AMINO ACIDS AND AMINO
ACID METABOLISM
All objectives listed are in the cognitive domain unless otherwise noted. The
student, at the end of the instructional period, is responsible for meeting
these objectives by achieving a cumulative score of 70% or better on all problem
sets, case studies, major exams, quizzes, and library assignments.
Objectives are indicated by a number followed by a purposeful question or
statement. The student, upon completion of the classroom component of clinical
biochemistry, will be responsible to successfully:
01
LIST THE ESSENTIAL AMINO ACIDS.
[1] arginine (ARG),
[6] lysine (LYS)
[2] histidine (HIS),
[7] methionine (MET)
[3] isoleucine (ILE),
[8] phenylalanine (PHE)
[4] leucine (LEU),
[9] threonine (THR)
[5] tryptophan (TRP)
[10] valine (VAL)
NOTE: Arginine and histidine may be
designated as semi-essential amino acids because they are synthesized in small
(but insufficient) amounts within the body. For the human body to do well,
these two amino acids must be included in the diet, especially for growing
children.
02
LIST THE NON-ESSENTIAL AMINO ACIDS.
[1] glycine (GLY)
[6] cysteine (CYS)
[2] alanine (ALA)
[7] asparagine (ASN),
[3] proline (PRO)
[8] glutamine (GLN),
[4] tyrosine (TYR)
[9] glutamic acid (GLU),
[5] serine (SER)
[10] aspartic acid (ASP)
03
WRITE THE GENERAL CHEMICAL STRUCTURE FOR AN AMINO
ACID
R - HC(NH2) - COOH
04
EXPLAIN THE PURPOSE OF TRANSAMINATION REACTIONS IN
AMINO ACIDS
Transamination reactions are catalyzed by a group of enzymes known as amino
transferases. These reactions generally provide a mechanism for redistribution
of nitrogen among the amino acids. It is a reaction sequence in the synthesis of
non-essential amino acids. Example:
Transamination reactions have equilibrium constants and the direction of the
reaction is determined by the intracellular concentration of the substrate and
product. This means that transamination reactions not only carry out amino acid
synthesis, they can also degrade amino acids. Example of a degrading reaction
using an α-keto acid as a substrate:
Transamination reactions go on constantly and there is a continuous
redistribution of amino groups among amino acids. NOTE: If radioactive
15N is
introduced into an amino acid pool via a single entity, it redistributes among
all the amino acids. It is an indicator that amino acids are constantly being
degraded and reassembled. All transaminase enzymes require the coenzyme
pyridoxal phosphate. The ability to transfer amino groups from one amino acid to
another helps to maintain a proper ratio of various amino acids.
05
DISCUSS PYRIDOXAL PHOSPHATE AS A COENZYME
Pyridoxal phosphate is derived from vitamin B6 (pyridoxine).
Nutritional
requirements for this vitamin are very low and deficiency disorders rarely
occur. This vitamin can be sequestered by drugs and poisons, which might induce
a deficiency state. Consider for example, a patient being treated with Isoniazid
for tuberculosis. Isoniazid will react covalently with pyridoxal, rendering it
unavailable for reactions. Prolonged treatment can induce a deficiency state
unless the patient supplements his diet with the vitamin. It is a unique
coenzyme, taking part in amino acid decarboxylations, racemizations, and
modification reactions of amino acid side chains. In the reaction process,
pyridoxal forms a Schiff base with the amino acid and a cation (a metal ion or
proton) is required to stabilize the reaction, forming a aldimine. See the
following example of the enzyme bound pyridoxal phosphate.
06
DISCUSS THE GLUTAMATE DEHYDROGENASE REACTION
This enzyme serves as a mechanism to dispose of surplus amino acids or to use
ammonia to form amino acids. In animal cells, this enzyme probably functions as
a catabolic enzyme to supply α-ketoglutaric acid for oxidation in the Kreb’s
cycle. Bacteria that use ammonia as a nitrogen source to assimilate nitrogen
into amino acids use this enzyme to form glutamic acid. NAD+ is the principle
cofactor in animals, however they can also use NADP+. This enzyme is located in
mitochondria. See reaction examples:
Glutamate is
deaminated to α-ketoglutarate and ammonia
α-ketoglutarate
is converted to glutamate
07
DISCUSS THE ROLE OF L-AMINO ACID OXIDASE
This enzyme is a flavoprotein enzyme found in liver and kidney. It will oxidize
an amino acid to its corresponding α-keto acid and ammonia. The reaction
requires molecular oxygen as the electron acceptor and oxygen will consequently
be reduced to H2O2. FAD is the coenzyme and mediates electron transfer and
activation of molecular oxygen for the reaction. See reaction scheme:
Since peroxide is a by-product of this reaction, it is potentially destructive
to renal and hepatic tissue, these tissues contain the enzyme catalase, which
decomposes the peroxide.
