Purine Nucleotide Synthesis

• Two-purine nucleotides of nucleic acids are adenosine monophosphate (AMP) and guanosine monophosphate (GMP).

Purine Nucleotide Synthesis

• De novo and salvage pathways are two routes for purine nucleotide synthesis.

De Novo Synthesis Pathway

• Purines synthesized are derived from the diet, or from the catabolism of nucleic acids, which are either oxidized to urate or reused.
• Purine synthesis occurs in all tissues, with the liver being the major site and to a limited extent, in the brain.
• Red blood cells (RBCs) and polymorphonuclear leukocytes cannot produce purines through de novo synthesis.
• Substrates include: Ribose-5-phosphate, glycine, glutamine, H2O, ATP, CO2, and aspartate.
• Tetrahydrofolate (THF) provides atoms for the purine ring and is required for the synthesis of all purines.
• Products include GMP, AMP, glutamate, fumarate, and H2O.
• De novo synthesis happens in two stages: synthesis of IMP, the parent nucleotide, and conversion of IMP to AMP and GMP.
• Purine rings cannot be broken down and must be modified to uric acid for excretion and to act as a natural antioxidant.
• Most dietary purines are converted to uric acid in intestinal mucosal cells, with intestinal bacteria degrading any remaining unabsorbed purines.

Salvage Pathway

• The salvage pathway recycles nitrogenous bases from nucleic acid degradation.
• Salvage pathways resynthesize purine nucleotides from purine bases like adenine, guanine, and hypoxanthine.
• Key enzymes include hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and adenine phosphoribosyl transferase (APRT).
• Purine synthesis via the salvage pathway occurs in all tissues, and is especially important in the brain and bone marrow.
• Substrates: Hypoxanthine, PRPP, guanine, and adenine.
• Products: GMP, AMP, and IMP
• The salvage pathway is simpler and requires less energy than de novo synthesis, consisting of a single reaction.

Disorders of Purine Catabolism

• Can result in hyperuricemia or hypouricemia.

Hyperuricemia

• Overproduction of uric acid can be caused by enzymatic defects (primary gout):
• PRPP synthetase (phosphoribosyl pyrophosphate).
• Glutamyl PRPP amidotransferase.
• HGPRTase.
• Secondary to other diseases (secondary gout).
• Elevated destruction of cells or decreased elimination of uric acid occurs cancer (leukemia, polycythemia), psoriasis and hyper catabolic states (starvation, trauma, etc.).
• Renal disorder can cause an elevated threshold for uric acid excretion.

Hypouricemia

• Diminished production of uric acid by enzymatic defects in:
• Adenosine deaminase.
• Purine nucleoside phosphorylase.
• Xanthine oxidase.
• Enhanced renal excretion of uric acid can be caused by familial renal hypouricaemia, and Fanconi’s syndrome.

Gout

• Metabolic disease from overproduction or under-excretion of uric acid, resulting from poor solubility and excess urate crystallizing.
• Urate deposits in soft tissues such as kidneys (causing nephropathy), toes, and joints (causing gouty arthritis).
• Gout is associated with hyperuricemia, but hyperuricemia isn't always associated with gout.
• Allopurinol, a hypoxanthine analog, inhibits xanthine oxidase to convert purines into uric acid. This reduces uric acid formation and increases soluble xanthine and hypoxanthine.
• Average normal blood serum uric acid level is 4 to 7 mg per 100 ml.

Lesch-Nyhan Syndrome

• It is an inherited X-linked disorder that affects only males, caused by a rare genetic mutation that causes HGPRTase deficiency.
• It is caused by a complete deficiency of HGPRTase, an enzyme that is involved in purine salvage pathway.
• Lack of nucleotide salvage(HGPRTase)
  • H = hyperuricemia
  • G=gout
  • P=psychiatric problems(self mutatin)
  • R=retardness( low IQ ,renal ston )
  • T=dystonia in muscles
• In the absence and lack of HGPRTase, the salvage pathway is inhibited.
• Purines cannot be reconverted to nucleotides but are degraded to uric acid.
• Lack of HGPRTase also causes an overproduction of PRPP, which stimulates purine biosynthesis.
• Increased purine synthesis increases the degradation product uric acid, resulting in increased uric acid in plasma and urine.
• Brain cells normally have higher levels of the purine salvage enzyme (HGPRTase).
• Brain utilizes the salvage pathway for synthesizing IMP and GMP as it cannot synthesize purine nucleotides by de novo pathways.
• In Lesch-Nyhan syndrome, synthesis of purine nucleotides (DNA precursors) by the brain is decreased.

How to diagnose Lesch-Nyhan Syndrome

• Elevated uric acid levels in blood and urine.
• Genetic testing for mutations in the HGPRT1 gene
• Enzyme activity assays for HGPRT
• Allopurinol reduces uric acid formation but does not alleviate neurological symptoms.

Xanthinuria

• Xanthinuria is related to the consequences of Xanthine Oxidase Deficiency

Immunodeficiency Disorders of Purine Metabolism

• Adenosine Deaminase (ADA) Deficiency causes Severe Combined Immune Deficiency (SCID).
• The disease is fatal and victims usually die in early childhood (within 2 years of age.)
• In ADA deficiency, both thymus-derived lymphocytes (T-cells) and bone marrow-derived lymphocytes (B-cells) are dysfunctional.

Pyrimidine Synthesis

• The pyrimidine nucleotides are cytidine monophosphate (CMP), uridine monophosphate (UMP), and thymidine monophosphate (TMP).
• Pyrimidine synthesis is less complex than purine synthesis.
• Aspartate, CO2, and glutamine are sources of atoms in the pyrimidine ring and are required for both purine and pyrimidine synthesis.
• The end products of pyrimidine catabolism are highly water-soluble: CO2NH3, β-alanine, and β-aminoisobutyrate.

Disorders of Pyrimidine Catabolism

• A hereditary disorder from a defective enzyme in pyrimidine synthesis.
• A defect in the multifunctional enzyme UMP synthase converts orotic acid to UMP, resulting in excretion of orotic acid in urine.
• UMP synthase contains both orotate phosphoribosyl transferase and orotidylate decarboxylase activity.
• There are two types of orotic acidemia:
  • Type I: both enzymes are deficient.
  • Type II: only orotidylate decarboxylase is deficient.
• Deficiency in UMP and other pyrimidine nucleotides results in inhibition of DNA and RNA synthesis.
• This leads to megaloblastic anemia and failure to thrive (growth retardation).

Reye's Syndrome

• Reye’s syndrome is secondary orotic aciduria (not due to defective pyrimidine nucleotide synthesis).
• Due to the inability of severely damaged mitochondria to utilize carbamoylphosphate in the formation of urea.
• Leads to cytosolic overproduction of orotic acid.
• Reyes syndrome associated with High blood ammonia and glutamine levels.