Basic principles of purine degradation
Purine nucleotide catabolism is a multi-step process that can be broadly divided into three types of biochemical reactions:
- Hydrolysis — the removal of inorganic phosphate from the parent nucleotide.
- Deamination — the removal of an amino group with the release of free ammonia.
- Oxidation — the introduction of oxygen into the molecule.
Interestingly, the utilization pathways of adenylic (AMP) and guanylic (GMP) nucleotides begin differently, but eventually converge at a single point — the formation of xanthine.
AMP degradation pathway
The degradation of adenylic nucleotides involves several sequential steps leading to the formation of hypoxanthine:
- First, a phosphate group is hydrolytically cleaved from AMP. This dephosphorylation reaction is catalyzed by the enzyme nucleotidase (5'-nucleotidase). The resulting nucleoside is adenosine.
- Next, adenosine deaminase comes into play. It performs the deamination of adenosine by cleaving off an ammonia molecule. The product of this reaction is inosine.
- The third step involves the cleavage of the N-glycosidic bond. The enzyme purine nucleoside phosphorylase performs phosphorolytic cleavage of inosine in the presence of phosphoric acid. Ribose is removed as ribose-1-phosphate, leaving only the nitrogenous base — hypoxanthine — from the nucleoside.
GMP degradation pathway
The catabolism of guanylic nucleotides is largely similar to AMP degradation, but has a different reaction order and specific enzymes:
- As with AMP, the process begins with the action of nucleotidase, which hydrolytically cleaves inorganic phosphate from GMP, converting it into guanosine.
- Then purine nucleoside phosphorylase acts. It cleaves off the carbohydrate component (ribose as ribose-1-phosphate), releasing the free nitrogenous base — guanine.
- In the final step of this branch, the enzyme guanase deaminates guanine into xanthine — the molecule where both degradation pathways converge.
Common oxidation step
Once hypoxanthine (from AMP) and xanthine (from GMP) are formed, the oxidation stage begins. The main player here is the enzyme xanthine oxidase, which belongs to the oxidoreductase class.
This enzyme sequentially catalyzes two reactions:
- Oxidation of hypoxanthine to xanthine.
- Oxidation of xanthine to uric acid (involves the introduction of a hydroxyl group at the C8 position of the purine ring).
Both reactions proceed with the obligatory participation of molecular oxygen and water. A side product, yet extremely critical to these oxidative processes, is hydrogen peroxide.
Enzymopathies and immunodeficiencies
Genetic defects in enzymes involved in purine degradation lead to severe disorders of the immune system:
- Adenosine deaminase (ADA) deficiency: Causes severe combined immunodeficiency (SCID / SCID). Both T lymphocytes and B lymphocytes are affected in this condition. The mechanism involves the accumulation of deoxyadenosine, which is converted to dATP. Excess dATP inhibits ribonucleotide reductase, completely blocking DNA synthesis and leading to massive lymphocyte apoptosis.
- Purine nucleoside phosphorylase (PNP) deficiency: Manifests as an immunodeficiency with predominant involvement of T-lymphocytes (B-cell function is often preserved). The underlying mechanism is the accumulation of dGTP, which has a pronounced toxic effect on T-cells.
Briefly: answers to topical questions
What cofactors and prosthetic groups are required for xanthine oxidase activity?
Several cofactors and prosthetic groups are required for xanthine oxidase activity. These include:
- FAD — a vitamin B2 derivative;
- Molybdenum (Mo) — a metal incorporated into the enzyme;
- Iron-sulfur clusters (Fe-S).
This enzyme is an aerobic dehydrogenase. It catalyzes two consecutive irreversible oxidation reactions of hypoxanthine and xanthine, transferring electrons to oxygen to form hydrogen peroxide.
What pathology develops with the excessive accumulation of uric acid in the body, and what are its causes?
Excessive accumulation of uric acid (hyperuricemia) leads to the development of gout (Gout).
The causes of the disease include:
- Genetically determined increase in synthesis or enhanced degradation of purine bases (including PRPP synthetase defects and decreased hypoxanthine-guanine phosphoribosyltransferase activity).
- Impaired renal excretion of uric acid.
- Excessive consumption of meat and purine-rich foods.
- Enhanced nucleic acid turnover in myelogenous leukemia, psoriasis, and hemoglobinopathies (secondary gout).
- Iatrogenic factors (cytostatic therapy, diuretics) and chronic renal failure.
Which drugs inhibit xanthine oxidase and what are they used for?
The sources identify allopurinol and febuxostat as xanthine oxidase inhibitors.
Allopurinol is used in gout: for the treatment of gout and the prevention of its flares as baseline therapy; it is not used for the immediate relief of pain during an acute attack. In normal renal function, allopurinol is recommended as first-line therapy.
Allopurinol is also used in tuberculosis patients receiving chemotherapy who develop hyperuricemia.
Mechanism of action of allopurinol: it is a structural analogue of hypoxanthine; xanthine oxidase oxidizes it to oxypurinol, which binds tightly to the active site of the enzyme. As a result, uric acid production decreases, and purine catabolism halts at the hypoxanthine stage, which is more soluble than uric acid.
During stable-dose therapy with xanthine oxidase inhibitors — allopurinol and febuxostat — monitoring of serum uric acid levels is required.
To remember the enzymes of the AMP degradation branch, use the acronym NAP: Nucleotidase — Adenosine deaminase — Purine nucleoside phosphorylase.
Class and exam questions
Which enzyme catalyzes the formation of uric acid?
The formation of uric acid is catalyzed by the enzyme xanthine oxidase. It sequentially oxidizes hypoxanthine to xanthine, and then xanthine to uric acid.
What is the function of purine nucleoside phosphorylase?
It catalyzes the phosphorolytic cleavage of nucleosides (inosine and guanosine). This cleaves off ribose as ribose-1-phosphate and releases the free nitrogenous base.
Why does adenosine deaminase deficiency lead to lymphocyte death?
Due to the absence of the enzyme, deoxyadenosine accumulates and is converted into dATP. High concentrations of dATP inhibit ribonucleotide reductase, halting DNA synthesis in cells.
What else to study on this topic
- Mechanisms of hyperuricemia development
- Pathogenesis of gout and urate deposition
- Structure of AMP and GMP purine nucleotides
- Regulation of xanthine oxidase activity
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