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Metabolism – Nucleotides

1- Which statement best describes Xanthine ?

a) It is a direct precursor of Guanine

b) It covalently binds to Allopurinol

c) It is oxidized to form Uric acid

d) It is oxidized to form Hypoxanthine

e) It is a substrate rather than a product of the enzyme Xanthine oxidase

2- Methylated heterocyclic bases of plants include all except-

a) Caffeine

b) Theophylline

c) Theobromine

d) Thymine

e) Dimethyl Xanthine

3- Which out of the following is not an anticancer drug ?

a) 5-fluorouracil

b) 5-iodouracil,

c) 3-deoxyuridine,

d) 6-thioguanine

e) Pseudouridine

4- Feedback inhibition of pyrimidine nucleotide synthesis can occur by which of the following ?

a) Increased activity of Carbamoyl phosphate synthetase

b) Increased activity of Aspartate transcarbamoylase

c) CTP allosteric effects

d) UMP competitive inhibition

e) TTP allosteric effects

5- Which base derivative can serve as a precursor for the synthesis of two of the other pyrimidine base derivatives ?

a) Cytidine triphosphate

b) Uridine mono phosphate

c) Adenosine mono phosphate

d) deoxy thymidine mono phosphate

e) deoxy Adenosine mono phosphate

6- Purine nucleotide biosynthesis can be inhibited by which of the following ?

a) Guanosine triphosphate

b) Uridine mono phosphate

c) Adenosine mono phosphate

d)Adenosine tri phosphate

e) Inosine diphosphate

7- Which of the following contributes nitrogen atoms to both purine and pyrimidine rings ?

a) Aspartate

b) Carbamoyl phosphate

c) Carbon dioxide

d) Glutamate

e) Tetrahydrofolate

8- Which out of the following  conditions is associated with hypouricemia ?

a) Lesch Nyhan syndrome

b) Adenosine deaminase deficiency

c) Over activity of PRPP synthetase

d) Over activity of amido transferase

e) Von Gierke’s disease

9- In patients with Lesch Nyhan Syndrome, purine nucleotides are overproduced and over excreted. The hypoxanthine analogue Allopurinol, which effectively treats gout , has no effect on the severe neurological symptoms of Lesch- Nyhan patients because it does not-

a) Decrease de novo purine synthesis

b) Decrease de novo pyrimidine bio synthesis

c) Decrease urate synthesis

d) Increase PRPP levels (Phosphoribosyl pyrophosphate)

e) Inhibit xanthine oxidase

10- A 4- year old presents to a pediatric clinic with megaloblastic anemia and failure to thrive. Blood biochemistry reveals “Orotic aciduria”. Enzyme measurement of the white blood cells reveals a deficiency of pyrimidine biosynthesis enzyme Orotate Phospho ribose transferase and abnormally high activity of the enzyme Aspartate transcarbamoylase. Which of the following treatment will reverse all symptoms ?

a) Blood transfusion

b) Dietary supplementation of PRPP

c) Oral thymidine

d) Oral Uridine

e) Plasmaphresis

11)- Which of the following is a required substrate for purine biosynthesis ?

a) 5- methyl thymidine

b) Ara -C

c) Ribose phosphate

d) PRPP

e) 5-Fluoro uracil

12)- Which of the following is an analogue of hypoxanthine ?

a) Ara C

b) Allopurinol

c) Ribose phosphate

d) PRPP

e)5-FU

13)-A  Pentose with a 5′ phosphate group, a 2′ OH group and 1′ pyrimidine group describes which of the following structures ?

a) Cytosine

b) Thymidine

c) Thymidylate

d) Cytidylate

e) Guanosine

14) Which is the rate limiting step of pyrimidine synthesis that exhibits allosteric inhibition by cytidine triphosphate-

a) Aspartate transcarbamoylase

b) Hypoxanthine Guanine phosphoribosyl Transferase

c) Thymidylate synthase

d) Xanthine oxidase

e) PRPP synthetase

15)- The conversion of Inosine mono phosphate-

a) To Adenosine mono phosphate (AMP) is inhibited by Guanosine mono phosphate(GMP)

b) To AMP requires uridine mono phosphate (UMP)

c) To GMP requires GMP kinase

d) To GMP requires Glutamine

e) To Guanosine di phosphate (GDP) requires ribonucleotide reductase

16)- A 56-year -old diabetic with end stage renal disease receives a kidney transplant from his son. His nephrologist is concerned for the possibility of transplant rejection and puts the patient on mycophenolic acid, that inhibits which of the following enzyme in the synthesis of nucleotides ?

a) PRPP synthetase

b) IMP dehydrogenase

c) Adenylo succinate synthetase

d) Ribonucleotide reductase

e) Adenylosuccinase

17- A physician evaluates a 32-year-old patient for fatigue. The patient is found to have an elevated white blood cell count and an enlarged spleen. A referral to an oncologist results in a diagnosis of chronic myelogenous leukemia. Treatment with hydroxyurea, a ribonucleotide reductase inhibitor is begun. The normal functioning of this enzyme is to do which of the followings ?

