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  #1  
Old 02-01-2012
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Biochemistry Why fair skin in Phenylketonuria

A boy is born with blonde hair, blue eyes and very fair complexion, dramatically lighter features than both of his parents. A PKU test is positive. The boy's lighter skin and hair is most likely due to which of the following?
A. Fewer melanocytes differentiating from the neural crest
B. Reduced proliferation of melanocytes in the basal layer of the epidermis
C. Elevated levels of tyrosinase in melanocytes
D. Deficiency in tyrosine in keratinocytes throughout the epidermis
E. Competitive inhibition of phenylalanine for tyrosinase in melanocytes
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  #2  
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Ans .......D...........
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Quote:
Originally Posted by Hitman View Post
Ans .......D...........


sure?? i think its E
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  #4  
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D.
Because - albinism --tyrosine is required for melanin synthesis.
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Quote:
Originally Posted by islandcrazy View Post
D.
Because - albinism --tyrosine is required for melanin synthesis.
yeah but keratinocytes dont synthesize melanin...
MELANOCYTES do
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yes true , keratinocytes dont produced melanin but tyrosine is the substrate for melanin which is decreased in PKU , thus lack of substrate in epidermis ( keratinocytes ) leads dec in melanin made by melanocyte ....

why would there be competitive inhibition of phenylalanine for tyrosinase in melanocytes in PKU ?? we have normal amt of phenylalanine but are not able to use it competitive inhibition would have been possible if phenylalanine was deficient ....... thats what i think .....
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Old 02-01-2012
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D. is the best answer here
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Quote:
Originally Posted by Hitman View Post
yes true , keratinocytes dont produced melanin but tyrosine is the substrate for melanin which is decreased in PKU , thus lack of substrate in epidermis ( keratinocytes ) leads dec in melanin made by melanocyte ....

why would there be competitive inhibition of phenylalanine for tyrosinase in melanocytes in PKU ?? we have normal amt of phenylalanine but are not able to use it competitive inhibition would have been possible if phenylalanine was deficient ....... thats what i think .....

but isnt phenylalanine a competitive inhibitor of tyrosinase too?
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Old 02-01-2012
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yup im correct

its E

heres the link


http://adsabs.harvard.edu/abs/1957Natur.179..199M
""N Jervis's introductory study, and in subsequent case reports of phenylketonuria (phenylpyruvic oligophrenia), a blonde hair colour and a fair skin have been noted in the large majority of the patients1-4. The most striking example described by Jervis was ``an idiot baby with blonde hair and blue eyes who belonged to a family of Sicilian extraction, all the members of which, for at least three generations, were of a very dark Mediterranean race''1. Three possible hypotheses may be advanced to explain the reduction of melanin formation: absence of the melanin-forming enzyme, tyrosinase; absence or decrease of the melanin precursor, tyrosine; and finally, inhibition of the tyrosine-tyrosinase reaction by phenylalanineor related abnormal aromatic metabolites which accumulate in the blood of patients with phenylketonuria .""


competitive inhibition of tyrosine-tyrosinase activity by phenylalanine is one of the causes of PKU

(think i remembered this from Kaplan qbank)

Last edited by mbbs2010; 02-01-2012 at 01:32 PM.
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it is D
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Quote:
Originally Posted by Hitman View Post
yes true , keratinocytes dont produced melanin but tyrosine is the substrate for melanin which is decreased in PKU , thus lack of substrate in epidermis ( keratinocytes ) leads dec in melanin made by melanocyte ....
now im sure the MLE is evil!! haha..
typically putting in things to confuse us...

remember.. on the test assume NOTHING... if they say keratinocytes.. they mean keratinocytes...

if they meant melanocytes.. they'd say melanocytes...

keratinocytes hv no relation with pigment formation .. n the question SPECIFICALLY ASKS.. about his skin pigmentation

so even though D maybe correct. it doesnt answer the question...

i used to get so confused with questions like these.. but then remembered someone on this forum saying -- IT ALL COMES DOWN TO THE DETAILs..
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  #12  
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At first I thought it was E, but knowing the author's history of posting very tricky questions I'd say something totally weird... like C

...another great one!
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if its phenylketonuria, why are we talking about tyrosinase? shouldn't we be discussing phenylalanine hydroxylase?
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D is the answer
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Ill go with D.......
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Quote:
Originally Posted by mbbs2010 View Post
yup im correct

its E

heres the link


http://adsabs.harvard.edu/abs/1957Natur.179..199M
""N Jervis's introductory study, and in subsequent case reports of phenylketonuria (phenylpyruvic oligophrenia), a blonde hair colour and a fair skin have been noted in the large majority of the patients1-4. The most striking example described by Jervis was ``an idiot baby with blonde hair and blue eyes who belonged to a family of Sicilian extraction, all the members of which, for at least three generations, were of a very dark Mediterranean race''1. Three possible hypotheses may be advanced to explain the reduction of melanin formation: absence of the melanin-forming enzyme, tyrosinase; absence or decrease of the melanin precursor, tyrosine; and finally, inhibition of the tyrosine-tyrosinase reaction by phenylalanineor related abnormal aromatic metabolites which accumulate in the blood of patients with phenylketonuria .""


competitive inhibition of tyrosine-tyrosinase activity by phenylalanine is one of the causes of PKU

(think i remembered this from Kaplan qbank)
yup this is the answer , it is e
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