Download B-Metabolism of Sulphur containing amino acids

Dr. Walaa AL-Jedda – 2016/2017
B-Metabolism of Sulphur containing amino acids
Sulphur containing amino acids are three:
1- L- Methionine: essential amino acid,
2- L- Cysteine
3- L- Cystine,
both are non-essential amino acids
Note: Other sources of sulphur in the body are the sulphur-containing vitamins:
thiamine (vit B1), lipoic acid and biotin.
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- Methionine, cysteine and cystine are the principal sources of sulphur in the body.
- Demethylation of methionine produces homocysteine which may be remethylated to form methionine again.
- Cystine is reversibly convertible to cysteine and homocystine to homocysteine
by oxidation- reduction.
- Both methionine and cysteine can undergo transamination reaction.
- Methionine is an essential amino acid and has to be supplied in the diet. Cysteine
is not essential and can be synthesized in the body from methionine.
- The presence of cysteine and cysteine in the diet reduces the requirement of
methionine (sparing action).
- Methionine before catabolized has to be activated first to "Active" methionine,
which can act as - CH3 group donor in the body.
Metabolic fate of L- Methionine:
Metabolic fate of L- methionine can be in three stages:
Stage 1: Activation of methionine and its demethylation to form LHomocysteine.
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Stage 2: Conversion of L- Homocysteine to L- Homoserine.
Stage 3: Degradation of L- Homoserine to end products L- propionyl- CoA
(which is glucogenic ) and α- amino butyrate (is excreted in urine).
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Metabolic role of Methionine:
1- Methionine is glucogenic: propionyl CoA the end product is glucogenic.
2- Cysteine formation
3- Lipotropic function : through active methionine which can donate methyl
group and can form choline from ethanolamine. Choline is lipotropic and
prevents accumulation of fat in liver.
4- Polyamine synthesis
5- Formation of methyl mercaptan and its clinical significance: patients with
severe liver disease, exhibit foul odour in breath called as "Foetor hepaticus".
It has been attributed to methyl mercaptan, which appears to be formed from
methionine. Methyl mercaptan has been found in urine of these patients.
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6- Transmethylation: the most important compounds with biologically labile
methyl group are:
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Metabolism of Cystine
Cystine metabolism proceeds through cysteine, to which it is readily converted.
The reaction is reversible and catalyzed by NADH- dependant oxido-reductase.
One molecule of cystine is reduced to form two molecules of cysteine and viceversa.
Cystine
(NADH-dependant oxidoreductase) +2H -2H
2 moles of cysteine
catabolized to pyruvic acid
A- Metabolic Fate of Cysteine
Cysteine is catabolized to form pyruvic acid, which can be converted to glucose.
Thus cysteine is a glucogenic amino acid.
Four possible pathways for pyruvic acid formation are as follows:
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B- Metabolic role of cysteine:
- Glucogenic: cysteine is catabolized to pyruvic acid which is glucogenic.
- Formation of glutathione: cysteine is required for synthesis of glutathione.
G-SH is the reduced form, active group is SH group. G-S-S-G is the oxidized
form.
- Formation of mercaptoethanolamine: which is an important constituent of
coenzyme A.
- Formation of taurine: cysteine is utilized in the formation of taurine, which
combines with cholic acid ( obtained from degradation of cholesterol in liver) to
form bile acid "taurocholic acid ".
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- Cysteine is particularly prominent amino acid in the proteins of nails, hairs,
hoofs and keratin of the skin(sclera-proteins).
- Cysteine is also a constituent of many other proteins, including certain protein
hormones like insulin, vasopressin etc., where it is of great importance in
maintaining secondary and tertiary structures of the proteins.
- Role of cysteine in detoxication.
Clinical Aspect
Inherited Disorders of S- containing amino acids
1- Cystinuria: An inherited disorder of cystine metabolism. Excretion of
cystine in urine increases 20-30 times of normal. Also there occurs increased
excretion of diabasic amino acids: Lysine., arginine and ornithine (COAL)
( specific diabasic aminoaciduria ).