a) Converts xanthine to uric acid

b) Converts ribonucleotides to deoxy ribonucleotides

c) Degrades guanine to xanthine

d) Degrades AMP to IMP

e) Converts PRPP to phosphoribosylamine

18- A child is noted to have recurrent respiratory infections that necessitate hospitalization. His lab tests demonstrate a decrease in T cells, B cells , natural killer cells and decreased antibodies. He is found to have severe combined immuno deficiency . The enzyme that is defective in this disorder is important in which of the following processes ?

a) Conversion of ribonucleotides to deoxy ribonucleotides

b) Formation of AMP

c) Synthesis of UMP

d) Conversion of dUMP to dTMP

e) Conversion of adenosine to inosine

19- A 7-year-old boy suffers from mental retardation and self-mutilation and has an increased levels of serum uric acid. These symptoms are characteristic of Lesch Nyhan syndrome, which is due  to defective-

a) Salvage pathway for pyrimidine biosynthesis

b) Denovo synthesis of pyrimidines

c) Xanthine oxidase

d) HGPRT (Hypoxanthine Guanine Phospho Ribosyl Transferase)

e) Formyl transferase

20- A 58-year-old man is awoken by a throbbing ach in his great toe. He had a similar attack earlier also, after indulging in a rich meal. On examination, he is noted to have an angry inflammed great toe and several nodules on the antihelix of his ear. Inhibition of which of the following enzymes might prevent the occurrence of such symptoms ?

a) Amido transferase

b) PRPP synthetase

c) Xanthine oxidase

d) Orotate phosphoribosyl transferase

e) Carbamoyl phosphate synthetase-II

 

Key to answers-  1)- c, 2)-d, 3)-c, 4)-c, 5)- b, 6)- c, 7)-a, 8)- b, 9)- a, 10)- a, 11)-d, 12)- b, 13)- d, 14)- a, 15)- d, 16)- b, 17)- b, 18)- e, 19)- d, 20)- c.

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 What is Orotic aciduria ?

“Deficiency of a Urea Cycle enzyme results in Orotic aciduria”, Justify the statement and elaborate on all the possible causes of Orotic aciduria.

Orotic aciduria refers to an excessive excretion of Orotic acid in urine. This is a disorder of pathway of pyrimidine biosynthesis.

Hereditary Orotic aciduria

Biochemical Defect– Orotic aciduria is a rare Autosomal recessive disorder. The usual form of hereditary Orotic aciduria is the buildup of Orotic acid due to the deficiency in one or both of enzymes that convert it to UMP. Either orotate phosphoribosyl transferase and orotidylate decarboxylase both are defective, or the decarboxylase alone is defective.

Type I orotic aciduria reflects a deficiency of both orotate phosphoribosyltransferase and orotidylate decarboxylase (reactions  5 and 6, Figure-1); the rarer type II orotic aciduria is due to a deficiency only of orotidylate decarboxylase (reaction 6)

Pathway of Pyrimidine Biosynthesis

 steps of pyrimidine biosynthesis

Figure-1-showing the steps of de novo pyrimidine nucleotide biosynthesis

The first step in de novo pyrimidine biosynthesis (Figure-1) is the synthesis of Carbamoyl phosphate from bicarbonate and glutamine in a multistep process, requiring the cleavage of two molecules of ATP. This reaction is catalyzed by Carbamoyl phosphate synthetase -II (CPS-II). Carbamoyl phosphate synthetase-II  primarily uses glutamine as a source of ammonia. A different enzyme mitochondrial carbamoyl phosphate synthase I catalyzes the first step of urea synthesis (Figure-2). Compartmentation thus provides two independent pools of carbamoyl phosphate.

Carbamoyl phosphate reacts with aspartate to form Carbamoyl aspartate in a reaction catalyzed by Aspartate Transcarbamoylase (Figure-1).Carbamoyl aspartate cyclizes to form Dihydro orotate, which then gets oxidized by Dihydro orotate dehydrogenase in the presence of NADto form orotate. At this stage, orotate couples to ribose, in the form of 5-phosphoribosyl-1-pyrophosphate (PRPP), a form of ribose activated to accept nucleotide bases. Orotate reacts with PRPP to form orotidylate (Orotate mono phosphate), a pyrimidine nucleotide. This reaction is driven by the hydrolysis of pyrophosphate. The enzyme that catalyzes this addition, pyrimidine phosphoribosyl transferase, is homologous to a number of other phosphoribosyl transferases that add different groups to PRPP to form the other nucleotides. 

Orotidylate is then decarboxylated to form uridylate (UMP), a major pyrimidine nucleotide that is a precursor to RNA. This reaction is catalyzed by orotidylate decarboxylase. UMP is the parent nucleotide; the other pyrimidine nucleotides are formed from UMP (Figure-1)

Clinical manifestations

This disorder usually appears in the first year of life and is characterized by growth failure, developmental retardation, megaloblastic anemia, and increased urinary excretion of Orotic acid.

UMP, The end product of this pathway, is the precursor of UTP, CTP and TMP. All of these end products normally act in some way to feedback inhibit the initial reactions of pyrimidine synthesis. Specially, the lack of CTP inhibition allows Aspartate Transcarbamoylase to remain highly active. This results in more and more production of Orotic acid which gets accumulated and is excreted in urine excessively.

