Vitamin D

Vitamin D aka CALCIFEROL discovered in 1922 by E McCollum with work on Rickets and effect of vitamin D and sunlight on the condition.

Vitamin D form:

• derived from a steroid
• considered a seco-steroid as 1 of 4 rings is broken
• 3 intact rings (A, C, D) with B broken between carbon 9 and 10
• 2 main forms:
• D2 ERGOCALCIFEROL
• D3 CHOLECALCIFEROL
• differ in structure of side chains but not metabolism or effect

SOURCES

Vit D as D3 is in relatively small number of foods of animal origin, largest quantity in fatty fish.
Vitamin is resistant to cooking, storage, or processing.
Plant sources are few with shitake mushrooms having 1 mcg / cup.
Fortified foods in US have D3 at 2.5 mcg (100 IU)/cup.
Supplements provide D3 primarily.
RDA 10 mcg or 400 IU
Skin source:

• 7-dehydrocholesterol (derived from cholesterol in sebaceous glands and secreted on skin)
• Conjugated double bond in ring B allows for absorption of specific UV wavelengths of light
• During direct exposure to sunlight (not through glass) UV B photon (~wavelength 285-320 nm) penetrate epidermis and dermis
• UVB causes ring B to open forming previtamin D3 (precholecalciferol)
• Unstable double bonds in previtamin D3 are rearranged in thermal isomerization resulting in vitamin D3
• Excessive vit D3 prevented by generation of inactive metabolites lumisterol and tachysterol
• Skin production of vitamin D3:
• 150-30 minutes of skin exposure = 10,000-20,000 IU (250-500mcg)

1mcg = 40IU
1IU = 0.025 mcg

ABSORPTION

Dietary vit D requires no digestion and absorbed from a micelle in association with fat with the aid of bile by passive difusion in the jejunum (and less so the duodenum) at 50% absorption rate.

Chylomicrons transport to blood.

TRANSPORT/STORAGE

Chylomicrons transport dietary vitamin D to nonhepatic tissues and the liver. In the skin, vitamin D3 is picked up and transported by vitamin D-binding protein (DBP).

In liver, chylomicron remnants or DBP taken up by endocytosis and the vitamin D is hydroxylated by cyt (450 hydoxylases (called mixed-function oxidases - the enzyme reduce one atom of molecular oxygen to water and one to the hydroxyl group; called CYP).

ENZYME 25-hydoxylase:

• In the liver mitochondria, 25-hydoxylase (CYP2A1) is NADPH-dependent, and hydroxylates vitamin D3 at carbon 25 to form 25-OH (vitamin) D or CALCIDIOL or 25-OH CHOLECALCIFEROL.
• In the liver microsomes, 25-hydoxylase (CYP2R1), also hydroxylases D3 as well as D2.
• 25-hydoxylase also found in lungs, intestines, kidneys
• Enzyme is unregulated, but is more efficient when vitamin D is deficient.

After hepatic synthesis, most 25-OH D is secreted from liver and transported in blood by DBP. The blood is the single largest pool of Vit D and has 1/2 life of 2-3 weeks. In adipose, vitamin D can be stored in nonhydroxylated form. In muscle vitamin D is stored in both forms,

Vitamin D polymorphisms of proteins, activation, and transport are thought to contribute to observed variations in 25-OH D and DBP concentrations (lower in AA than caucasians).

25-OH is taken up by tissues (mostly kidneys) in response to increased concentrations of PARATHYROID HORMONE (PTH).  

25-OH D-DBP Complex binds to CUBULIN-MEDGALIN MEMBRANE RECEPTOR on the proximal tubule of the kidney to form MEGALIN-DBP-25 0H D Complex.

Complex is internalized by endocytosis where the 25-OH D is released and hydroxylated at position 1 to form vitamin's active form 1,25-OH D (CALCITRIOL) or 1,25 dihydroxyxholecalciferol.

