Vitamin D

Question 1

Vitamin D3 (cholecalciferol), is normally formed in the skin from ____(a)____ in a photochemical reaction driven by the UV component of sunlight. ____(a)____ (provitamin D3) is photolyzed by the ultraviolet light of sunlight to ____(b)____, which spontaneously isomerizes to vitamin D3.

Answer 1

(a) 7-dehydrocholesterol
(b) previtamin D3

Question 2

Vitamin D3 is not itself biologically active, but it is converted by hydroxylation reactions in the ____(a)____ to ____(b)____ (1,25-dihydroxycholecalciferol), a hormone that regulates calcium uptake in the intestine and calcium levels in kidney and bone.

Answer 2

(a) kidney
(b) calcitriol

Question 3

____________ (ergocalciferol) is a commercial product formed by UV irradiation of the ergosterol of yeast.

Answer 3

Vitamin D2

Question 4

____________ is a rich source of vitamin D.

Answer 4

Cod-liver oil

Question 5

State the role of vitamin D.
[Remember: Calcitriol/1,25-dihydroxycholecalciferol. 7-dehydrocholesterol/provitamin D3. Vitamin D3/cholecalciferol]

Answer 5

1. Vitamin D plays an essential role in the control of calcium and phosphorus metabolism and is required for proper calcification of bones.
2. Calcitriol hormone (remember how it is formed?) regulates calcium uptake in the intestine and calcium levels in kidney and bone. [NB. It works with parathyroid hormone in calcium homeostasis.]
3. Like steroid hormones, calcitriol regulates gene expression by interacting with specific nuclear receptor proteins to form a complex that functions as a transcription factor.
4. Acting through nuclear receptors, calcitriol activates the synthesis of an intestinal Ca2+ binding protein essential for uptake of dietary Ca2+.

Question 6

Outline the outcomes of deficiency of vitamin D.

Answer 6

• Vitamin D deficiency in childhood produces rickets, a disease characterized by inadequate calcification of cartilage and bone.
• In adults, vitamin D deficiency leads to softening and weakening of bones, a condition called osteomalacia.
• In women past the age of menopause and in elderly men the production of calcitriol decreases.
• Deficiency can also occur in pregnant women
• This may be a cause of the serious bone loss termed osteoporosis, where the rate of resorption of bone by osteoclasts exceeds that of bone formation by osteoblasts
• Since it is 1,25-dihydroxy derivative that is essential for control of calcium ion metabolism, patients with damaged kidneys often suffer severe demineralization of their bones (renal osteodystrophy).
  How? Kidneys form 1,25 DHCC (i.e. calcitriol). So if kidneys are damaged, that won’t happen, they’ll be no proper control of calcium ion metabolism and hence renal osteodystrophy will occur.

Remember:
Deficiency in children - rickets (inadequate calcification of cartilage and bone)
Deficiency in adults - osteomalacia (softening and weakening of bones)

Question 7

Administration of synthetic 1α,25-dihydroxyvitamin D3 or a synthetic analog, provides an effective treatment for patients with renal osteodystrophy and also for children with an inherited defect in production of calcitriol. TRUE or FALSE?

Answer 7

TRUE

Question 8

Choose the correct answer.
The major role of vitamin D3 (cholecalciferol) in the body is
A. regulation of calcium and phosphorous metabolism required in proper calcification of bones
B. transcription of specific genes that mediate growth and development
C. biological antioxidant
D. post translational modification of blood coagulation factors
E. is required in oxidation-reduction reactions

Answer 8

Choice A
B - vitamin A, particularly retinoic acid
C - vitamin E, and Vitamin C helps
D- vitamin K involved in synthesis of those factors
E - vitamin B3

Question 9

The active form of Vitamin D, 1,25-DHCC regulates calcium in the following parts of the body except?
A. Bone
B. Kidney
C. Skin
D. Intestinal mucosa
E. Blood

Answer 9

Choice C.