Lack of CTP, TMP, and UTP leads to a decreased nucleic acid synthesis and decreased erythrocyte formation resulting in Megaloblastic anemia. 

Physical and mental retardation are frequently present. The anemia is refractory to vitamin B12 or folic acid.

Laboratory Diagnosis

The diagnosis of this disorder is suggested by the presence of severe Megaloblastic anemia with normal serum B12 and Folate levels and no evidence of TC-II deficiency (Transcobalamine- II). A presumptive diagnosis is made by finding increased urinary orotic acid. Confirmation of the diagnosis, however, requires assay of the Transferase and decarboxylase enzymes in the patient’s erythrocytes .

Treatment

Uridine treatment is effective because Uridine can easily be converted into UMP by omnipresent tissue kinase, thus allowing UTP, CTP, and TMP to be synthesized and feedback inhibit further Orotic acid production.

Deficiency of Ornithine trans carbamoylase (Urea cycle disorder)

Increased excretion of orotic acid, uracil, and uridine accompanies a deficiency in liver mitochondrial ornithine transcarbamoylase (reaction 2, Figure 2). Excess carbamoyl phosphate exits to the cytosol, where it stimulates pyrimidine nucleotide biosynthesis. The resulting mild orotic aciduria is increased by high-nitrogen foods.

OTC deficiency

Figure-2- Showing the  block at the level of Ornithine transcarbamoylase that results in diffusion of carbamoyl phosphate to cytoplasm to be utilized in the pathway of pyrimidine bio synthesis causing Orotic aciduria.

Reye Syndrome

The orotic aciduria that accompanies Reye syndrome probably is a consequence of the inability of severely damaged mitochondria to utilize carbamoyl phosphate, which then becomes available for cytosolic overproduction of orotic acid.

Drug induced Orotic aciduria

1) Allopurinol  is an alternative substrate for orotate phosphoribosyltransferase (reaction 5, Figure-1), competes with orotic acid. Orotate phosphoribosyltransferase (reaction 5, Figure 1) converts the drug allopurinol  to a nucleotide (Oxypurinol ribonucleotide). The resulting nucleotide product also inhibits orotidylate decarboxylase (reaction 6, figure-1), resulting in orotic aciduria and orotidinuria (Figure-3)

Allopurinol induced orotic aciduria

Figure-3- showing Allopurinol induced Orotic aciduria

2) 6-Azauridine, following conversion to 6-azauridylate, also competitively inhibits orotidylate decarboxylase (reaction 6, Figure-1), enhancing excretion of orotic acid and orotidine.

3) The anticancer drug 5-fluorouracil is also phosphoribosylated by orotate phosphoribosyl transferase.

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1) Mechanism of action of cAMP

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter17/animation__second_messenger__camp.html

2) Steps of synthesis of pyrimidine nucleotides

http://www.wiley.com/college/fob/quiz/quiz22/22-5.html

3) Regulation of pyrimidine biosynthesis

http://www.wiley.com/college/fob/quiz/quiz22/22-7.html

 

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Q.1- Name the disease which results from a deficiency of hypoxanthine-guanine phosphoribosyl transferase. Briefly describe the symptoms.

Q.2- Describe two diseases caused by abnormal nucleotide metabolism and explain their biochemical basis.

Q.3- Outline briefly how deoxythymidylate can be synthesized from uridylate.

Q.4- Briefly describe the biochemical basis of the following (a) Lesch Nyhan syndrome (b) Orotic aciduria.

Q.5- Explain how azaserine and sulfonamides inhibit purine nucleotide synthesis?

Q.6- Describe 2 savage pathways for purines

Q.7-Describe the role of folic acid in the nucleotide biosynthesis and describe the step which is inhibited by Methotrexate, an anticancer drug.

Q.8- Describe the steps and regulation of Pyrimidine biosynthesis and highlight the site of defect for orotic aciduria.

Q.9- What are important differences between Carbamoyl phosphate synthetase 1 and II.

Q.10- Deficiency of urea cycle enzymes especially Ornithine Trans Carbamoylase leads to Orotic aciduria, explain the biochemical basis for this finding.

Q.11- Uridine and Cytidine are given as a part of treatment for orotic aciduria. What is the biochemical basis for this supplementation?

Q.12- Describe the steps of degradation of Purines and discuss the clinical significance if any of this pathway.

Q.13- What is gout? Discuss the causes, clinical manifestation and treatment of gout.

Q.14- Name the enzymes, the abnormal activities of which can cause hyperuricemia; explain the reactions catalyzed and discuss the basis for hyperuricemia in each of these conditions.

Q.15-Explain the biochemical basis of hyperuricemia in chronic alcoholism

Q.16- Explain the biochemical basis of hyperuricemia upon excessive fructose / fruit ingestion

Q.17- Explain the biochemical basis of hyperuricemia in Von-Gierke’s disease.

Q.18- Show by means of a reaction the conversion of Ribonucleotides to deoxy Ribonucleotides.

Q.19-Show the steps of conversion of IMP(Inosine mono phosphate) to AMP (Adenosine monophosphate and  GMP (Guanosine mono phosphate).

Q.20 -Show by means of reaction the counter regulation in the  de novo synthesis of purine nucleotides.