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It is an inherited disorder and occurs at frequency of (1:7000) individuals
Defect: it is considered to be due to a renal transport defect in the reabsorption of
the above four amino acids do not occur, a single reabsorptive site is involved.
Complications: Cystine is relatively insoluble amino acid, which may precipitate
in renal tubules, ureters and bladder to form cystine calculi.
Cystine stones account for 1-2% of all urinary tract calculi. It forms a major
complication of the disease.
A "mixed disulfide" consisting of L- cysteine and L- homocysteine has been found
in urine. This is more soluble and thus reduces the tendency to formation of cystine
crystals / and calculi.
Normally, the concentration of urinary cysteine is well within its solubility but in
Cystinuria, the solubility maybe exceeded leading to cysteine crystallization and
forming kidney stones.
Diagnosis:
Urine examination: detection of hexagonal, flat crystals in urinary deposit in a
patient who is not taking sulpha drugs is pathognomonic.
- Oral hydration is an important part of treatment for this disorder in addition to
alkalization of urine, because cystine is more soluble in alkaline than acidic urine.
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2- Homocystinuria Type-1 (classical type)
-The Homocystinuria are a group of disorders involving defect in the metabolism of
methionine and its metabolic intermediates, homocysteine / and homocystine.
Enzyme deficiency: genetic deficiency in the enzyme cystathionine synthetase,
which leads to accumulation of homocystine. Plasma level of homocystine increases
and excreted in urine ("overflow" aminoaciduria), 50-100 mg or more excreted in
urine per day. In some cases, S- adenosyl methionine is also excreted.
Clinical features: mental retardation, liver is enlarged (hepatomegaly), skeletal
deformities, ectopia lentis, life threatening arterial / venous thrombosis.
In addition to classical type, two more types have been described:
Homocystinuria Type-2, Homocystinuria Type-3
Relation of B-complex Vitamins, Homocysteine and Heart
Attack
Recently it has been shown that some cases of CHD where there are no obvious risk
factors like increased cholesterol/ TG / LDL, obesity, hypertension etc. are due to
increased level of homocysteine in the blood.
Individuals with the highest homocysteine levels have three times the risk of
precipitating heart attack, even if all other risk factors are under control, the reason still
unclear, but it is postulated that it increases the possibility of thrombosis, and promotes
also plaque formation thus damaging the arteries.
It is also shown that an insufficient concentration of three B-vitamins (folic acid, vit
B12 and pyridoxine (B6 )) cause increase in homocysteine level in the blood.
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Sufficient amounts of folic acid, B12 and B6 reduce the blood levels of homocysteine.
The higher the concentration of folic acid in the diet, the lower the homocysteine
levels in blood.
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Metabolism of other amino acids
Glycine
Glycine is the simplest amino acid. Chemically it is " amino acetic acid ".
It is non-essential amino acid and can be synthesized in animal tissues. Though it is
non- essential but it is an important amino acid as it forms many biologically
important compounds in the body.
A- Metabolic fate:
1- Deamination: producing glyoxylic acid( glyoxylate),which convert to oxalic acid or
formic acid and thus enters" one-carbon pool".
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2- Glycine Cleavage
3- Conversion to serine
4- Oxidation to form Aminoacetone
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B- Metabolic Role of Glycine:
1- Synthesis of Heme: glycine is necessary in the first reaction of heme synthesis.
2- Synthesis of Glutathione: glutathione is a tripeptide formed from three amino
acids; glutamic acid, cysteine and glycine.
3- Synthesis of Purine Nucleus.
4- Synthesis of Creatine.
5- Conjugation: with 1-benzoic acid to form hippuric acid and excreted in urine.
2-cholic acid to form glycocholic acid, a bile acid which is
excreted in bile as sodium salts.
