Purine and Pyrimidine Purine and Pyrimidine MetabolismMetabolism

Harliansyah, Ph.DHarliansyah, Ph.DDepartment of BiochemistryDepartment of Biochemistry

Faculty of MedicineFaculty of MedicineYarsi UniversityYarsi University

Metabolism of purines and pyrimidines are divided into 4 sections:

1. De novo synthesis of nucleotides from basic metabolites, which provides a sources of nucleotides from basic building blocks in growing cells.

2. Salvage pathways that recycle preformed bases and nucleosides and providean adequate supply of nucleotides in cells at rest.

3. Catabolic pathways for excretion of nucleotide degradation products, a processthat is essential to limit the accumulation of toxic levels of nucleotides withincells.

4. Biosynthesis pathways for conversion of the ribonucleotides into the deoxy- ribonucleotides, providing precursors for deoxyribonucleic acid (DNA) synthesis.

NucleotidesNucleotides

BIOMEDICAL IMPORTANCE

Nucleotides—the monomer units or building blocks of nucleic acids—serve multiple additional functions.

1. Part of many coenzymes and serve as donors of phosphoryl groups (eg, ATP or GTP), of sugars (eg,UDP- or GDP-sugars), or of lipid (eg, CDP-acylglycerol).

2. Regulatory nucleotides include the second messengers cAMP and cGMP, the control by ADP of oxidative phosphorylation, and allosteric regulation of enzyme activity by ATP, AMP, and CTP.

3. Synthetic purine and pyrimidine analogs that contain halogens,thiols, or additional nitrogen are employed for chemotherapy of cancer and AIDS and as suppressors of the immune response during organ transplantation.

Breakdown of purines and Breakdown of purines and pyrimidines pyrimidines

Nucleases digest DNA and RNA into their Nucleases digest DNA and RNA into their                     constituent nucleotides.                     constituent nucleotides.

Nucleotide breakdown is a three step process: Nucleotide breakdown is a three step process:         1) Removal of phosphate to form nucleoside. 1) Removal of phosphate to form nucleoside.         2) Removal of pentose leaving the 2) Removal of pentose leaving the

                    "preformed base".                     "preformed base".         3) Formation of final excretory products: 3) Formation of final excretory products:                                 Purines : uric acid Purines : uric acid

Pyrimidines : CO2, NH3 Pyrimidines : CO2, NH3

Purines and pyrimidines are nitrogen-containing heterocycles, cyclic compounds whose rings contain both carbon and other elements (hetero atoms).

Structures of purines and Structures of purines and pyrimidinespyrimidines

Mononucleotide structure Mononucleotide structure Nitrogenous base: Nitrogenous base:                 Purines Purines                         Adenine, Guanine Adenine, Guanine                 Pyrimidines Pyrimidines                         Cytosine, Uracil, Thymine. Cytosine, Uracil, Thymine.         Pentose sugar: Pentose sugar:                         Ribose or deoxyribose. Ribose or deoxyribose.         Phosphate groups. Phosphate groups.

                    Note: Note:                         N-base + sugar = nucleoside. N-base + sugar = nucleoside.                         N-base + sugar + phosphate =  nucleotide. N-base + sugar + phosphate =  nucleotide.

Synthesis of Phosphoribosyl Pyrophosphate: Synthesis of Phosphoribosyl Pyrophosphate:         PRPP represents "activated ribose", and is PRPP represents "activated ribose", and is

    a precursor of the purines and pyrimidines.     a precursor of the purines and pyrimidines.

Purine SynthesisPurine Synthesis Purine synthesis is critical to fetal development, Purine synthesis is critical to fetal development,

therefore defects in enzymes will result in a nonviable therefore defects in enzymes will result in a nonviable fetus.fetus.

Phosphoribosyl Phyrophosphate (PRPP) synthetase Phosphoribosyl Phyrophosphate (PRPP) synthetase defects are known and have severe consequences.defects are known and have severe consequences.

PRPP synthetase superactivity has been documented, PRPP synthetase superactivity has been documented, resulting in increased PRPP, elevated levels of resulting in increased PRPP, elevated levels of nucleotides, and increased excretion of uric acid.nucleotides, and increased excretion of uric acid.

Phosphoribosyl Pyrophosphate Phosphoribosyl Pyrophosphate (PRPP) Synthetase Defects(PRPP) Synthetase Defects

PRPP deficiency results in convulsions, PRPP deficiency results in convulsions, autistic behavior, anemia, and severe autistic behavior, anemia, and severe mental retardation. mental retardation.