 

 

 

 

 

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Case Details

At the age of 11 months, a boy showed signs of delayed motor development and was brought for consultation. His mother had noticed sand like crystals on the diaper of the baby but reported only when asked particularly about it. History revealed that the child had a compulsive urge to bite his lips and fingers. Upon questioning the mother revealed that she had a brother with similar symptoms.

The Lesch Nyhan syndrome was suspected, urinary and serum uric acid levels were estimated. Both were abnormally high for the boy’s age. The diagnosis was confirmed by estimating enzyme levels in skin fibroblasts; the enzyme activity was 50 % of the normal.

Which enzyme is deficient in Lesch Nyhan Syndrome?

What is the basis for Hyperuricemia, hyperuricosuria and self-mutilation in this disorder?

Lesch–Nyhan syndrome (LNS)

Case discussion

Lesch–Nyhan syndrome (LNS), also known as Nyhan’s syndrome and Juvenile gout, is a rare inherited disorder.

Biochemical Defect

LNS is an X-linked recessive disorder, caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT), and produced by mutations in the HGPRT gene. This enzyme is normally present in each cell in the body, but its highest concentration is in the brain, especially in the basal ganglia. The HGPRT gene has been localized to the long arm of the X chromosome (q26-q27). The disorder appears in males; occurrence in females is extremely rare.

Pathogenesis

Formation of DNA (during cell division) requires nucleotides, molecules that are the building blocks for DNA. The purine bases (adenine and guanine) and Pyrimidine bases (thymine and cytosine) are bound to deoxyribose and phosphate is incorporated as necessary. Normally, the nucleotides are synthesized de novo from amino acids and other precursors. A small part, however, is ‘recycled’ from degraded DNA of broken-down cells. This is termed the “salvage pathway”.(Figure) 

HGPRT is the “salvage enzyme” for the purines: it channels hypoxanthine and guanine back into DNA synthesis. Failure of this enzyme has two results:

  • Cell breakdown products cannot be reused, and are therefore degraded. This gives rise to increased uric acid, a purine breakdown product.
  • The de novo pathway is stimulated due to an excess of PRPP (5-phospho-D-ribosyl-1-pyrophosphate or simply phosphoribosyl-pyrophosphate).

In the absence of HGPRT, these purine bases cannot be salvaged, and instead are degraded and excreted as uric acid. In addition to the failure of purine recycling, the synthetic rate for purines is accelerated, presumably to compensate for purines lost by the failure of the salvage process. The failure of recycling together with the increased synthesis of purines is the basis for the overproduction of uric acid.

Figure-1 showing Salvage pathway.

The increased production of uric acid leads to hyperuricemia. Since uric acid is near its physiological limit of solubility in the body, the persistent hyperuricemia increases the risk of uric acid crystal precipitation in the tissues. Uric acid crystal deposition in the joints produces an inflammatory reaction and gouty arthritis. The kidneys respond to the hyperuricemia by increasing its excretion into the urinary tract, increasing the risk of forming urate stones in the urinary collecting system. These stones may be passed as a sandy sludge or as larger particles that may obstruct urine flow and increase the risk for hematuria and urinary tract infections.

Frequency

The reported worldwide prevalence is 1 case per 380,000 population.

Clinical Manifestations

LNS is characterized by three major hallmarks: neurological dysfunction, cognitive and behavioral disturbances including self-mutilation, and uric acid overproduction (hyperuricemia). Some may also be afflicted with macrocytic anemia. Virtually all patients are males; males suffer delayed growth and puberty, and most develop shrunken testicles or testicular atrophy. Female carriers are at an increased risk for gouty arthritis, but are usually otherwise unaffected.

1) Overproduction of uric acid

One of the first symptoms of the disease is the presence of sand-like crystals of uric acid in the diapers of the affected infant.

The overproduction of uric acid is present at birth, but may not be recognized by routine clinical laboratory testing methods. The serum uric acid concentration is often normal initially, as the excess purines are promptly eliminated in the urine. But as the disease progresses the hyperuricemia may be observed.

The crystals usually appear as an orange grainy material, or they may coalesce to form either multiple tiny stones or distinct large stones that are difficult to pass. The stones, or calculi, usually cause hematuria (blood in the urine) and increase the risk of urinary tract infection. Some victims suffer kidney damage due to such kidney stones. Stones may be the presenting feature of the disease, but can go undetected for months or even years.

2) Nervous system impairment

The most common presenting features are abnormally decreased muscle tone (hypotonia) and developmental delay, which are evident by three to six months of age. Lack of speech is also a very common trait associated with LNS.

Irritability, loss of motor control, involuntary movements and arching of the spine (opisthotonus) are also there

3) Self-injuring behavior

The age at onset of self-injury may be as early as 1 yr and occasionally as late as the teens. The self-injury begins with biting of the lips and tongue; as the disease progresses, affected individuals frequently develop finger biting and head banging. The self-injury can increase during times of stress. Self-mutilation is a distinguishing characteristic of the disease and is apparent in 85% of affected males.

Diagnosis

The gross overproduction of uric acid is often evident in routine blood and urine studies.

1) Uric acid levels in the blood typically are elevated,

2) Urinary uric acid excretion also is increased typically.