08
DISCUSS AMMONIA
AND HOW THE BODY HANDLES THIS TOXIC ELEMENT
Ammonia is produced in all tissues of the body. The normal plasma concentration
ranges from 25 μmol/L to 60 μmol/L. The major source of ammonia is from the GI
tract. Ammonia is toxic to body cells, especially nervous tissue. Ammonia is a
gas and readily diffuses across cell membranes. Ammonia from the GI tract is the
result of bacterial action in the colon and lower part of the small intestine
(ileum). Primary enzymes that responsible for gut ammonia production are
proteases, ureases, and oxidases. Normally, ammonia laden blood from the
intestines passes first through the liver (the site of major ammonia
detoxification). Ammonia will enter the Krebs-Henseleit Urea Cycle in the hepatocyte and be converted to urea. See simplified schematic of this urea or ornithine cycle.
NOTE
Inherited deficiencies of urea cycle enzymes are major causes of hyperammonemia
in infants. The most common cause of hyperammonemia is advanced liver disease.
Both chronic and acute liver problems can be characterized by impaired ammonia
metabolism.
The deamination of amino acids produces ammonia. The protective mechanisms of
the body will covert the ammonia to other metabolites. Consider the role of
glutamine in transporting ammonia in a non-toxic form. Glutamine is an amino
acid amide used for protein synthesis. See reaction schematic:
Glutamine circulates to the kidney to complete the process of eliminating
ammonia from the body. Consider the following reaction schematic for elimination
of ammonia.
Ammonia is a gas and readily diffuses across the membranes of the tubular cells
into the lumen where it readily combines with H+ to form the less toxic ammonium
ion. Once the ammonium ion is formed, it is trapped in the lumen and is excreted
in the urine. It is excreted with other anions (PO4-, Cl-, SO4-). 60% of the
hydrogen ion excreted in the kidney’s is excreted as NH4+.
Consider a safety mechanism that occurs when there is systemic acidosis.
Glutamine is released from the muscle. The liver will slow or cease its uptake
of glutamine and also release additional glutamine. The cumulative effect is to
increase the glutamine levels in blood. Glutamine will be taken up by the renal
tubular cells and converted first to glutamate. The tubular cells will also
convert glutamate to α-ketoglutaric acid. This release of ammonia binds to the
H+ to form NH4+ and reduce the amount of hydrogen ion in the body.
09
EXPLAIN THE ROLE OF ASPARAGINE IN REGULATING AMMONIA
LEVELS
Asparagine does not have any specific function in the body other than to be
incorporated into polypeptides and proteins. It can be synthesized using
ammonia, but is usually synthesized from aspartate using glutamine as an amine
donor. See following reaction.
NOTE
Some forms of leukemia cells cannot synthesize their own asparagine as is in the
case of the typical animal cell. Asparaginase is an enzyme that converts
asparagine to aspartate. Using asparaginase therapeutically lowers serum
asparagine and deprives neoplastic cells of a growth element.
10
EXPLAIN
HOW THE BRAIN DETOXIFIES AMMONIA
Tissues of the brain have an abundant supply of glutamine synthetase which can
convert glutamate to glutamine. The blood contains high levels of glutamine
following the ingestion of protein. The body catabolize proteins into amino
acids and amino acids are a source of ammonium ions. Glutamine is thought to be
a storage and transport form of ammonia. See following reaction:
11
EXPLAIN HOW HYPERAMMONEMIA MIGHT OCCUR AND IF
IT DOES, WHAT HAPPENS
Hyperammonemia occurs whenever there is an increase in blood ammonia greater
than 60 μmol/L. Normal range is 30 - 60 μmol/L. Most ammonium ions in portal
blood are detoxified in the hepatocytes by their conversion to urea. If the
detoxifying system should fail and urea ceases to be formed at a sufficient
rate, the patient will experience blurred vision, tremors, and slurred speech. If hyperammonemia continues, then coma occurs followed by death. If the hyper-ammonemia
is due to a malfunctioning liver or a portal blood flow problem, then the
condition is known as hepatic coma.
In the newborn, delayed development of the urea cycle has been observed, in
which case the infant experiences a transient hyperammonemia.