6- Glycine is Glucogenic.
7- Source of formate (" one carbon pool") and oxalate.
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Histidine
Nutritionally semi-essential amino acid. Histidine is required in the diet in growing
animals and in pregnancy and lactation. Under these conditions, the amino acid
becomes essential. Chemically it is " α-amino β- imidazole propionic acid".
A- Metabolic Role
- It is glucogenic through formation of glutamate to α-ketoglutarate.
- Histamine formation: decarboxylation of histidine produces histamine.
- Formate can serve as one carbon moiety. The 'one carbon ' fragment of histidine is
taken up by folic acid and metabolized by transformylation reaction normally.
In deficiency of folic acid, the histidine derivative, formiminoglutamic acid, ("figlu")
accumulates and excreted in urine, used as a test for folic acid deficiency (Refer folic
acid).
- Other histidine compounds:
-Ergothioneine: present in R.B. cells and liver. It is reducing substance.
- Carnosine and Anserine: both occur in muscles. In myopathies anserine is found in
urine.
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Tryptophan
- It is an essential amino acid. Omission of tryptophan in diet of man and animals is
followed by tissue wasting and negative nitrogen balance.
- It is both glucogenic and ketogenic.
- Tryptophan can synthesize niacin ( nicotinic acid ), a vitamin of B-complex group.
- It is a hetero cyclic amino acid and chemically it is" α-amino β-3-indole propionic
acid". It is the only amino acid with an indole ring.
A- Metabolic Fate
In this pathway, tryptophan is finally converted to glutaric acid, which in turn gives
two molecules of acetyl CoA( thus it is ketogenic) from acetoacetyl CoA. It also
produces alanine which on transamination can form pyruvic acid ( thus it is
glucogenic).
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B- Metabolic Role
1- Tryptophan is both glucogenic and ketogenic.
2- Nicotinic acid formation
3- Formation of Tryptamine
4- Transamination
5- Formation of xanthurenic acid: xanthurenic acid excretion in urine is an index for
B6- deficiency.
Clinical interpretation:
a- Normal persons excrete only 1-3 mg of xanthurenic acid per day and 2-11 mg in 24
hours urine after taking a 2 gm load of tryptophan (loading test).
b- In Pyridoxine deficiency, much greater amount of xanthurenic acid is excreted in
urine. Following a "loading " test up to 60 mg be excreted in 24 hours urine.
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6- Formation of serotonin: serotonin is a vasoconstrictor substance. It is present in
the blood and is produced in tissues like gastric mucosa, intestine, brain, mast cells
and platelets.
Serotonin Functions:
- It is a potent vasoconstrictor.
- Produces contraction of smooth muscles.
- Stimulator of cerebral activity.
Effect of serotonin on brain:
Serotonin does not pass blood-brain barrier to any significant amount. For action it
has to be produced locally from amino acid.
- Excess of serotonin in brain tissues produces stimulation of cerebral activity
(excitation).
- Deficiency of serotonin produces depressant effect.
CLINICAL ASPECT
Inherited Disorder
Hartnup Disease: A hereditary disorder associated with defective tryptophan
metabolism.
The disease occurs probably due to impaired formation of " transport proteins" for
tryptophan and neutral amino acids in intestinal mucosal, renal tubular epithelial cells
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and the brain. There is defective intestinal and renal transport of tryptophan and other
neutral amino acids.
Clinical features: these are characterized by:
- Mental retardation
- Intermittent cerebellar ataxia and other neurological symptoms.
- Pellagra-like skin rash-cutaneous hypersensitivity to sunlight.
Blood: plasma level of tryptophan and other neutral amino acids are reduced.
Urine: the neutral amino acids, including tryptophan are excreted in urine and faeces, at
least 5-10 times of normal average. Urine also shows greatly increased amounts of
indoleacetic acid.