ENZYME 1-HYDROXYLASE (CYP27B1)

• NADPH-dependant mitochondrial enzyme
• Expressed in highest concentrations in kidney, but in macrophages, skin, intestines, bone
• Kidneys release into blood for other tissues to use

Calcitriol Synthesis Regulation:

• Tightly regulated by PTH and FIBROBLAST-LIKE GF (FGB-25)
• PTH 
• Secreted when [Calcium] or [Mg] are low
• Stimulates 1-hydroxylase enzyme
• FGB-25 
• secreted by osteocytes
• Reduces 1-hydroxylase activity
• Stimulates 24-hydroxylase to promote synthesis of 24,25-(OH)2 D.
• High serum [Ca2+] and [phosphorous] also inhibit CALCITRIOL formation
• CALCITRIOL binds to vitamin D response element (VDRE) on the promoter region of 1-hydroxylase gene to decrease its own formation

Enzyme 24-hydroxylase:

• Generates 24,25-(OH)2 D from hydroxylation of 25-OH D, and 1,24,25-(OH)3 D from 1,25 (OH)2 D.
• Promotes inactivation of Vit D, further oxidized to excretory products

CALCITRIOL made in kidneys released to blood LOOSELY bound to DBP, with  half life 2-6 hours. Normal plasma [Calcitriol-DBP] is 20-40 ng/mL. 

Target tissues: widespread.

MECHANISM of ACTION

Functions

1. Serum calcium homeostasis
2. Serum phosphate homeostasis
3. Cell differentiation, proliferation, growth
4. Muscle structure and function

MOA:

• NONGENOMIC
• GENOMIC

Serum Calcium Homeostasis
(1) EFFORTS TO INCREASE CA2+

• Calcitriol and Kidneys
• Calcitriol and Intestines
• Calcitriol and Bone

(2) EFFORTS TO DECREASE CA2+

Calcitriol causes increase Ca which blocks PTH

Calcitonin (thyroid) will stimulate bone absorption of Ca by inhibiting osteoclasts and increasing urine calcium

Phosphorus Homeostasis: calcitriol increases intestinal absorption of Ph and resorption in bones to increase [Ph].

Cell differentiation, Proliferation, and Growth

Premyeloid WBC and stem cells to differentiate into macrophages and monocytes.
Stimulates stem cells in bone marrow to mature osteoclasts.
Stimulates skin epidermal cell differentiation (rx of psoriasis)
Impact cell cycle

Cacitriol and Muscle

Deficiency causes atrophy of type 2 muscle fibers ~ voltage gated calcium channels in sarcoplasm.

Other Roles

Decreases risks of :

• Cardiovascular risk reductions
• Diabetes risk
• Cancer risk
• Autoimmune

INTERACTIONS

Calcium
Phospohorous
Vitamin K dependant proteins

EXCRETION

Caclitriol hydroxylation at carbon 24 by 24-hydroxylase generates trihydroxy metabolite 24,25-(OH)2 D which is further oxidized to undergo side chain cleavage to end product Calcitrioc acid.

Excretion is by the bile.

RDA

Assumes sun exposure, suggests intake of 600 IU (15 ng)
Over age 70, 800 IU (20 ng)

5-15 min sun at midday a few times a week sufficient, increased serum [25-OH D] 45 ng/mL.


DEFICIENCY

Bone-related conditions:
Rickets - failure of bone to mineralize
Osteomalacia - failure to mineralize already formed bone
Biochemical indicators:

• elevated bone Alk Phos
• elevated PTH
• Low 24 hr -urine Ca2+ 
• Low serum Ca2+ and phosphorous and 25-OH D.

Other causes of low vit D:

• Insufficient sun exposure
• Age-associated 7-dehydroxycholesterol content in skin
• Reduced renal 1-hydroxylase activity
• Fat malabsorption (Crohn's, celiac disease, cystic fibrosis, pancreatitis, liver disease, bariatric procedures)
• Synthesis disorders ~ PTH, liver, kidneys
• Anticonvulsant drugs

RX:

• <10 ng/mL:
• 50,000 IU (1,250 mcg) 1 time a week for 8-12 weeks to achieve [25 OH-D > 30 ng/mL
• 20-29 ng/mL
• 1-2,000 IU (25-50 mcg) daily for 2-3 months
• For every 1,000 IU ingested, [] increase by 6-10 ng/mL.
• Maintenance: 800-2,000 IU (20-50 mcg) daily to maintain 20+ ng/mL

TOXICITY

TUL = 4,000 IU (100 mcg) daily - 10,000 IU (2,500 ug) daily

One of the most likely vitamins to cause toxicity, commonly from supplements for months.
Sun exposure is  protected against toxicity due to shunting to inactive metabolites.
Manifestations:

• high ionized [Ca2+] > 5.7 mg/dL
• Hypercalcuria
• Hyperphosphatemia
• Calcinosis of soft tissue including organs (kidneys, heart, lungs, blood vessels

ASSESSMENT of STATUS

Circulating vitamin D in 25-OH concentration reflects status which varies depending on intake and sunlight. Usually 30-40 ng/mL (to convert to nmol/L, multiple by 2.496).