12
DISCUSS THE GLYCOGENIC AND KETOGENIC EFFECTS OF
AMINO ACID METABOLISM
In the early research studies of amino acid metabolism, comparisons were made
between fat and carbohydrate metabolism. Physiological experimentation found
that some amino acids caused an overall increase in glucose levels. These amino
acids were designated as glycogenic. Continued research identified amino acids
that increased the quantity of circulating ketone bodies. These amino acids were
designated as ketogenic. Some of the amino acids were noted to cause both
increased glucose and ketone bodies. Most amino acids are glycogenic. Only leucine and lysine are completely ketogenic. Isoleucine, phenylalanine,
tyrosine, and tryptophan are thought to be both glycogenic and ketogenic.
AMINO ACIDS
Glucogenic and
Ketogenic Ketogenic Glucogenic
Phenylalanine Leucine Alanine Arginine
Tyrosine *Lysine Cysteine
Aspartic acid
*Tryptophan Serine Asparagine
Isoleucine Glycine Glutamic acid
Glutamine Hydroxyproline
Threonine Histidine
* These are thought to fall Proline Methionine
in this category. Valine Tryptophan
13
BRIEFLY DISCUSS ALANINE METABOLISM
Alanine is a glycogenic, non-essential amino acid and has limited biological
functions. It is incorporated into proteins and participates in transamination
reactions. The principle reaction is as follows:
14
ILLUSTRATE A SUMMARY/OVERVIEW OF AMINO ACID
METABOLISM
15
BRIEFLY DISCUSS BRANCHED-CHAIN AMINO ACID METABOLISM
The amino acids; leucine (essential and ketogenic), isoleucine (ketogenic,
glycogenic, and essential) and valine (glycogenic and essential) are involved.
When these amino acids are in excess of what is physiologically needed, they
will be transaminated to their corresponding branched-chain α-keto acid.
Leucine is converted to 2-ketoisocaproate by leucine-α-keto-glutarate
transaminase. This keto acid will undergo an additional four catabolic reactions
to produce 3-hydroxy-3-methyl-glutarylCoA (HMG-CoA), which is the end-product in
the leucine catabolic pathway. Leucine metabolism occurs in the mitochondria.
Here HMG-CoA will be metabolized to acetoacetate (ketogenic) and acetyl-CoA (ketogenic).
Because steroid synthesis occurs in the cytoplasm, the catabolic products of
leucine do not enter into steroid synthesis.
Isoleucine is converted to α-keto-β-methylbutyrate by
leucine-α-keto-glutarate
transaminase. There will be four additional reaction steps to form
methylacetoacetyl-CoA. The next reaction step will be to form propionyl-CoA
(glycogenic) and acetyl-CoA (ketogenic). Propionyl is carboxylated to form
methylmalonyl-CoA, which can be converted to succinyl-CoA and enter the Kreb’s
cycle.
Valine is converted to 2-ketoisovalerate by branch-chain amino acid transaminase.
There are five additional catabolic steps to form methylmalonyl semialdehyde.
The final reaction step forms propionyl-CoA, which is glycogenic.
16
BRIEFLY DISCUSS BRANCHED-CHAIN AMINOACIDURIA
Metabolic deficiencies caused by defective branched-chain metabolism are
uncommon. In most cases, this type of aminoaciduria is usually detected through
a urine test. The most common deficiency is due to a defect in branched-chain
keto acid dehydrogenase which results in the accumulation of α-keto acids and
corresponding hydroxy acids. These products give the urine a characteristic
maple syrup odor, hence “maple-syrup urine disease”. If the disorder is allowed
to go undetected and without medical intervention, the following symptoms
(single or combination) appear early in the infants development (first or second
week).
a. hypotonia (loss of muscle tone) e. feeding difficulties
b.
↓ intraocular (eye) pressure f. hypoglycemia
c. vascular relaxation g. convulsions
d. lethargy h. mental retardation
The laboratory role is to detect this disorder through urine testing, abnormal
amino acid electrophoresis patterns, and/or enzyme deficiencies.
17
DISCUSS THE METABOLISM OF THE HYDROXY AMINO ACIDS, SERINE, AND THREONINE
These are glucogenic amino acids because they can be degraded to pyruvic acid
and then to acetyl-CoA.
Serine is an active metabolic compound and can be catabolized to a variety of by
products. Because it is freely interconvertible with glycine, alternate
metabolic routes are available for synthesis of other metabolites. See
illustration.
There are two pathways of
threonine degradation. One pathway begins with serine/threonine
dehydratase to form α-ketobutyrate which is reduced by a dehydrogenase to
propionyl-CoA (glycogenic pathway because it is converted to succinyl-CoA). The
other pathway uses threonine dehydrogenase to convert threonine to α-amino-β-ketobutyrate,
which undergoes a spontaneous reduction to aminoacetone which is converted to
pyruvate in two additional reaction steps.
