Purine Degradation and Pyrimidine Synthesis (LEC)

Purine nucleotide catabolism pathway

AMP is converted to adenosine, which is then deaminated to form inosine by adenosine deaminase.

Conversion of inosine to hypoxanthine

Inosine undergoes ribose removal by purine nucleoside phosphorylase (PNP) to form hypoxanthine.

Hypoxanthine to uric acid pathway

Hypoxanthine is converted to xanthine and then to uric acid by xanthine oxidase.

Guanine degradation

Guanine is converted to xanthine by guanase and subsequently forms uric acid through xanthine oxidase.

Final product of purine degradation

The end product of purine degradation is uric acid.

Blood uric acid levels

Blood uric acid levels are close to saturation limits, which can lead to conditions like gout.

Causes of hyperuricemia: Overproduction

Hyperuricemia can occur due to overproduction of uric acid as a result of various factors.

Idiopathic causes of hyperuricemia

In many cases, the cause is unknown (idiopathic), but males and obesity are known risk factors.

PRPP synthetase gene mutations

Mutations in the PRPP synthetase gene can lead to increased Vmax and decreased Km, resulting in elevated levels of PRPP, which is an activator of purine synthesis, leading to excessive turnover of purines.

HGPRT deficiency (Lesch-Nyhan syndrome)

This genetic disorder results in less recycling (salvaging) of purine bases, leading to elevated PRPP levels that stimulate purine biosynthesis and increased purine turnover.

Impact of chemotherapy on uric acid levels

Patients undergoing chemotherapy may experience increased cell death, leading to the breakdown of DNA and RNA, which results in elevated uric acid levels.

Inherited disorders causing hyperuricemia

Inherited conditions such as von Gierke disease and hereditary fructose intolerance can lead to hyperuricemia due to disrupted metabolism.

Causes of hyperuricemia: Underexcretion

Hyperuricemia may be caused by underexcretion of uric acid, which is more common than overproduction.

Blood uric acid levels

Blood uric acid levels greater than 6.8 mg/dL are associated with conditions like gout due to monosodium urate crystals forming in joints and soft tissues.

Risk factors for idiopathic underexcretion

In many cases, the cause of underexcretion is not defined, but males and obesity are known risk factors.

Renal disease implications for uric acid

Renal disease can impair the excretion of uric acid, contributing to elevated levels in the blood.

Lactic acidosis and uric acid competition

In lactic acidosis, urate competes with lactate for excretion, leading to increased uric acid levels.

Causes of hyperuricemia: Overproduction

Hyperuricemia can also occur due to overproduction of uric acid, though it is less common than underexcretion.

Urate lowering therapy

Urate lowering therapy is aimed at lowering uric acid levels in the management of hyperuricemia.

Allopurinol mechanism of action

Allopurinol inhibits xanthine oxidase and decreases uric acid formation, leading to lower uric acid levels in the body.

Conversion of Allopurinol

Allopurinol is a hypoxanthine analog that is metabolized to oxypurinol, which acts as a non-competitive irreversible inhibitor of xanthine oxidase.

Solubility effects

Hypoxanthine and xanthine are more soluble than urate, which helps prevent precipitation of monosodium urate crystals in the joints and soft tissues.

Febuxostat mechanism of action

Febuxostat is a xanthine oxidase inhibitor that also lowers uric acid levels by decreasing its formation.

Autosomal recessive SCID (ADA deficiency)

Adenosine deaminase deficiency affects lymphocytes, leading to adenosine accumulation and increased [dATP] levels, which inhibits ribonucleotide reductase and reduces lymphocyte cell division, resulting in decreased T- and B-cell immunity.

Clinical features of ADA deficiency

Patients experience repeated infections, which can be fatal and life-threatening due to impaired immunity.

Treatment options for ADA deficiency

Treatment includes enzyme replacement therapy, bone marrow transplantation, or gene therapy, which has been successful in restoring immune function.

PNP deficiency

Inherited deficiency of purine nucleoside phosphorylase (PNP) is less common and leads to a less severe immunodeficiency that primarily affects T-cells.

Clinical features of PNP deficiency

Patients with PNP deficiency experience repeated infections, and lab tests indicate T-cell deficiency due to impaired nucleotide metabolism.

Donors of C and N atoms to the pyrimidine ring

The donors of C and N atoms to the pyrimidine ring are aspartate, glutamine, and CO2.

Pyrimidine biosynthesis

Pyrimidine biosynthesis involves the formation of uracil (RNA), cytosine, and thymine (DNA) from a common precursor.

