CLINICAL CASES
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Regulation of the dietary intake of amino acids can also be important
when considering the treatment of certain defects in amino acid biosynthesis.
Phenylalanine is an essential amino acid that is also used to generate the
nonessential amino acid tyrosine. The enzyme that carries out this reaction is
the mixed function oxidase phenylalanine hydroxylase (PAH). Inherited
deficiencies in PAH are associated with a condition known as phenylke-
tonuria (PKU; see Case 38). The absence of PAH results in elevations of
phenylalanine and various phenylketones, the accumulation of which is asso-
ciated with the neurologic defects seen in this disorder. PKU can be treated by
controlling the dietary intake of phenylalanine. Diets low in phenylalanine will
help prevent excessive elevations in phenylalanine. Phenylalanine can not be
completely eliminated from the diet because it is an essential amino acid
needed for protein synthesis. In the absence of PAH activity, tyrosine becomes
an essential amino acid because it cannot be generated from phenylalanine.
C O M P R E H E N SIO N Q U E ST IO N S
[41.1] All amino acids are needed for the production of proteins in cells and
for the synthesis of important biomolecules. Which of the following
amino acids, all of which can be synthesized by the human body, must
be taken in the diet because it is not synthesized in sufficient quantities
to meet the body’s needs?
A. Asparagine
B. Glutamine
C. Methionine
D. Proline
E. Tyrosine
[41.2] A young child is in an automobile accident that requires surgical inter-
vention and substantial recovery time in the hospital. A consultation
with a nutritionist results in a specific dietary plan. The plan included
the supplementation of an amino acid that is typically considered a
nonessential amino acid. Which of the following amino acids is an
essential amino acid under conditions of enhanced growth or surgical
recovery?
A. Alanine
B. Arginine
C. Glycine
D. Serine
E. Tyrosine
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