18
DISCUSS THE METABOLISM OF GLYCINE
Glycine is oxidized by simple enzyme systems. In one system, d-amino acid
oxidase converts glycine to glyoxalate which is a precursor of oxalate. An
excess of oxalate tends to favor the formation of oxalate renal stones. In a
second system, glycine is converted to serine by serine transhydroxymethylase to
serine, which in turn is converted to pyruvate by serine dehydratase. Glycine
can be combined with succinyl-CoA to form δ-aminolevulate, the first step in the
formation of protoporphyrins for the synthesis of heme in hemoglobin. Refer to
the previous objective (#17) for an overview of other metabolic fates of glycine.
19
EXPLAIN THE CLINICAL SIGNIFICANCE OF NON-KETOGENIC HYPERGLYCINEMIA
CAUTION: This is NOT hyperglycemia. This is a serious disease in which the blood
levels of glycine are significantly elevated. This is an inherited disorder in
which the enzyme propionyl-CoA carboxylase (ATP-hydrolyzing) (propionyl-CoA:carbon-dioxide
ligase [ADP-forming] is defective. The prognosis for this patient is very poor,
with death in early infancy. It is characterized by severe mental retardation, neutropenia, and ketosis. Glycine can function as an inhibitory neurotransmitter
in the brain and spinal cord. If glycine is in high concentration, then a
constant inhibitory effect occurs which may induce problems.
NOTE
Elevated levels of glycine will result in glycinuria. Excess glycinuria may be
deaminated to form glyoxylate, which is a precursor of oxalate. If there is a
defect in the mechanism to catabolize glyoxylate, it will be oxidized to
oxalate. Increased oxalate may favor the formation of urinary stones. Glycinuria
may also be due to a defect in the tubular reabsorption system for glycine.
20
DISCUSS THE METABOLISM OF PROLINE, ORNITHINE, AND ARGININE
The common denominator in the metabolism of these three amino acids is this,
they are built directly from glutamate.
Proline (a glycogenic amino acid) can be oxidized to dehydroproline (without
deamination) and the second oxidation step opens the ring to yield glutamate,
which can be transaminated to α-ketogluterate (a Kreb’s cycle intermediate).
Arginine (a glycogenic amino acid) is catabolized to ornithine by the enzyme arginase, which hydrolytically removes the guanidino group as urea. Most of the arginine that is produced in the body or ingested in the diet is not needed in
protein synthesis. It is an intermediate in the urea cycle. Arginine is a
precursor to polyamines (spermidine and spermine). Polyamines have a functional
role in cell proliferation and growth; stabilization of intact cells,
organelles, and membranes; function as polyanions to DNA and RNA; and inhibit
certain enzymes. Arginine is also a source of nitric oxide (NO). Nitric oxide is
produced by cytotoxic macrophages, by neutrophils (in which case, NO is known to
act as a platelet inhibitory factor). If arginine is a nitrous oxide source, it
is metabolized to citrulline.
Ornithine is an intermediate in arginine metabolism. It is also an intermediate
in the urea cycle. Refer to the following illustration for the catabolism of
arginine.
21
DISCUSS THE OVERALL METABOLIC FATE OF METHIONINE AND CYSTEINE
Both of these amino acids are sulfur containing amino acids and are glycogenic.
The catabolism of methionine (an essential amino acid) occurs through
homocysteine and continued catabolism yields α-ketobutyrate. Cysteine is
catalyzed to pyruvic acid and then to acetyl-CoA.
Methionine serves as a source of sulfur and methyl groups in human metabolism.
When a methyl group is transferred, methionine become homocysteine. If methionine becomes in short supply, homocysteine can be resynthesized. See
illustration for degradation pathway for methionine:
Cysteine is the major source of sulfur for the body. The sulfur of cysteine is
made available for detoxifying reactions and to be incorporated into proteins
such as ferredoxin.
22
DESCRIBE THE METABOLIC FATE OF HOMOCYSTEINE
There are three major routes by which homocysteine is metabolized.
FIRST: If the body needs cysteine, homocysteine will first condense with serine
. See the following reaction.
SECOND: If methionine is in short supply, homocysteine will be remethylated. See
the following reaction.
THIRD: If methionine and cysteine are in ample supply, homocysteine is
catabolized to α-ketobutyrate, ammonia, and hydrogen sulfide.
23
EXPLAIN THE CLINICAL SIGNIFICANCE OF THE DISORDER HOMOCYSTINURIA
Homocystinuria is caused by a deficiency of cystathionine synthase (in the
degradation pathway for methionine) and allows homocysteine to be excreted in
the urine. One of the first symptoms that something is wrong is the appearance
of mental retardation. How homocysteine effects so many abnormalities is not
understood. It is postulated that homocysteine reacts with and blocks the lysyl
aldehyde groups on collagen, interfering with cross-linking. Ocular abnormalies
occur, including lens dislocation. Skeletal abnormalities such as osteoporosis,
joints not being able to fully extend, and spongy bone formation in the vertebra
are reported. Thrombo-embolism and vascular occlusions are characteristic,
causing infarcts throughout the body. Vitamin B12 is poorly absorbed in the
intestines. Prognosis for this disorder is poor because of the tendency for thromboembolisms and coronary infarctions that can occur at any age.