Excessive PRPP activity causes gout Excessive PRPP activity causes gout (deposition of uric acid crystals), along (deposition of uric acid crystals), along with various neurological symptoms, such with various neurological symptoms, such as deafness. as deafness.

Biosynthesis of Purine

IMP to AMP & GMP

Pyrimidine SynthesisPyrimidine Synthesis

Pyrimidine Synthesis is critical to fetal Pyrimidine Synthesis is critical to fetal development just as purine metabolism is development just as purine metabolism is critical. Therefore an absolute deficiency critical. Therefore an absolute deficiency of an enzyme of pyrimidine synthesis of an enzyme of pyrimidine synthesis would be fatal.would be fatal.

A very low level of the enzyme UMP A very low level of the enzyme UMP synthase has been documented, resulting synthase has been documented, resulting in the condition orotic aciduria.in the condition orotic aciduria.

Orotic AciduriaOrotic Aciduria Deficiency in UMP synthetase activityDeficiency in UMP synthetase activity Due to the demand for nucleotides in the Due to the demand for nucleotides in the

process of red blood cell synthesis, patients process of red blood cell synthesis, patients develop the condition of megaloblastic anemia, develop the condition of megaloblastic anemia, a deficiency of red blood cells.a deficiency of red blood cells.

Pyrimidine synthesis is decreased and excess Pyrimidine synthesis is decreased and excess orotic acid is excreted in the urine (hence the orotic acid is excreted in the urine (hence the name orotic aciduria)name orotic aciduria)

Biosynthesis of Pyrimidine

Purine DegradationPurine Degradation

Purine Nucleotides from ingested nucleic Purine Nucleotides from ingested nucleic acids or turnover of cellular nucleic acids acids or turnover of cellular nucleic acids is excreted by humans as uric acid.is excreted by humans as uric acid.

Humans excrete about 0.6 g uric acid Humans excrete about 0.6 g uric acid every 24 hours.every 24 hours.

The enzyme “nucleotidase” is also known as

purine nucleotide phosphorylase

(PNP)

Adenosine Deaminase (ADA) and Purine Adenosine Deaminase (ADA) and Purine Nucleoside Phosphorylase (PNP) Deficiency.Nucleoside Phosphorylase (PNP) Deficiency.

A deficiency of either ADA or PNP causes a A deficiency of either ADA or PNP causes a moderate to complete lack of immune function.moderate to complete lack of immune function.

Affected children cannot survive outside a Affected children cannot survive outside a sterile environment. sterile environment.

They may also have moderate neurological They may also have moderate neurological problems, including partial paralysis of the problems, including partial paralysis of the limbs. limbs.

When a compatible donor can be found, bone When a compatible donor can be found, bone marrow transplant is an effective treatment. marrow transplant is an effective treatment.

Catabolysm of Pyrimidines

Pyrimidines are generally degraded to intermediates of carbon metabolism (for example, succinyl-CoA) and ammonia (NH4+).

NH4+ is packaged as urea through the urea cycle and excreted by humans

Defects in enzymes of pyrimidine degradation havebeen documented, resulting in increased levels of pyrimidines and neurological disorders.

No treatments are available and mechanisms are unknown

Pyrimidine and Purine SalvagePyrimidine and Purine Salvage

Free Purine and Pyrimidine bases are Free Purine and Pyrimidine bases are constantly released in cells during the constantly released in cells during the metabolic degradation of nucleotides.metabolic degradation of nucleotides.

Free Purine and Pyrimidine bases are in large Free Purine and Pyrimidine bases are in large part salvaged and reused to make nucleotides.part salvaged and reused to make nucleotides.

Salvage of free nucleotides consumes much Salvage of free nucleotides consumes much less energy than de novo nucleotide synthesis less energy than de novo nucleotide synthesis and is the energetically preferred source of and is the energetically preferred source of nucleotides for nucleic acid synthesis.nucleotides for nucleic acid synthesis.

