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Biological function and synthesis of Calcitriol

Jun 17，2022

Calcitriol is a calciotropic hormone secreted by renal proximal tubular cells. It stimulates intestinal calcium absorption and bone mineralization. The drug form is used for the treatment of osteoporosis and renal failure.

Discovery

In 1971, 1α,25-dihydroxyvitamin D3 was purified using radiolabeled vitamin D3 incubated with the intestine of vitamin D-deficient chicks.The configuration of the hydroxyl group at position 1 was deduced by the cochromatography with chemically synthesized 1α,25- and 1β,25-dihydroxyvitamin D3 compounds.

Structure

Calcitriol is a secosteroid hormone essential for bone and mineral homeostasis.

Article illustration

Properties

Molecular formula C27H44O3,Mr 416.6.White crystalline powder, MP 111–115°C. UV λmax (abs. ethanol): 264nm (ε 19,000). [α]D 25+48 (methanol). Slightly solubleinmethanol, ethanol, ethylacetate, and THF. Air- and light-sensitive.

Synthesis and release

Metabolism

Vitamin D is a secosteroid whose actions are dependent on specific metabolic steps catalyzed by cytochrome P450 enzymes (CYPs). The first step occurs in the liver, where vitamin D3 25-hydroxylase (CYP2R1) catalyzes 25-hydroxyvitamin D3 (25(OH)D3) synthesis, the main circulating form of vitamin D. 25(OH)D3 then acts as the substrate for 25(OH)D3 1α-hydroxylase (CYP27B1) that catalyzes the conversion of 25(OH)D3 into 1α,25- dihydroxyvitamin D3 in the kidney, an active form of vitamin D3.2 25(OH)D3 and 1α,25-dihydroxyvitamin D3 are hydroxylated by CYP24A1 at C-24, resulting in their degradation. CYP24A1 is the dominant catabolic enzyme of calcitriol and catalyzes multiple oxidation reactions.

Biological function

Target cells/tissues and functions VDR is present in the cells of the intestinal epithelium, renal tubules, parathyroid gland cells, epidermis, mammary epithelium, pancreatic cells, pituitary gland, bone cells, spleen cells, lymph node, and germ tissues. VDR regulates the expression of numerous genes involved in calcium and phosphorus homeostasis, cellular proliferation and differentiation, and immune response, largely in a ligand-dependent manner.

Phenotype in gene-modified animals

CYP27B1 knockout mice exhibit an identical phenotype to hereditary vitamin D-dependent rickets type I (VDDR-I) while VDR knockout mice exhibit an identical phenotype to hereditary vitamin D-dependent rickets type II (VDDR-II) in humans.

Use for treatment

Calcitriol and alfacalcidiol are used in the treatment of patients with osteoporosis and renal osteodystrophy. Maxacalcitrol ointment is used to treat plaque psoriasis.

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