Distinction between purine and pyrimidine biosynthetic pathways

Purine biosynthesis involves building the base onto a ribose-phosphate backbone, while pyrimidine biosynthesis involves assembling the pyrimidine ring first, and then adding ribose-phosphate later.

Donors of C and N atoms for pyrimidine

The donors of C and N atoms for pyrimidine synthesis are aspartate, glutamine, and CO2.

Thymidine synthesis requirements

Thymidine synthesis requires methylene tetrahydrofolate (THF), which provides the necessary carbon group for the conversion of uridine to thymidine.

Initial assembly of pyrimidine ring

In pyrimidine biosynthesis

Regulation of pyrimidine biosynthesis

The regulation of pyrimidine biosynthesis primarily involves CPS-II (carbamoyl phosphate synthetase-II) as the regulatory enzyme.

CPS-II vs. CPS-I

CPS-II is compared with CPS-I of the urea cycle; CPS-II is the regulatory step in pyrimidine biosynthesis.

Inhibitors of CPS-II

CPS-II is inhibited by UTP (end-product) through feedback inhibition.

Activators of CPS-II

CPS-II is activated by ATP and PRPP, ensuring an equal concentration of purine and pyrimidine nucleotides.

Other regulatory enzymes

Other key enzymes in pyrimidine biosynthesis include OPRT and OMP decarboxylase (UMP synthase), which play roles in the pathway.

Thymidine synthesis

Thymidine monophosphate (dTMP) is synthesized from deoxyuridine monophosphate (dUMP) by thymidylate synthase, which requires methylene tetrahydrofolate (THF) as a 1-carbon donor.

Conversion of THF

Methylene tetrahydrofolate is converted to dihydrofolate, which is subsequently converted to tetrahydrofolate (THF) by dihydrofolate reductase.

dTMP's unique requirement

dTMP is the only pyrimidine that requires methylene tetrahydrofolate for its synthesis.

Mechanism of action of 5-fluorouracil

5-Fluorouracil is converted to 5-fluoro-dUMP, which irreversibly binds to thymidylate synthase, inactivating it as a suicide inhibitor.

Mechanism of action of methotrexate

Methotrexate, a folate analog, competitively inhibits dihydrofolate reductase, preventing the formation of THF, which inhibits purine and thymidine biosynthesis.

Clinical application of 5-fluorouracil

5-Fluorouracil is used in cancer treatment for its ability to inhibit thymidine synthesis, thus hindering tumor cell proliferation.

Clinical application of methotrexate

Methotrexate is employed in cancer therapy and certain autoimmune diseases due to its inhibition of folate metabolism and subsequent effect on DNA synthesis.

Enzyme deficiency in orotic aciduria

OPRT and/or OMP decarboxylase deficiency (UMP synthase) leads to orotic aciduria.

Clinical features of orotic aciduria

A 1-year-old infant may present with weakness, anemia (low hemoglobin levels), macrocytic megaloblastic anemia, poor height and weight growth, orotic acid excretion in urine, and normal blood ammonia levels.

Metabolic basis of clinical features in orotic aciduria

Pyrimidine deficiency due to enzyme deficiency results in decreased RBC formation, leading to reduced DNA synthesis and cell division, which manifests as macrocytic megaloblastic anemia.

Effect of uridine administration

Uridine administration improves anemia as uridine is a common precursor for the synthesis of thymidine and cytidine, alleviating pyrimidine deficiency.

Regulatory role of UTP

UTP acts as a feedback inhibitor of CPS-II, which helps lower orotic acid levels.

Normal blood ammonia levels

The normal blood ammonia levels indicate that the urea

Comparison of CPS-I and CPS-II roles

CPS-I (carbamoyl phosphate synthetase-I) is involved in the urea cycle, facilitating the conversion of ammonia to urea, while CPS-II (carbamoyl phosphate synthetase-II) is involved in pyrimidine biosynthesis, producing carbamoyl phosphate for the synthesis of pyrimidine nucleotides.

Subcellular location of CPS-I and CPS-II

CPS-I is located in the mitochondria, where it participates in the urea cycle, whereas CPS-II is located in the cytosol for pyrimidine biosynthesis.

Orotic aciduria and hyperammonemia type II

Orotic aciduria can also result from hyperammonemia type II, which is due to a deficiency of ornithine transcarbamoylase in the urea cycle, leading to excess carbamoyl phosphate that diffuses into the cytosol for pyrimidine biosynthesis.

Differentiation between causes of orotic aciduria

To differentiate between orotic aciduria due to a urea cycle defect and that due to pyrimidine biosynthesis defect, one can measure blood ammonia levels; levels are normal in pyrimidine biosynthesis defects but elevated in urea cycle defects.