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Old 02-11-2012
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Biochemistry 1 α-hydroxylation of 25-hydroxycholecalciferol

In which of the following conditions 1α-hydroxylation of 25-hydroxycholecalciferol is intact?
A) End-stage renal disease
B) Fanconi renal syndrome
C) Vitamin D-resistant rickets
D) Distal renal tubular acidosis
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Ans .... C.........
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.... D.....
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C makes the most sense to me.
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C..........?
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Quote:
Originally Posted by khushi.chahal View Post
C makes the most sense to me.
Low enzyme levels in this disease... this one is not the correct answer
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Correct Answer Answer is D

Quote:
Originally Posted by Evergreen View Post
In which of the following conditions 1α-hydroxylation of 25-hydroxycholecalciferol is intact?
A) End-stage renal disease
B) Fanconi renal syndrome
C) Vitamin D-resistant rickets

D) Distal renal tubular acidosis
In A, B, and C, 1alpha-hydroxylase activity is decreased.
Source: Kaplan, Biochemistry
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What exactly is vitD resistant rickets? It siunds like its decreased end organ sensitivity to vitamin D, no?
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Quote:
Originally Posted by khushi.chahal View Post
What exactly is vitD resistant rickets? It siunds like its decreased end organ sensitivity to vitamin D, no?
Two rare genetic diseases can cause rickets in children. The critical enzyme to synthesize calcitriol from 25-hydroxyvitamin D is 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase).

1) When this enzyme is defective and calcitriol can no longer be synthesized, the disease 1alpha-hydroxylase deficiency develops. The disease is also known as vitamin D-dependent rickets type 1 or pseudovitamin D deficiency rickets.
2) When the Vitamin D Receptor is defective, the disease hereditary vitamin D-resistant rickets, also known as vitamin D-dependent rickets type 2, develops.

Both diseases are rare autosomal recessive disorders characterized by hypocalcemia, secondary hyperparathyroidism, and early onset severe rickets. Note that in both conditions 1alpha-hydroxylation of 25-Hydroxyvitamin D does not occur in kidney.
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Originally Posted by Evergreen View Post
Two rare genetic diseases can cause rickets in children. The critical enzyme to synthesize calcitriol from 25-hydroxyvitamin D is 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase).

1) When this enzyme is defective and calcitriol can no longer be synthesized, the disease 1alpha-hydroxylase deficiency develops. The disease is also known as vitamin D-dependent rickets type 1 or pseudovitamin D deficiency rickets.
2) When the Vitamin D Receptor is defective, the disease hereditary vitamin D-resistant rickets, also known as vitamin D-dependent rickets type 2, develops.

Both diseases are rare autosomal recessive disorders characterized by hypocalcemia, secondary hyperparathyroidism, and early onset severe rickets. Note that in both conditions 1alpha-hydroxylation of 25-Hydroxyvitamin D does not occur in kidney.
so the vitamin D receptors on the bone (on osteoblasts) is defective? hence, vitamin D resistance? how does this decrease vitamin D production? shouldn't the production of vitamin D remain normal (if not, increased via negative feedback)?
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Quote:
Originally Posted by khushi.chahal View Post
so the vitamin D receptors on the bone (on osteoblasts) is defective? hence, vitamin D resistance? how does this decrease vitamin D production? shouldn't the production of vitamin D remain normal (if not, increased via negative feedback)?
Actually in Kaplan, Biochemistry, It has been mentioned that Vitamin D-resistant rickets is one of the reasons for decreased 1alpha-hydroxylation in Kidney.
I also found in the website that in the form of X-linked Vitamin D-resistant rickets (X-linked hypophosphatemia), there would be a decreased activity of 1alpha-hydroxylase.

http://www.ncbi.nlm.nih.gov/pmc/arti...00151-0232.pdf

http://en.wikipedia.org/wiki/X-linked_hypophosphatemia
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Quote:
Originally Posted by khushi.chahal View Post
so the vitamin D receptors on the bone (on osteoblasts) is defective? hence, vitamin D resistance? how does this decrease vitamin D production? shouldn't the production of vitamin D remain normal (if not, increased via negative feedback)?
Because the active form of the vitamin is the one that is missing here (Lack of 1α-hydroxylation of 25-hydroxycholecalcifero), hence, even do you have vit D within your blood, it is not in the active form of the vitamin. Remember that in order to be functional, Vit D has to undergo 2 hydroxilations: one in liver and one in kidney.
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Because the active form of the vitamin is the one that is missing here (Lack of 1α-hydroxylation of 25-hydroxycholecalcifero), hence, even do you have vit D within your blood, it is not in the active form of the vitamin. Remember that in order to be functional, Vit D has to undergo 2 hydroxilations: one in liver and one in kidney.
i dont think im understanding what receptors here are resistant. is it the receptors in the kidney which will read 25-vit.D or the receptors in the bone that will read 1,25-vit.D?
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