26
chapter 2
Amino Acids
Glutamine
This amide of glutamic acid has properties similar to
those of asparagine. The y-ami do nitrogen, derived from
ammonia, can be used in the synthesis of purine and pyrim-
idine nucleotides (Chapter 27), converted to urea in the
liver (Chapter 17), or released as NH
3
in the kidney tubu-
lar epithelial cells. The last reaction, catalyzed by the en-
zyme glutaminase, functions in acid-base regulation by
neutralizing H+ ions in the urine (Chapter 39).
Glutamine is the most abundant amino acid in the
body. It composes more than 60% of the free amino acid
pool in skeletal muscle. It is metabolized in both liver
and gut tissues. Glutamine, along with alanine, are sig-
nificant precursors of glucose production during fasting
(Chapter 15). It is a nitrogen donor in the synthesis of
purines and pyrimidines required for nucleic acid syn-
thesis (Chapter 27). Glutamine is enriched in enteral and
parenteral nutrition to promote growth of tissues; it also
enhances immune functions in patients recovering from
surgical procedures. Thus, glutamine may be classified as a
conditionally essential amino acid during severe trauma
and illness.
cocr
I
+ H3N— C— H
I
CH2
I
CH2
I
Glutamine
Unusual Amino Acids
Several L-amino acids have physiological functions as free
amino acids rather than as constituents of proteins. Exam-
ples are as follows:
1
. /
1
-Alanine is part of the vitamin pantothenic acid.
2. Homocysteine, homoserine, ornithine, and citrulline
are intermediates in the biosynthesis of certain other
amino acids.
3. Taurine, which has an amino group in the /1-carbon
and a sulfonic acid group instead of COOH, is present
in the CNS and as a component of certain bile acids
participates in digestion and absorption of lipids in
the gastrointestinal tract.
4. y-Aminobutyric acid is an inhibitory
neurotransmitter.
5. Hypoglycin A is present in unripe akee fruit and
produces severe hypoglycemia when ingested.
6
. Some D-amino acids are found in polypeptide
antibiotics, such as gramicidins and bacitracins, and
in bacterial cell wall peptides.
Amino Acids Used as Drugs
D-Penicillamine (D-/l,/l-dimethylglycine), a metabolite of
penicillin, was first isolated in the urine specimens from
patients treated with penicillin with liver disease. It is an ef-
fective chelator of metals such as copper, zinc, and lead. It
is used in the chelation therapy of
Wilson’s disease,
which
is characterized by excess copper accumulation leading to
hepatolenticular degeneration
(Chapter 37).
H3C— C— CH— COO’
I
I
+
SH NH3
D-penicillamine
A-Acetylcysteine
is
administered
in
the
ace-
toaminophen toxicity. It replenishes the hepatic stores
of glutathione (Chapter 17).
A-Acetylcysteine is also
used in the treatment of pulmonary diseases including
cystic fibrosis (Chapter 12). In patients with chronic
renal
insufficiency,
prophylactic
oral
administration
of A-Acetylcysteine have been used in the prevention
of further renal impairment due to administration of
radiographic contrast agents. In this setting presum-
ably A-Acetylcysteine functions as an antioxidant and
augments the vasodilatory effect of nitric oxide via the
formation of S-nitrosothiol (Chapter 17).
COOH
o
I
II
HS— CH— CH— NH— C— CH
3
N-acetylcysteine
Gabapentin is y-aminobutyrate covalently linked to cy-
clohexane to make it lipophilic and to facilitate its transport
across the blood-brain barrier. It is used as an anticonvul-
sant and in amyotrophic lateral sclerosis (ALS).
Gabapentin