Purine SalvagePurine Salvage Salvage of the free purine bases guanine and Salvage of the free purine bases guanine and

hypoxanthine (the deamination product of hypoxanthine (the deamination product of adenine) often involves the enzyme adenine) often involves the enzyme hypoxanthine-guanine hypoxanthine-guanine phosphoribosyltransferase (HGPRT)phosphoribosyltransferase (HGPRT)

Salvage of free adenine is accomplished by the Salvage of free adenine is accomplished by the enzyme adenine phosphoribosyltransferase enzyme adenine phosphoribosyltransferase (APRT), converting free adenine and PRPP to (APRT), converting free adenine and PRPP to adenosine monophosphate (AMP)adenosine monophosphate (AMP)

Purine SalvagePurine Salvage

Inter-relationship between the different processes of purine metabolism

Pyrimidine SalvagePyrimidine Salvage

Pyrimidine salvage defects have not been clinically documented

Lesch-Nyhan SyndromeLesch-Nyhan Syndrome Hypoxanthine Guanine Phosphoribosyltransferase Hypoxanthine Guanine Phosphoribosyltransferase

(HGPRT) deficiency(HGPRT) deficiency X-linked genetic conditionX-linked genetic condition Severe neurologic disease, characterized by self-Severe neurologic disease, characterized by self-

mutilating behaviors such as lip and finger biting mutilating behaviors such as lip and finger biting and/or head bangingand/or head banging

Up to 20 times the uric acid in the urine than in normal Up to 20 times the uric acid in the urine than in normal individuals. Uric acid crystals form in the urine.individuals. Uric acid crystals form in the urine.

Untreated condition results in death within the first Untreated condition results in death within the first year due to kidney failure.year due to kidney failure.

Treated with allopurinol, a competitive inhibitor of Treated with allopurinol, a competitive inhibitor of xanthine oxidase.xanthine oxidase.

Lesch-Nyhan SyndromeLesch-Nyhan SyndromeLesch-Nhyan syndrome, an X-linked disorder that results from a mutation in the gene for hypoxanthine-guanine phosphoribosyltransferase, is characterized by severe neurological defects.• mental retardation

• self-mutilation

• Aggressive behaviour• cerebral palsy

• elevated uric acid (gout)

Allopurinol and HypoxanthineAllopurinol and Hypoxanthine

GoutGout Elevated uric acid levels in Elevated uric acid levels in

the bloodthe blood Uric acid crystals will form in Uric acid crystals will form in

the extremities with a the extremities with a surrounding area of surrounding area of inflammation. This is called inflammation. This is called a a tophustophus and is often and is often described as an arthritic described as an arthritic “great toe”.“great toe”.

Can be caused by a defect Can be caused by a defect in an enzyme of purine in an enzyme of purine metabolism or by reduced metabolism or by reduced secretion of uric acid into secretion of uric acid into the urinary tract.the urinary tract.

tophus

GOUTGOUT GENETIC DEFECT OF PURINE METABOLISM:GENETIC DEFECT OF PURINE METABOLISM:

OVER SECRETION OF URIC ACID OVER SECRETION OF URIC ACID oror RENAL DEFECT = DECREASED EXCRETION RENAL DEFECT = DECREASED EXCRETION

OF URIC ACID OF URIC ACID oror BOTHBOTH

ELEVATED URIC ACIDELEVATED URIC ACID

GOUTGOUT

CAUSES:CAUSES: SEVERE DIETING/STARVATIONSEVERE DIETING/STARVATION EXCESSIVE INTAKE OF FOODS HIGH IN EXCESSIVE INTAKE OF FOODS HIGH IN

PURINESPURINES HEALTH CONDITIONS WHICH CAUSE AN HEALTH CONDITIONS WHICH CAUSE AN

INCREASE IN CELL TURNOVER INCREASE IN CELL TURNOVER (LEUKEMIA, MULTIPLE MYELOMA, SOME (LEUKEMIA, MULTIPLE MYELOMA, SOME ANEMIAS, etc.)ANEMIAS, etc.)

Hyperuricemia is a risk factorHyperuricemia is a risk factor

for gout, but some patients with for gout, but some patients with normal serum uric acid levels normal serum uric acid levels

develop acute gouty arthritisdevelop acute gouty arthritis..

HYPERURICEMIA (ELEVATED URIC ACID)HYPERURICEMIA (ELEVATED URIC ACID)

Greater than 7 mg/dlGreater than 7 mg/dl Symptoms related to sudden increase of uric acid levelsSymptoms related to sudden increase of uric acid levels Urate crystals precipitate within a joint – inflammatory response Urate crystals precipitate within a joint – inflammatory response

occursoccurs Tophi – accumulations of urate crystalsTophi – accumulations of urate crystals