3) Definitive diagnosis is obtained most often by measurement of HGPRT enzyme activity in blood or tissue

Diagnosis is confirmed by identifying a molecular genetic mutation in the HGPRT gene. Molecular genetic diagnosis provides an ideal tool for carrier detection and prenatal screening of at-risk pregnancies.

4) Macrocytic anemia, sometimes profound, is relatively common. Vitamin B-12, folate, and iron results are typically normal.

Other Tests

Noninvasive imaging studies of the kidneys and other parts of the urogenital system are warranted because of the marked increase in the risk for kidney stones.

Treatment

Treatment for LNS is symptomatic. Gout can be treated with Allopurinol to control excessive amounts of uric acid. Kidney stones may be treated with lithotripsy, a technique for breaking up kidney stones using shock waves or laser beams. There is no standard treatment for the neurological symptoms of LNS

Prognosis

The prognosis for individuals with severe LNS is poor. Death is usually due to renal failure or complications from hypotonia, in the first or second decade of life. Less severe forms have better prognosis.

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A 4-year-old girl presented to the clinic with Megaloblastic anemia and failure to thrive. History revealed that the child was born normally. The red blood cell count was 2.55 millions/cmm and hemoglobin was 6g/dl. He was given antibiotics and transfusions. Despite that the anemia worsened. There was no response following treatment with B12, Folic acid, or pyridoxine.

A prominent feature of the child’s urine was a crystalline sediment, which was found to be orotic acid. Orotic acid in amounts as high as 1500 mg (9.6 mmol) was excreted daily (Normal 1.4 mg/day, 9 μmol). Enzyme measurements of white blood cells revealed a deficiency of the pyrimidine biosynthesis enzyme orotate phosphoribosyltransferase and abnormally high activity of enzyme Aspartate Transcarbamoylase.

What is the  nature of the disease?

How can this be treated?

 

Case Discussion

The child is suffering from Orotic aciduria.

Orotic aciduria

Basic concept

Orotic aciduria refers to an excessive excretion of Orotic acid in urine.This is a disorder of pathway of pyrimidine biosynthesis.

Biochemical Defect

Orotic aciduria is a rare Autosomal recessive disorder. The usual form of hereditary Orotic aciduria is the build up of Orotic acid due to the deficiency in one or both of enzymes that convert it to UMP. Either orotate phosphoribosy ltransferase and orotidylate decarboxylase both are defective, or the decarboxylase alone is defective. It can also arise secondary to blockage of the urea cycle, particularly ornithine Trans carbamoylase deficiency.

Pathway of Pyrimidine Biosynthesis

 

 Figure-showing the steps of de novo pyrimidine nucleotide biosynthesis

The first step in de novo pyrimidine biosynthesis is the synthesis of Carbamoyl phosphate from bicarbonate and glutamine in a multistep process, requiring the cleavage of two molecules of ATP. This reaction is catalyzed by Carbamoyl phosphate synthetase -II (CPS-II ). Carbamoyl phosphate synthetase-II  primarily uses glutamine as a source of ammonia.

Carbamoyl phosphate reacts with aspartate to form Carbamoyl aspartate in a reaction catalyzed by aspartate Transcarbamoylase. Carbamoyl aspartate cyclizes to form Dihydro orotate, which then gets oxidized by  Dihydro orotate dehydrogenase in the presence of NAD+  to form orotate. At this stage, orotate couples to ribose, in the form of 5-phosphoribosyl-1-pyrophosphate (PRPP), a form of ribose activated to accept nucleotide bases. Orotate reacts with PRPP to form orotidylate (Orotate mono phosphate), a pyrimidine nucleotide. This reaction is driven by the hydrolysis of pyrophosphate. The enzyme that catalyzes this addition, pyrimidine phosphoribosyl transferase, is homologous to a number of other phosphoribosyl transferases that add different groups to PRPP to form the other nucleotides. 

Orotidylate is then decarboxylated to form uridylate (UMP), a major pyrimidine nucleotide that is a precursor to RNA. This reaction is catalyzed by orotidylate decarboxylase. This enzyme is one of the most proficient enzymes known. In its absence, decarboxylation is extremely slow and is estimated to take place once every 78 million years; with the enzyme present, it takes place approximately once per second, a rate enhancement of many folds !

UMP is the parent nucleotide; the other pyrimidine nucleotides are formed from UMP.

Clinical manifestations

This disorder usually appears in the first year of life and is characterized by growth failure, developmental retardation, megaloblastic anemia, and increased urinary excretion of Orotic acid.

UMP, The end product of this pathway, is the precursor of UTP, CTP and TMP. All of these end products normally act in some way to feedback inhibit the initial reactions of pyrimidine synthesis. Specially, the lack of CTP inhibition allows Aspartate Transcarbamoylase to remain highly active. This results in more and more production of Orotic acid which gets accumulated and is excreted in urine excessively.

Lack of CTP, TMP, and UTP leads to a decreased nucleic acid synthesis and decreased erythrocyte formation resulting in Megaloblastic anemia. 

Physical and mental retardation are frequently present. The anemia is refractory to vitamin B12 or folic acid.