24
DESCRIBE THE FUNCTION OF RHODANESE
Rhodanese is an enzyme that catalyzed the transfer of sulfur from thiocysteine
(and other sulfane donors) to a variety of acceptors. One example: if cyanide (CN¯)
is present, cyanide will react with rhodanse and thiocysteine to form the
non-toxic thiocyanate. See illustration.
25
ILLUSTRATE THE MAJOR PATHWAY FOR THE CONVERSION OF CYSTEINE TO PYRUVATE AND
TAURINE
26
DESCRIBE TAURINE AND ITS FUNCTION
Taurine (also 2-aminoethane sulfonate) forms conjugates with bile acids (glycocholic
acid and glycochenodeoxycholate acid) to form primary bile acids, Taurine is
also known to inhibit nerve impulse transmission. Conjugation occurs in the
liver. In the GI tract, microflora can de-conjugate these two bile acids to
deoxycholic acid and lithocholic acid.
27
DISCUSS THE CLINICAL SIGNIFICANCE OF CYSTINURIA
This disorder that involves a defect in the stereospecific tubular transport of
the disulfide cystine and three other basic amino acids (arginine, ornithine,
and lysine) from the glomerular filtrate into the peritubular capillaries. Cysteine in extracellular fluids is oxidized to cystine. There is not a defect
in the metabolism of these amino acids. Cystine has low solubility in urine and
this predisposes the patient toward the formation of calculi. Patients, to
counteract this disorder will drink large amounts of water or take medication to
promote soluble derivatives of cystine. If a patient is suspected of having this
disorder, a simple screening test will be to collect a first morning specimen
and examine the centrifuged urine sediment for cystine crystals.
28
DISCUSS THE CLINICAL SIGNIFICANCE OF CYSTINOSIS
This is a very serious disease (also known as cystine storage disease) in which
the disulfide cystine collects/accumulates in lysosomes. It is thought to be due
to a autosomal recessive trait. This is an problem of impaired efflux of cystine
from lysosomes due to a defect in lysosomal function. The cystine content of
tissues can be 100 times that of normal if the disorder manifests in infants. If
the disease manifests in adults, it is less serious and cystine content in the
tissues will be around 30 times greater than normal. The adult form of
cystinosis is benign. In the infantile form, death usually occurs before the
tenth year. The infantile form is characterized by the features of dwarfism,
rickets, polyuria, dehydration, metabolic acidosis, aminoaciduria,
pyelonephritis, and ocular abnormalities. The adult form of cystinosis symptoms
is usually limited to ocular abnormalities.
29
DISCUSS THE METABOLISM OF PHENYLALANINE
Phenylalanine is an essential aromatic amino acid and is both glycogenic and
ketogenic. Normally, all phenylalanine is degraded to tyrosine in the liver.
After this single reaction step, phenylalanine is degraded in the same pathway
as for tyrosine. See the following illustration for the conversion of
phenylalanine to tyrosine and refer to Objective #30 for the degradation scheme
for tyrosine.
Tetrahydrobiopterin is a coenzyme for phenylalanine and serves as a cofactor or
other hydroxylases. If phenylalanine hydroxylase is absent then a serious
disorder known as phenylketonuria (PKU) is resultant.
30
DISCUSS AND/OR ILLUSTRATE THE TYROSINE CATABOLISM ROUTE TO ACETOACETATE AND
FUMARATE
Tyrosine is degraded by a major pathway (see illustration) but can be degraded
to other metabolites by other minor pathways. Tyrosine is a aromatic,
non-essential amino acid that is both ketogenic and glucogenic.
31
DISCUSS THE CLINICAL DISORDER, PHENYLKETONURIA (PKU)
This disorder is so named because of the following metabolites found in urine:
phenylpyruvate, phenyllactate, and phenylacetate. The three metabolites give PKU
its distinctive mousy or barnyard odor. This recessive, autosomal condition
expresses itself in about one in 10,000 births. The deficient enzyme,
phenylalanine hydroxylase, causes phenylalanine to accumulate in increased
quantities, resulting in phenylalanine being degraded in other minor pathways to
produce the three metabolites, resulting in their being excreted in urine in
very large amounts (up to 2.0 grams/day). If phenylalanine levels in blood rise
to 20 mg/dL or greater, the patient is deemed to have PKU. If PKU is allowed to
go undetected and untreated the following results: (1) mental retardation,
setting in within a few days of birth and becoming significant in a few weeks.