Overproduction of urOverproduction of uric acidic acid• • Primary idiopathic hyperuricemiaPrimary idiopathic hyperuricemia • • Hypoxanthine-guanine phosphoribosyl-transferase Hypoxanthine-guanine phosphoribosyl-transferase deficiencydeficiency • • Phosphoribosylpyrophosphate synthetase overactivityPhosphoribosylpyrophosphate synthetase overactivity • • Hemolytic processesHemolytic processes • • Lymphoproliferative diseaseLymphoproliferative disease • • Myeloproliferative diseaseMyeloproliferative disease • • Polycythemia veraPolycythemia vera • • Psoriasis (severe)Psoriasis (severe) • • Paget's diseasePaget's disease • • RhabdomyolysisRhabdomyolysis • • ExerciseExercise • • AlcoholAlcohol • • ObesityObesity • • Purine-rich dietPurine-rich diet

Decreased excretion of uric Decreased excretion of uric acidacid • • Primary idiopathic hyperuricemiaPrimary idiopathic hyperuricemia

• • Renal insufficiencyRenal insufficiency • • Polycystic kidney diseasePolycystic kidney disease • • Diabetes insipidusDiabetes insipidus • • HypertensionHypertension • • AcidosisAcidosis

--Lactic acidosis--Lactic acidosis --Diabetic ketoacidosis--Diabetic ketoacidosis

• • Down syndromeDown syndrome • • Starvation ketosisStarvation ketosis • • BerylliosisBerylliosis • • SarcoidosisSarcoidosis • • Lead intoxicationLead intoxication • • HyperparathyroidismHyperparathyroidism • • HypothyroidismHypothyroidism • • Toxemia of pregnancyToxemia of pregnancy • • Bartter's syndromeBartter's syndrome

TREATMENTTREATMENT Serum uric acid concentrations may be reduced Serum uric acid concentrations may be reduced

with nonpharmacologic therapy. Useful with nonpharmacologic therapy. Useful dietary dietary and lifestyle changes include weight and lifestyle changes include weight reduction, decreased alcohol ingestion, reduction, decreased alcohol ingestion, decreased consumption of foods with a decreased consumption of foods with a high purine content, and control of high purine content, and control of hyperlipidemia and hypertensionhyperlipidemia and hypertension. Used . Used alone, however, these measures will probably not alone, however, these measures will probably not reduce serum uric acid levels to normal, which is reduce serum uric acid levels to normal, which is the treatment goal for the treatment goal for the prevention of acute gout the prevention of acute gout attacksattacks. Symptomatic hyperuricemia usually . Symptomatic hyperuricemia usually requires medication. requires medication.

Acute Gouty ArthritisAcute Gouty Arthritis

Three treatments currently available Three treatments currently available for acute gouty arthritis attacks are for acute gouty arthritis attacks are nonsteroidal anti-inflammatory drugs nonsteroidal anti-inflammatory drugs (NSAIDs)(NSAIDs), colchicine and , colchicine and corticosteroids.corticosteroids.

NSAIDsNSAIDs short-acting, short durationshort-acting, short duration,, potent antiinflammatory potent antiinflammatory effecteffect

Indomethacin( indocin Indomethacin( indocin )):50mg tid-qid:50mg tid-qid Diclofenic acid: Cataflam, VorenDiclofenic acid: Cataflam, Voren NaproxenNaproxen:750mg st:750mg st250mg tid250mg tid

Special NSIADs:Special NSIADs: Specific COX-2 inhibitors: celecoxib, Specific COX-2 inhibitors: celecoxib, rofecoxibrofecoxib Meloxicam: preferential COX-2 inhibitor, least GI side Meloxicam: preferential COX-2 inhibitor, least GI side

effecteffect Sulindac: least nephrotoxicity Sulindac: least nephrotoxicity Cinopal: least GI toxicityCinopal: least GI toxicity

ColchicineColchicine mechanism mechanism

PMN: Polymorphonuclear neutrophil

CorticosteroidsOral Prednisone, 0.5 mg per kg

on day 1, taper by 5.0 mg each day thereafter

Fluid retention; impaired wound healing

Intramuscular Triamcinolone acetonide (Kenalog), 60 mg intramuscularly, repeat in 24 hours if necessary

May require repeat injections; risk of soft tissue atrophy

Intra-articular Large joints: 10 to 40 mg* Small joints: 5 to 20 mg*

Preferable route for monoarticular involvement

ACTH† 40 to 80 IU intramuscularly; repeat every 8 hours as necessary

Repeat injections are commonly needed; requires intact pituitary-adrenal axis; stimulation of mineralocorticoid release may cause volume overload

ACTH=adrenocorticotropic hormone

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