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Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. [1] [2] In humans, the most important compounds within this group are vitamin D 3 (cholecalciferol) and vitamin D 2 (ergocalciferol). [2] [3]
25-Hydroxyvitamin D 1-alpha-hydroxylase (VD 1A hydroxylase) also known as calcidiol 1-monooxygenase [5] or cytochrome p450 27B1 (CYP27B1) or simply 1-alpha-hydroxylase is a cytochrome P450 enzyme that in humans is encoded by the CYP27B1 gene.
CYP24A1 also is able to catalyze another pathway which starts with 23-hydroxylation of 1,25-(OH) 2 D 3 and culminates in 1,25-(OH) 2 D 3-26,23-lactone. [ 6 ] The side chains of the ergocalciferol (vitamin D 2 ) derivatives, 25-OH-D 2 and 1,25-(OH) 2 D 2 , are also hydroxylated by CYP24A1.
A metabolic network is the complete set of metabolic and physical processes that determine the physiological and biochemical properties of a cell.As such, these networks comprise the chemical reactions of metabolism, the metabolic pathways, as well as the regulatory interactions that guide these reactions.
CYP2R1 is cytochrome P450 2R1, an enzyme which is the principal vitamin D 25-hydroxylase. [5] [6] In humans it is encoded by the CYP2R1 gene located on chromosome 11p15.2. [7]It is expressed in the endoplasmic reticulum in liver, where it performs the first step in the activation of vitamin D by catalyzing the formation of 25-hydroxyvitamin D. [8]
7-Dehydrocholesterol (7-DHC) is a zoosterol that functions in the serum as a cholesterol precursor, and is photochemically converted to vitamin D 3 in the skin, therefore functioning as provitamin-D 3. The presence of this compound in human skin enables humans to manufacture vitamin D 3 (cholecalciferol).
An anabolic pathway is a biosynthetic pathway, meaning that it combines smaller molecules to form larger and more complex ones. [ 10 ] : 570 An example is the reversed pathway of glycolysis, otherwise known as gluconeogenesis , which occurs in the liver and sometimes in the kidney to maintain proper glucose concentration in the blood and supply ...
Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. [8] [9] [10] It is also known as 1,25-dihydroxycholecalciferol.It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. [11]