*Note: There is decreased synthesis of serotonin and nicotinic acid, which accounts
for neurological symptoms and Pellagra like rash respectively.
Metabolic Role of Other Amino Acids
Role of Nitric Oxide
Nitric oxide (NO) is formed in the body from amino acid arginine. Endothelium
derived relaxing factor ( EDRF ) which produces vasodilatation is now proved to be
nitric oxide.
Formation of NO: Arginine is acted upon by an enzyme called " nitrogen oxide
synthase", a cytosolic enzyme and converts arginine to citrulline and nitric oxide
( NO).
 Nitricoxide is formed from arginine by the enzyme nitric oxide synthase (NOS)
that contains heme, FAD, FMN, NADPH and tetrahydrobiopterine(FH4).
 NOS catalyze five- electron oxidation of the nitrogen of arginine.
 Calmodullin is required to modulate NOS activity.
 From the molecular oxygen, one atom is added to (NO•), and the other into
citrulline, therefore, the enzyme NOS is a di-oxygenase.
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Functions of Nitric Oxide
- It acts as vasodilator and causes relaxation of smooth muscles.
- It has important role in regulation of blood flow and maintaining blood pressure.
- It is involved in penile erection.
- Acts as a neurotransmitter in the brain and peripheral autonomic nervous system
- May have also role in relaxation of skeletal muscles.
- Inhibits adhesion, activation and aggregation of platelets.
- May constitute part of primitive immune system and may mediate bactericidal
actions of macrophages.
- Low level of nitric oxide may be involved in causation of pylorospasm of infantile
hypertrophic pyloric stenosis.
Metabolic fate:
-Nitric oxide has a very short half-life (3-4 seconds).
- NO combines with oxygen to form NO2 and reacting with hemoglobin and
converted to NO3.
-Nitrates (NO, NO2 and NO3) are excreted in urine.
- Very low quantity of NO• is expelled through lung.
- On exposure to superoxide anion (O•2-), NO• is converted to a highly reactive
free radical, peroxy nitrites (OONO•), which causes, lipid peroxidation, cell injury
and cell death.
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Clinical Aspect
- Nitroglycerine: the important coronary artery vasodilator used in Angina Pectoris
acts to increase intracellular release of EDRF( now proved to be NO) and cGMP
- In septic shock: bacterial lipopolysaccharide present in blood causes uncontrolled
production of NO leading to dilation of blood vessels and lowering of BP.
- In eclampsia and pre- eclampsia : the hypertension is due to decrease production of
nitric oxide ( NO ) due to probably formation of ADMA( asymmetric dimethyl
arginine).
- Iron supplements: iron supplements can dramatically reduce dry cough symptoms in
heart patients. Cardiac patients using ACE inhibitors, widely prescribed for
hypertension, heart failure and other cardiac conditions often suffer from a dry cough.
It is the biggest reason for people stopping taking their medication. Iron supplements act
by decreasing the production of nitric oxide, which is linked to inflammation of the
bronchial cells in the lungs.
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Mechanism of action:
-NOS is activated by acetyl choline, which attaches with the receptor, and the
signal is transmitted through inositol triphosphate, leading to release of calcium
ions, that activates NOS.
- Tumor necrosis factor- alpha (TNF-α) increases the activity of the enzyme.
- Thus NO produced in one cell, diffuse to the adjacent smooth muscle and
activates guanylatecyclase.
- Increased level of cyclic GMP activates protein kinase in smooth muscles,
which causes phosphorylation of myosin light chains, leading to relaxation of
muscles.
- Thus NO is a vasodilator.
Isoenzymes of NOS:
There are 3 isoforms of NOS
1-Neuronal NOS (NOS-1 or n NOS)
-seen in central and peripheral neurons (cerebellum and GIT).
-mainly cytoplasmic enzyme
-activated by calcium
-seen in chromosome 12
-implicated in the long QT interval syndrome
2- Macrophage NOS (NOS-2 or i NOS (inducible NOS)
-mainly seen in macrophages and neutrophils, also in hepatocytes.