(2) hypo-pigmentation of skin and hair, and (3) eczema. This disorder is best
detected by evaluating the blood of the infant about 2-4 days following birth.
Allowing the baby to establish its diet eliminates most false negatives.
Acceptable testing methods include the (1) Guthrie test, (2) fluorescence
technique, (3) electrophoresis, and (4) chromatography methods. Urine screening
techniques using ferric chloride are not sensitive enough. By the time PKU
metabolites can be detected in urine, irreversible damage to the central nervous
system has begun. The PKU disorder is expressed earlier in boy than in girls,
because of the slower rise in phenylalanine metabolites in females. It is known
that serotonin levels are low in these patients.
32
BRIEFLY EXPLAIN WHY HYPOPIGMENTATION IS A SYMPTOM IN PHENYLKETONURIA
Hair and skin pigmentation is due to the presence of melanin. Melanin is
synthesized from tyrosine. Impairment in the tyrosine pathway affects melanin
production, producing hypo-pigmentation.
33
ILLUSTRATE THE THREE METABOLITES OF FAULTY
PHENYLALANINE CATABOLISM
34
DISCUSS THREE VARIANTS OF CLASSIC PHENYLKETONURIA
Transient Phenylketonuria. This is observed in a few patients at birth. It will
spontaneously disappear within a few weeks. It is assumed that the problem is
due to a delay in the maturation of the enzyme phenylalanine hydroxylase.
Persistent hyperphenylalaninemia. It has been found that there is a small
population of patients found in mass screenings, whose phenylalanine blood
values range from 4 to 16 mg/dL. Normal is 0.8 to 1.8 mg/dL. This is thought to
be a mild, benign form of PKU and does not require medical intervention.
Malignant hyperphenylalaninemia. These patients demonstrate phenylketonuria, yet
they do not respond to therapy. They usually develop neurological defects and/or
seizures. They generally die within the first few years of life. There is one
thing that is distinctive about these patients, a pteridine coenzyme
dihydrobiopterin reductase (resembles folic acid) cannot be demonstrated. It is
estimated that 3.0% of patients diagnosed with PKU have this form.
35
EXPLAIN WHAT ALCAPTONURIA
HAS TO DO WITH TYROSINE
Alcaptonuria has the distinction of being the first disorder to be identifies as
a genetic disease, an inborn error of metabolism. This disorder is due to a
defect in homogenistic acid oxidase which allows for the accumulation of
homogenistic acid (HGA) and it subsequent abnormal secretion in urine. Tyrosine,
as it is being catabolized to its end products, has homogenistic acid as an
intermediate product. The urine is initially a normal color but on standing, the
colorless hydroquinone (homogenistic acid) will auto-oxidize to a quinone and
form a dark color. Its presence in urine can be detected by testing with ferric
chloride (a transient blue color produced) or silver-nitroprusside (a black
color produced). A non-diabetic urine specimen continuing this metabolite if
tested with a strip, will produce a negative glucose test, but the clinitest
tablet test will produce a orange-red color. One of the first clues that
something is wrong with an infant is the appearance of brown or black stained
diapers. One serious feature of alcaptonuria is the oxidation of HGA to assorted
pigments that deposit into bones, connective tissue, and other body organs. This
produced such complications as arthritis, cardiac disorders, and liver problems.
It has been known to deposit in ear cartilage forming black pigment spots. This
pigmented ear condition is known as Ochronosis. This disorder is somewhat benign
and there is not an effective treatment for it.
NOTE
Other chemicals known to cause the darkening of urine are
(1) melanin, (2) phenolic compounds, and (3) gentistic acid (a
salicylate metabolite).
36
EXPLAIN WHAT ALBINISM HAS TO DO WITH TYROSINE
METABOLISM
Skin coloration is genetically controlled and there are a number of genetic loci
in the human known to effect skin pigments. The chemical basis of skin
pigmentation has been established only for classical albinism. Melanin is the
skin color pigment. Melanin is derived from tyrosine in one of the minor
biosynthetic pathways. It is known that a tryrosinase deficiency will result in
the non-production of melanin.