Laboratory Diagnosis

The diagnosis of this disorder is suggested by the presence of severe Megaloblastic anemia with normal serum B12 and Folate levels and no evidence of TC-II deficiency ( Transcobalamine- II).A presumptive diagnosis is made by finding increased urinary orotic acid. Confirmation of the diagnosis, however, requireassay of the Transferase and decarboxylase enzymes in the patient’s erythrocytes

Treatment

Uridine treatment is effective because Uridine can easily be converted into UMP by omnipresent tissue kinase, thus allowing UTP, CTP, and TMP to be synthesized and feedback inhibit further Orotic acid production.

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A 46-year-old male presented to the emergency department with severe right toe pain. The patient was in usual state of health until early in the morning when he woke up with severe pain in his right big toe. The patient denied any trauma to the toe and no previous history of such pain in other joints. He did say that he had a “few too many” beers with the guys last night. Patient’s past medical history was significant for hypertension, diabetes mellitus, chronic Alcoholism and renal stones for which he underwent left nephrectomy about 25 years ago. Family history was non-contributory.

On examination, he was found to have a temperature of 38.2°C (100.8°F) and in moderate distress secondary to the pain in his right toe. The right big toe was swollen, warm, red, and exquisitely tender. The remainder of the examination was normal. Synovial fluid was obtained and revealed rod- or needle-shaped crystals that were negatively birefringent under polarizing microscope. The laboratory investigation report revealed;

Hemoglobin – 8.9gm/dl,

ESR -124 mm at the end of first hour,

Leucocyte count -7400/cmm with normal differential count

Random blood sugar-139 mg/dl

Creatinine- 1.6 mg/dl

Serum uric acid level- 10.9 mg/dl

His 24 hour urinary uric acid excretion was 446 mg/dl.

Serum calcium, phosphorus, LFT, electrolytes and lipid profile were normal.

What is the likely diagnosis?

How would you make a definite diagnosis?

What is the Pathophysiology of this disorder?

 

Case Details

The patient is suffering from Gouty Arthritis. The patient reported with pain in the big toe. Pain in the big toe precipitated by alcohol is very typical of history of gout. The patient had past history of alcoholism and renal stones. High serum urate levels and synovial fluid analysis are diagnostic of gout.

Gout is a metabolic disease most often affecting middle-aged to elderly men and postmenopausal women. It is the result of an increased body pool of urate with hyperuricemia. It is typically characterized by episodic acute and chronic arthritis, due to deposition of Mono Sodium Urate crystals in joints and connective tissues with the risk for deposition in kidney interstitium or uric acid nephrolithiasis.

Acute arthritis is initially monarticular and often involves the first metatarsophalangeal joint. Symptoms include acute pain, tenderness, warmth, redness, and swelling.

Diagnosis requires identification of crystals in synovial fluid. Treatment of acute attacks is with anti-inflammatory drugs. The frequency of attacks can be reduced by regular use of NSAIDs, colchicine, or both and by treating hyperuricemia with Allopurinol or uricosuric drugs.

Gout

 Overview of Uric Acid Metabolism

Uric acid is the final breakdown product of purine degradation in humans.

Purine bases are used in many important biological processes including the formation of nucleic acids (ribonucleic acid [RNA] and deoxyribonucleic acid [DNA]), energy currency (adenosine triphosphate [ATP]), cofactors (nicotinamide adenine dinucleotide [NAD], flavin adenine dinucleotide [FAD]), and cellular signalling (cAMP and cGMP). Purines are both synthesized de novo and taken in through the diet. Their degradation is a ubiquitous process; however, increased levels of the enzymes that carry out the metabolism of purine bases suggest that purine catabolism is higher in the liver and the gastrointestinal tract. Abnormalities in purine biosynthesis and degradation are associated with numerous disorders suggesting that the regulation of purine levels is essential.

Degradation of purine nucleotides, nucleosides and bases follow a common pathway. During purine catabolism, the purine nucleotides

 

 Figure-1 showing purine nucleotide catabolism.

 

Adenosine monophosphate (AMP) and GMP are generated from the dephosphorylation of ATP and GTP, respectively. AMP is then deaminated to IMP by AMP deaminase. Subsequently, GMP and IMP are dephosphorylated by specific 5’Nucleotidase to produce the nucleosides guanosine and Inosine respectively.

Alternatively, AMP can be dephosphorylated to form adenosine, which is then deaminated by adenosine deaminase (ADA) to form Inosine. Inosine and guanosine are further broken down by the cleavage of the purine base from the ribose sugar to yield ribose 1-phosphate and hypoxanthine and guanine, respectively. Similar reactions are carried out for the degradation of purine deoxy Ribonucleotides and deoxyribonucleoside. Guanine is deaminated to form Xanthine, whereas hypoxanthine is oxidized to form Xanthine by the enzyme Xanthine oxidase. Xanthine is further oxidized, again by Xanthine oxidase, to form uric acid, which is excreted in the urine.

Uric acid has a pKa of 5.4 and is in the ionized urate form at physiologic pH. Urate is not very soluble in an aqueous environment and the concentration of urate in human blood is very close to saturation. Therefore, conditions that lead to excessive degradation of purine bases can lead to the formation of urate crystals.


Hyperuricemia

Hyperuricemia can result from increased production or decreased excretion of uric acid or from a combination of the two processes. Sustained hyperuricemia predisposes some individuals to develop clinical manifestations including gouty arthritis, urolithiasis, and renal dysfunction.

