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Vitamin D

Helen L. Henry

10.1002/cphy.cp070318

Source: Supplement 22: Handbook of Physiology, The Endocrine System, Endocrine Regulation of Water and Electrolyte Balance

Originally published: 2000

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Metabolism of Vitamin D
1.1 Synthesis of Vitamin D
1.2 Synthesis of 25‐Hydroxyvitamin D3
1.3 Synthesis of the Dihydroxylated Metabolites of 25‐Hydroxyvitamin D3
1.4 24R,25‐Dihydroxyvitamin D3
1.5 Regulation of Hydroxylation of 25‐Hydroxyvitamin D3 in the Kidney
1.6 Catabolism of Vitamin D Metabolites
1.7 Vitamin D Binding Protein
2 Actions of 1α,25‐Dihydroxyvitamin D3
2.1 Intestine
2.2 Bone
2.3 Kidney
2.4 Parathyroid Gland
2.5 Growth and Differentiation
2.6 Immune System
3 Mechanisms of 1α,25‐Dihydroxyvitamin D3 Action
3.1 Genomic
3.2 Rapid Actions
4 Biological Activity of 24R,25‐Dihydroxyvitamin D3
Figure 1. Figure 1.

Synthesis and activation of vitamin D. The full pathway for vitamin D3 is shown. As described in the text, along with the details of this pathway, vitamin D2 is believed to be metabolized by the same pathway.
Figure 2. Figure 2.

Top: Generalized schematic diagram of mitochondrial steroid hydroxylases. Bottom: Some typical mitochondrial steroid hydroxylases. Two hydroxylations, at carbons 20 and 22, precede cleavage of the side chain of cholesterol, the first step in the synthesis of sex steroids, glucocorticoids, and mineralocorticoids. Production of cortisol involves hydroxylation at the 11β position. The 1α‐hydroxylation of 25(OH)D3 is catalyzed by an enzyme system similar to these classical steroid hydroxylases.
Figure 3. Figure 3.

Electron transport chain for mitochondrial steroid hydroxylases. FPox is the oxidized form of NADPH‐ferredoxin reductase and FPred is its reduced form, a flavoprotein; NHI, the non‐heme iron protein ferredoxin, which transports electrons between the Flavoprotein and cytochrome P‐450. In its reduced form, cytochrome P‐450, when bound to carbon monoxide (CO), displays the spectral characteristics from which its name is derived. R is the substrate and R‐OH is the product of the hydroxylation reaction. Molecular oxygen and H2.O are the co‐substrate and co‐product, respectively.
Figure 4. Figure 4.

Amino acid sequences of the N terminus of 25(OH)D3‐1α‐hydroxylase and the conserved region surrounding the cysteine (CYS), which provides one of the coordination sites for the heme of cytochrome P‐450. These sequences are identical in mice and rats, while the human sequence varies from both at three positions in the N terminus and one in the conserved cysteine region.
Figure 5. Figure 5.

Catabolic pathways of 1,25α(OH)2D3. Pathway A: 24‐hydroxylation and oxidation, followed by 23‐hydroxylation and cleavage off our carbons from the side chain to yield calcitroic acid. Pathway B: 23‐ and 26‐hydroxylation, followed by the 25, 26, 23‐lactone. Similar pathways occur for 25(OH)D3.


Figure 1.

Synthesis and activation of vitamin D. The full pathway for vitamin D3 is shown. As described in the text, along with the details of this pathway, vitamin D2 is believed to be metabolized by the same pathway.



Figure 2.

Top: Generalized schematic diagram of mitochondrial steroid hydroxylases. Bottom: Some typical mitochondrial steroid hydroxylases. Two hydroxylations, at carbons 20 and 22, precede cleavage of the side chain of cholesterol, the first step in the synthesis of sex steroids, glucocorticoids, and mineralocorticoids. Production of cortisol involves hydroxylation at the 11β position. The 1α‐hydroxylation of 25(OH)D3 is catalyzed by an enzyme system similar to these classical steroid hydroxylases.



Figure 3.

Electron transport chain for mitochondrial steroid hydroxylases. FPox is the oxidized form of NADPH‐ferredoxin reductase and FPred is its reduced form, a flavoprotein; NHI, the non‐heme iron protein ferredoxin, which transports electrons between the Flavoprotein and cytochrome P‐450. In its reduced form, cytochrome P‐450, when bound to carbon monoxide (CO), displays the spectral characteristics from which its name is derived. R is the substrate and R‐OH is the product of the hydroxylation reaction. Molecular oxygen and H2.O are the co‐substrate and co‐product, respectively.



Figure 4.

Amino acid sequences of the N terminus of 25(OH)D3‐1α‐hydroxylase and the conserved region surrounding the cysteine (CYS), which provides one of the coordination sites for the heme of cytochrome P‐450. These sequences are identical in mice and rats, while the human sequence varies from both at three positions in the N terminus and one in the conserved cysteine region.



Figure 5.

Catabolic pathways of 1,25α(OH)2D3. Pathway A: 24‐hydroxylation and oxidation, followed by 23‐hydroxylation and cleavage off our carbons from the side chain to yield calcitroic acid. Pathway B: 23‐ and 26‐hydroxylation, followed by the 25, 26, 23‐lactone. Similar pathways occur for 25(OH)D3.

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Article Title: Vitamin D
 

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Helen L. Henry. Vitamin D. Compr Physiol 2011, Supplement 22: Handbook of Physiology, The Endocrine System, Endocrine Regulation of Water and Electrolyte Balance: 699-718. First published in print 2000. doi: 10.1002/cphy.cp070318