37
EXPLAIN THE CLINICAL SIGNIFICANCE OF FUMARYCLACETOACETATE TRANSFERASE
DEFICIENCY
Designated as Type I, hepatorenal tyrosinemia, this disorder is a serious
disease that is characterized by liver failure, polyneuropathy, rickets, and
generalized renal tubular dysfunction (renal failure). Urine will contain (1)
tyrosine, (2) n-acetyltyrosine, (3) p-hydroxyphenyl acetate, (4) p-hydroxyphenyl
lactate, (5 ) p-hydroxyphenyl pyruvate, (6) and tyramine. There are a variety of
other amino acids excreted. The most reliable indicator of this disorder is the
presence of p-hydroxyphenyl pyruvic acid (PHPPA), in which a 25 fold increase in
urine excretion occurs. Most cases have been found in a French-Canadian
population in Quebec. There is an increased risk of hepatoma development in
children.
38
DESCRIBE AND/OR ILLUSTRATE HOW TYROSINE CAN BE EMPLOYED IN THE SYNTHESIS OF CATECHOLAMINES
The catecholamines are neurotransmitters in the sympathetic neurons. They
consist of dopamine, norepinephrine, and epinephrine. The sympathetic neurons
and adrenal glands are the principle sites of synthesis. Their oxidation pathway
is as follows:
L-DOPA (3,4-dihydroxyphenethylamine), catalyzed from tyrosine by
tyrosine hydroxylase is the first physiological agent in this sequence of reactions.
There are two catabolic pathways from DOPA, one pathway forms melanin and the
other pathway forms catecholamines.
Norepinephrine (also called noradrenalin) is stored in the terminal axons of
sympathetic postganglionic nerves and also in certain synapses of the CNS. It
stimulates the alpha and beta receptors of its target cells to effect
vasoconstriction of the arterioles of the skin, pupil dilation, and relaxation
of the intestinal tract. Norepinephrine is inactivated by O-methylation and N-deamination
to produce two principle metabolites: normetanephriene and vanillylmandelic acid
(VMA).
Epinephrine (also called adrenalin) is stored in chromaffin granules and
released in response to hypoglycemia, fear, anger, or stress. It strongly
stimulates the sympathetic nervous system and enacts the following: (1)
glycogenolysis in the liver and muscles, (2) lipolysis in adipose tissue, and
(3) increases the heart rate. It is an effective drug in the treatment of
anaphylaxis. It is inactivated by O-methylation and N-deamination to produce to
principle metabolites: metanephrine and vanillylmandelic acid (VMA). Urinary
assays for the metabolites of norepinephrine and epinephrine is useful in
assaying the function of the adrenal gland. It can also confirm the presence of
pheochromocytoma (a chromaffin-positive tumor of the adrenal gland and certain
ganglionic tissues of the body that can secrete epinephrine and/or
norepinephrine.)
39
BRIEFLY EXPLAIN THE CLINICAL RELATIONSHIP OF
TYROSINE TO PARKINSON’S DISEASE
Tyrosine is the precursor of dopamine, see Objective 38. It has been established
that there are certain cells in the substantia nigra and locus coeruleus (small
regions of the brain) that produce dopamine as a neural transmitter. As these
cells deteriorate over time, so does the production of dopamine. Some degree of
success has been obtained in treating Parkinson’s disease with L-DOPA (trade
name = levodopa), as a precursor of dopamine. It has not been successful in
certain populations of patients because of the side-effects of nausea, vomiting,
hypotension, cardiac arrhythmia’s and other central nervous system symptoms.
40
CORRELATE THE CLINICAL RELATIONSHIP OF TRYPTOPHAN TO
5-HYDROXYINDOLEACETIC ACID
Tryptophan can be converted to 5-hydroxytryptamine (serotonin). Serotonin is
found in the CNS, where it functions as a transmitter in relationship to
sleeping. Serotonin is degraded to 5-hydroxy-indoleacetic acid (5-HIAA), which
is excreted in the urine. If there is a malignant carcinoid tumor of the
argentaffin cells, then excess serotonin is produced, which in turn elevates the
urinary 5-HIAA levels. Carcinoid tumors tend to distribute in the GI tract,
pancreatic ducts, bronchial tree, thymus, ovary, thyroid gland, uterus, and
salivary glands. If urinary 5-HIAA exceeds 25 mg/dL, then the test is positive
for the tumor. When testing for 5-HIAA, the patient is to avoid pineapples,
tomatoes, avocados, bananas, chocolate, walnuts, eggplant, and plums because of
their serotonin content. Medications, such as glycerol guaiacolate (also called
guaifenesin), an expectorant and reserpine (an antipsychotic) are to be ceased. Other medications include: phenobarbital, phenacetin, ephedrine, methocarbamol,
and nicotine are to
be avoided. Carcinoid tumors will produce in addition to 5-HIAA: histamine,
insulin, ACTH, catecholamines, prostaglandins, and growth hormones. See
following illustration of catabolic pathway.