Hyperuricemia is defined as a plasma (or serum) urate concentration >408 mol/L (6.8 mg/dL). The risk of developing gouty arthritis or urolithiasis increases with higher urate levels and escalates in proportion to the degree of elevation.

Incidence

Hyperuricemia is present in between 2.0 and 13.2% of ambulatory adults and is even more frequent in hospitalized individuals.

Causes of Hyperuricemia

Hyperuricemia may be classified as primary or secondary depending on whether the cause is innate or is the result of an acquired disorder. However, it is more useful to classify hyperuricemia in relation to the underlying pathophysiology, i.e., whether it results from increased production, decreased excretion, or a combination of the two.

A) Increased Urate Production-

The common causes are as follows-

1) Diet contributes to the serum urate in proportion to its purine content. Foods high in nucleic acid content include liver, “sweetbreads” (i.e., thymus and pancreas), kidney, and anchovy. Excessive consumption leads to hyperuricemia.

2) Endogenous sources of purine production also influence the serum urate level. De novo purine biosynthesis is an 11-step process that forms Inosine monophosphate (IMP). The rates of purine biosynthesis and urate production are determined, for the most part, by amidotransferase, which combines phosphoribosylpyrophosphate (PRPP) and glutamine. Metabolic abnormalities that lead to the overproduction of purine nucleotides through the de novo pathway lead to increased purine degradation and subsequent hyperuricemia. An example of this is an increase in the activity of 5-phosphoribosyl-1-pyrophosphate (PRPP) synthetase. This enzyme is responsible for the production of PRPP, which is an important precursor of both purine and pyrimidine de novo biosynthesis. Elevations in PRPP lead to increased purine nucleotide production that can in turn increase the rate of degradation and hence increased uric acid production. Similarly increased activity of amidotransferase also leads to similar effects of increased uric acid production.

3) Hyperuricemia can also result from defects in the purine salvage pathway. The enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is responsible for reforming IMP and GMP from hypoxanthine and guanine, respectively. In this manner purine bases are salvaged back into the purine nucleotide pool. Lesch-Nyhan syndrome results from an inherited deficiency in HGPRT. This syndrome is associated with mental retardation and self-destructive behavior, which may be associated with inadequate production of purine nucleotides through the salvage pathway in certain neuronal cells. In addition, Lesch-Nyhan patients have gout resulting from the inability to salvage purine bases, which leads to increased levels of uric acid.

4) Over production of urate may also be due to glucose-6-phosphatase deficiency (von Gierke disease) , In von Gierke disease its both overproduction and decreased excretion responsible for hyperuricemia. Overproduction is due to over active HMP pathway to utilize the excess load of glucose-6 phosphate and decreased excretion is due to lactate accumulation as a result of anaerobic Glycolysis in muscles. Lactate is excreted while uric acid reabsorbed through anion exchange transporters.

5) Accelerated purine nucleotide degradation can also cause hyperuricemia, i.e., with conditions of rapid cell turnover, proliferation, or cell death, as in leukemia, Cytotoxic therapy for malignancy, hemolysis, or rhabdomyolysis. 

6) Overproduction of uric acid may also occur, hemolytic anemias, pernicious anemia, ineffective erythropoiesis (as in B-12 deficiency) and obesity.

 

B) Decreased Uric Acid Excretion

Over 90% of individuals with sustained hyperuricemia have a defect in the renal handling of uric acid.

 

Common causes of secondary gout due to under excretion of uric acid include-

·          Primary idiopathic

·          Renal insufficiency

·          Polycystic kidney disease

·          Diabetes insipidus

·          Hypertension

·          Acidosis

·          Lactic acidosis

·          Diabetic ketoacidosis

·          Starvation ketosis

·          Lead intoxication

·          Hyperthyroidism

·          Hypothyroidism

·          Toxemia of pregnancy

·          Down syndrome

·          Low dose salicylates

·          Diuretics

·          Alcohol

·          Nicotinic acid

C)   Combined Mechanism

·          Alcohol

·          Glucose- 6 phosphatase deficiency

·          Shock

Alcohol promotes hyperuricemia because of increased urate production and decreased uric acid excretion. Excessive alcohol consumption accelerates hepatic breakdown of ATP to increase urate production. Alcohol consumption can also induce hyperlacticacidemia, which blocks uric acid secretion. The higher purine content in some alcoholic beverages such as beer may also be a factor.

Decreased renal excretion is by far the most common cause of hyperuricemia.

 

Pathophysiology

Urate precipitates as needle-shaped monosodium urate (MSU) crystals, which are deposited extracellularly  in cartilage tendons, tendon sheaths, ligaments, walls of bursae and skin around cooler distal joints and tissues (eg, ears). In severe, long-standing hyperuricemia, MSU crystals may be deposited in larger central joints and in the parenchyma of organs such as the kidney. At the acid pH of urine, urate precipitates readily as small plate like or irregular crystals that may aggregate to form gravel or stones, which may cause obstruction. Tophi are MSU crystal aggregates that most often develop in joint and cutaneous tissue.

