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Why fair skin in Phenylketonuria

7K views 15 replies 10 participants last post by  scopusmount 
#1 ·
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
 
#6 ·
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 .....:eek:
 
#9 · (Edited)
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)
 
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