41
DISCUSS THE CATABOLISM OF TRYPTOPHAN
Tryptophan catabolism takes place in the hepatocytes and gives rise to alanine
(making tryptophan glucogenic) and to acetyl-CoA (allowing it to have a
ketogenic role). The major pathway is through Kynurenine to gluteryl-CoA. See
the following illustration:
Tryptophan also serves as a source of nicotinic acid (classified as a vitamin
and known as niacin). A deficiency of both tyrosine and niacin produces a
condition known as pellagra. Tryptophan is a precursor for serotonin.
42
DISCUSS THE CLINICAL IMPORTANCE OF SEROTONIN
Serotonin is a powerful, smooth muscle stimulant and is an important
neurotransmitter. It is involved in behavioral patterns, sleep, pain perception,
and mental depression. It is found in a variety of body organs, especially the
platelets and mast cells. It causes vasoconstriction of the smooth muscles in
arterioles and bronchioles. It is thought to act as a transmitter in the GI
tract to effect the release of peptide hormones.
43
DISCUSS THE PHYSIOLOGICAL IMPORTANCE OF HISTIDINE
Histidine is more important in one carbon metabolism than it is in energy
production. Histidine is a glucogenic and essential amino acid. Its major route
of metabolism is the formation of glutamate where it enters the Kreb’s cycle as
α-ketoglutarate. See illustration:
44
DISCUSS THE CLINICAL SIGNIFICANCE OF HISTIDINEMIA
Histidinemia, due to a deficiency in histidase, results in increased blood
levels of histidine. It is characterized by speech defects and sometimes mental
retardation. It can be detected by (1) skin biopsies and testing for the absence
of histidase, (2) the absence of urocanate in sweat or urine, and (3) presence
of imidazolepyruvate in urine using the ferric chloride test. If medical
intervention is needed, which is not often, then histidine is withheld from the
diet.
45
DISCUSS THE CATABOLISM OF ASPARTATE
Aspartic acid or aspartate is a non-essential amino acid and is glucogenic.
Aspartate is metabolically active because it can be interconverted with the C4 dicarboxylic acids within the Kreb’s cycle. Transamination is the major route of
synthesis and degradation of aspartate. See following example:
Aspartate, as a nitrogen donor, can be deaminated to fumarate in the urea cycle
by first forming argininosuccinate (see Objective 8). By being deaminated,
aspartate is being degraded to form arginine. In the presence of asparagine
synthetase, aspartate can be synthesized to asparagine. Aspartate can be
converted to homoserine, which is a precursor for threonine and isoleucine.
46
DISCUSS THE CATABOLISM OF ASPARAGINE
Asparagine is an non-essential amino acid and is glucogenic. Asparagine is
deaminated to aspartate, which is in turn transaminated to oxalacetate. There
are no known metabolic defects known with this pathway.
47
DISCUSS THE METABOLISM OF LYSINE
Lysine is an essential amino acid and is glucogenic. The α-amino group of lysine
does not equilibrate with the other amino acids of the body. The amino group may
be transferred to other amino acids, but other amino acids cannot transfer their
amine groups to lysine. Threonine is the only other amino acid displays this
equilibration characteristic of lysine. Most of lysine is degraded in a pathway
unique to lysine and forms a Schiff base in one of the reaction steps. See
following illustration:
48
BRIEFLY DESCRIBE TWO METABOLIC DISORDERS OF LYSINE
This first disorder, hyperlysinemia, is considered to be benign. It is due to a
deficiency in the enzyme, α-amino adipic semialdehyde synthase. Urine specimens
are noted to contain increased amounts of saccharopine and lysine.
The second disorder, familial lysinuric protein intolerance, is more serious. There is a defect in the transport mechanism for lysine, arginine and ornithine
(dibasic amino acids) within the cells of the intestine and nephron. Clinical
studies show that plasma levels for the dibasic amino acids are decreased from
1/3
to 1/2 of normal. Symptoms include thin hair, muscle wasting, and osteoporosis. If
the patient eats a meal, they usually develop hyperammonemia. The hyper-ammonemia
can be prevented by supplementing the diet with citrulline. It is thought that
there is a deficiency of ornithine and arginine (uric acid cycle) in the
hepatocytes.
49
DISCUSS THE MAJOR METABOLIC FATE OF GLYCINE
Glycine is utilized in the biosynthesis of tetrapyrroles. The four classes of tetrapyrroles are heme, chlorophyll, phycobilin, and cobalamine. All of these
compounds are synthesized from δ-aminolevulinic acid. Heme is the best
understood of the synthesis pathways. Seven reactions are required for heme
synthesis; first step begins in the mitochondria, then the next three reactions
steps occur in the cytoplasm, then the three remaining steps occur in the
mitochondria. Examine the following simplified schematic for heme synthesis.
