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Old 07-31-2011
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Biochemistry Hyperphenyalaninemia and Hyperprolactinemia!

A newborn is diagnosed with hyperphenyalaninemia at birth & placed on a special phylalanine-restricted diet. Several months later, the infant has a normal serum phenylalanine level but there are some neurological abnormalities observed. His serum prolactin level is elevated. Which of the following enzymes is most likely deficient in this patient ?

A) Phenylalanine hydroxylase
B) Dihydrobiopterin reductase
C) Dopamine hydroxylase
D) Homogentisate oxidase
E) N-methyltransferase
F) Tyrosinase
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Old 07-31-2011
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yep
Dopamine hydroxylase sounds right

Too tired to think right now .....
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Last edited by axax; 07-31-2011 at 09:50 AM.
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Old 07-31-2011
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I think its B
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Phenylalanine - Tyrosine ---> Dopamine
Dopamine ---> (-) PRL

A) Phenylalanine hydroxylase===NO (special phylalanine-restricted diet -> normal serum phenylalanine = should have normal Dopamine)

So I think the answer is C) Dopamine hydroxylase
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Old 07-31-2011
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i'll go with b
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Old 07-31-2011
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my answer C) Dopamine hydroxylase(serum prolactin level is elevated)
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Old 07-31-2011
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dopamine hydroxlase sure ... and there is prolactinemia because dopamine normally inhibits prolactin secretion so no dopamin leads to increased prolactin which may present as galatorrhea and amenorrhea in patients .... and this enzyme dopamine hydroxylase requires ascorbic acid for its activity also ... and plz tell me about that this enzyme requires vitamin b6 also or not?
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This is a tricky question guys...

I will post the answer in the morning!
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i think its C as well.
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I think it is dopamine hydroxylase due to elevated prolactin
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B) Dihydrobiopterin reductase because its deficiency will decrease the dopamine level and so increase prolactin level....

And dopamine hydroxylase deficiency will increase the dopamine level so C is definitely incorrect.
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how come dopamine hydroxlase guys???
it converts dopamine into norepinephrine....
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Old 08-01-2011
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i also thought dopamine hydroxylase, but looks like it doesn't participate in dopamine synthesis itself (just checked with the book ) must be B) Dihydrobiopterin reductase.
a tricky qn!
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Old 08-01-2011
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Answer is B
I am 1000% sure
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Correct Answer The answer is B!

28% of ppl chose Phenylalanine hydroxylase, 25% of ppl chose Dopamine hydroxylase and 22% chose Dihydrobiopterin reductase.

The correct answer is in fact B) Dihydrobiopterin reductase.

Look at the attached image to get a good understanding of the pathway. The cofactor for the enzymes phenylalanine hydroxylase and tyrosine hydroxylase is Dihydrobiopterin reductase. This is the enzyme deficient as the patient is unable to convert Tyrosine to Dopa leading to low levels of Dopamine and high levels of Prolactin.

In classic PKU (phenylalanine hydroxylase deficiency), pts do not have any metabolic errors while forming dopamine and moreover, this pts phenylalanine levels have been maintained due to diet therapy.

The answer is not Dopamine hydroxylase because this enzyme is used to convert Dopamine to Norepinephrine. If its deficient or non-functional, there will be decreased norepinephrine and high levels of dopamine would accumulate causing decreased levels of prolactin.

lol this question was tricky! Its from UW and I got it wrong as well

Hyperphenyalaninemia and Hyperprolactinemia!-uw212.png
click image to enlarge
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Old 08-01-2011
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Star A

A) Phenylalanine hydroxylase
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Old 08-01-2011
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@hope2pass, thanks!

Can you pls explain sth?

Several enzymes are involved with dopamine formation:
A) Phenylalanine hydroxylase ,
B) Dihydrobiopterin reductase
C) Tyrosin hydoxylase
D) dopamine carboxylase

So the question is why dihydrobiopterin reductase, not other enzymes?
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Correct Answer Further explanation

Quote:
Originally Posted by usluipek View Post
@hope2pass, thanks!

Can you pls explain sth?

Several enzymes are involved with dopamine formation:
A) Phenylalanine hydroxylase ,
B) Dihydrobiopterin reductase
C) Tyrosin hydoxylase
D) dopamine carboxylase

So the question is why dihydrobiopterin reductase, not other enzymes?

A) Phenylalanine hydroxylase - Its not the right answer because this enzyme is deficient in classic Pheynlketonuria. It converts ingested phenylalanine to tyrosine. The pt in question has normal phenylalanine levels after the diet therapy but now has low dopamine levels. PKU pts do not have any metabolic error when producing dopaime, therefore this option is eliminated.

C) Tyrosine hydroxylase & D) Dopamine decarboxylase - these options were not given in the question stem. Refer to the diagram provided in the explanation.

Tyrosinase is an enzye that synthesizes melanin from tyrosine and is deficient in Albinism.

Dopamine hydroxylase is the enzyme that catalyzes the synthesis of norepinephrine from dopamine. Deficiency of this would lead to accumulation of high levels of dopamine not a decrease in dopamine.

The best answer is Dihydrobiopterin reductase. This enzyme converts BH2 to BH4 and a deficiency of the enzyme leads to a deficiency of BH4 (tetrahydrobiopterin) which is used as a cofactor for synthesis of tyrosine, DOPA, dopamine and NO.

Hope this helps
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