Symptoms and Signs

Acute gouty arthritis usually begins with sudden onset of pain (often nocturnal). The metatarsophalangeal joint of a great toe is most often involved, but the ankle, knee, wrist, and elbow are also common sites. Rarely, the hip, shoulder, sacroiliac, sternoclavicular, or cervical spine joints are involved. The pain becomes progressively more severe, usually over a few hours, and is often excruciating. Swelling, warmth, redness, and exquisite tenderness may suggest infection. The overlying skin may become tense, warm, shiny, and red or purplish. Fever, tachycardia, chills, and malaise sometimes occur. Coexisting hypertension, hyperlipidemia, and obesity are common.

Figure-2-showing swollen metatrasophalangeal joint

 

Course: The first few attacks usually affect only a single joint and last only a few days. Later attacks may affect several joints simultaneously or sequentially and persist up to 3 wk if untreated. Subsequent attacks develop after progressively shorter symptom-free intervals. Eventually, several attacks may occur each year.

Tophi: They are usually firm yellow or white papules or nodules, single or multiple. They can develop in various locations, commonly the fingers, hands, feet, and around the olecranon or Achilles tendon. Tophi can also develop in the kidney and other organs and under the skin on the ears. Tophi may even erupt through the skin, discharging chalky masses of urate crystals. Tophi may eventually cause deformities.

 

Figure-3 -showing Tophi

Chronic gout: Chronic gouty arthritis can cause pain, deformity, and limited joint motion. Inflammation can be flaring in some joints while subsiding in others. About 20% of patients with gout develop urolithiasis with uric acid stones or Ca oxalate stonese. Untreated progressive renal dysfunction, most often related to coexisting hypertension or, less often, some other cause of nephropathy, further impairs excretion of urate, accelerating crystal deposition in tissues.

Cardiovascular disease and the metabolic syndrome are common among patients with gout.

i) Diagnosis of Acute gouty arthritis

  • Clinical criteria
  • Synovial fluid analysis
  • Serum uric acid level

Gout should be suspected in patients with acute single joint involvement, particularly older adults or those with other risk factors.

Synovial fluid analysisSynovial fluid analysis can confirm the diagnosis by identifying needle-shaped, strongly negatively birefringent urate crystals that are free in the fluid or engulfed by phagocytes.

Serum urate levelAn elevated serum urate level supports the diagnosis of gout but is neither specific nor sensitive; at least 30% of patients have normal serum urate at the time of an acute attack. However, the serum urate level reflects the size of the extracellular miscible urate pool. The level should be measured on 2 or 3 occasions in patients with newly proven gout to establish a baseline; if elevated (> 7 mg/dL [> 0.41 mmol/L]), 24-h urinary urate excretion can also be measured. Normal 24-h excretion is about 600 to 900 mg on a regular diet. Quantification of urinary uric acid can indicate whether hyperuricemia results from impaired excretion or increased production and help guide any serum urate–lowering therapy. Patients with elevated urine excretion of urate are at increased risk of urolithiasis.

X-raysX‑rays of the affected joint may be taken to look for bony tophi but are probably unnecessary if the diagnosis has been established by synovial fluid analysis.

ii) Diagnosis of chronic gouty arthritisChronic gouty arthritis should be suspected in patients with persistent joint disease or subcutaneous or bony tophi. Plain x‑rays of the first metatarsophalangeal joint or other affected joint may be useful. Bony lesions are not specific or diagnostic but nearly always precede the appearance of subcutaneous tophi.

Prognosis

With early diagnosis, therapy enables most patients to live a normal life. Gout is generally more severe in patients whose initial symptoms appear before age 30.

Treatment

  • Termination of an acute attack with NSAIDs(Non steroidal anti inflammatory drugs) or corticosteroids
  • Prevention of recurrent acute attacks with daily colchicine or an NSAID
  • Prevention of further deposition of MSU crystals and resolution of existing tophi by lowering the serum urate level
  • Treatment of coexisting hypertension, hyperlipidemia, and obesity.

Lowering the serum urate level:

Neither colchicine, NSAIDs, nor corticosteroids, retard the progressive joint damage caused by tophi. Such damage can be prevented and, if present, reversed with urate-lowering drugs. Tophaceous deposits are resorbed by lowering serum urate. Lowering serum urate may also decrease the frequency of acute arthritic attacks. This decrease is accomplished by

  • Blocking urate production with allopurinol
  • Increasing urate excretion with a uricosuric drug
  • Using both types of drugs together in severe tophaceous gout

Uricase can also be used but not yet routinely. Uricase is an enzyme that converts urate to allantoin, which is more soluble.

 

Other treatments:


·         Fluid intake ≥ 3 L/ day is desirable for all patients, especially those who chronically pass urate gravel or stones.

·         Alkalinization of urine (with K citrate, or acetazolamide) is also occasionally effective for those with persistent uric acid urolithiasis despite Hypouricemic therapy and adequate hydration.

·         Extracorporeal shock wave lithotripsy may be needed to disintegrate renal stones.

·         Large tophi in areas with healthy skin may be removed surgically.

·         Dietary restriction of purines is less effective, but high intake of high-purine food and alcohol (beer in particular) should be avoided.

·         Carbohydrate restriction and weight loss can lower serum urate in patients with insulin resistance because high insulin levels suppress urate excretion.

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