-induced by cytokines (interleukin-1 and (TNF-α) and during inflammation.
-Cytoplasmic enzyme
- not activated by calcium
-seen in chromosome 17.
3-Endothelial NOS (NOS-3 or e NOS)
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-seen in endothelial cells, platelets, endocardium and myocardium.
- localized in the plasma membrane.
-activated by calcium.
-seen in chromosome 7.
Metabolism of Creatine
Two closely related nitrogenous compounds which are connected with protein
metabolism are: creatine and creatinine.
The reaction is:
- Irreversible
-It is non-enzymatic
- Creatinine has ring structure.
Occurrence and Distribution:
A- Creatine: it is a normal constituent of the body. It is present in muscle, brain,
liver, tests and in blood. Can occur in "free" form and also as "phosphorylated"
form.
Total amount in adult human body is approximately 120 gm. 98% of total amount is
present in muscles, of which 80% occurs in phosphorylated form, 1.3% in nervous
system (brain) and 0.5-0.7% in tissues.
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Blood level:
- In whole blood creatine level varies from 2-7 mg %.
- In plasma it is less than 1mg%. In male it varies from 0.2-0.6 mg%. In female 0.35-0.9
mg%.
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B- Creatinine: creatinine is the anhydride of creatine, and it is in this form that
creatine is excreted in normal health. Total creatinine in muscle is only 0.01% (10 mg).
Blood: Whole blood creatinine level varies from 1-2 mg%
Urinary excretion: in normal health, urinary excretion in the form of creatinine and it
is only 2% of the total. In males(1.5-2 gm)in 24 hrs urine, and in females(0.8- 1.5 gm).
Urine of normal healthy adults male contains creatinine but no creatine, and this is:
- independent of amount of proteins taken in the diet.
- excretion is greater in muscular persons and appears to be related to muscular
development and muscular activity.
- after severe exercise, it may increase, but total amount remains constant from day to
day.
Creatinuria: excretion of creatine in urine is called Creatinuria. Creatine excretion
occurs:
- In children: reason probably lack of ability to convert creatine to creatinine.
- In adults females in pregnancy and maximum after parturition (2-3 weeks).
- In febrile conditions.
- In thyrotoxicosis, probably due to associated myopathies.
- In muscular dystrophies, mytositis, and myasthenia gravis.
- Lack of carbohydrate in diets and DM.
- In wasting diseases e.g. in malignancies.
- In starvation.
Biosynthesis of Creatine
Three amino acids are required in biosynthesis of creatine. They are: Glycine, Arginine
and Methionine. Substrates to start are glycine and arginine.
Site of synthesis: - in kidney, - in liver
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Role of creatine in muscle:
1- Creatine is the reservoir of energy in muscles. When muscles contract, energy is
derived from breakdown of ATP to ADP and Pi. ATP must be reformed quickly, to
supply the energy, which initially comes from creatine-P, subsequently from glycolysis
(contracting muscle).
In the resting condition, creatine-P is formed, the enzyme that catalyzes the reaction is
ATP-creatine transphosphorylase.
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2- A further source of ATP in muscle is by the myokinase reaction.
Creatinine Clearance:
Endogenous creatinine clearance is used as renal function test. At normal levels of
creatinine in the blood, this metabolite is filtered at the glomerulus but neither secreted
nor re-absorbed by tubules. Hence its clearance measures the glomerular filtrate rate
(GFR).
GFR is a convenient method for estimation because:
-it is a normal metabolite in the body
- it does not require the intravenous administration of any test material.
-estimation of creatinine is simple.
Ccr = U * V / P U:urine creatinine concentration in mg/dl
P: serum creatinine in mg/ dl
V: volume of urine in ml/min.
Normal values: 95- 105 ml/min.
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