of inactive metabolites.Cutaneously [1][2][3][4][5][6][7][8][9]11,12 25-hydroxyvitamin D is the major circulating form of vitamin D, and its serum levels are widely used as a reflection of total body stores of vitamin D. It is biologically inactive, however, and requires hydroxylation in the kidney to form the biologically active 1,25-dihydroxyvitamin D (1,25[OH 2 ]D) ( Figure 1). [1][2][3][4][5][6][7][8][9]11,12 Megalin and cubilin are endocytic receptors in the proximal tubule cells of the kidney.…”

“…In recent years, vitamin D has been studied for its potential role in the prevention of cancers, cardiovascular diseases, autoimmune diseases, and other chronic conditions. [1][2][3][4][5][6][7][8][9] Vitamin D has both endocrine and autocrine functions. The endocrine effects of vitamin D are involved in serum calcium homeostasis.…”