General pathogenesis: how purines destroy immunity
Normally, purine nucleosides are gradually cleaved down to end products, specifically nitrogenous bases and uric acid. In genetic deficiency of specific catabolic enzymes, this process slows down dramatically.
The cascade of biochemical disorders unfolds as follows:
- Due to the blockage of salvage pathways, a huge amount of ribonucleotides and deoxyribonucleotides begins to accumulate in cells.
- In tissues with high mitotic activity (which include lymphocytes), the concentration of dATP and dGTP reaches critical levels.
- An excess of these substances acts as a potent allosteric inhibitor for the enzyme ribonucleotide reductase (RNR).
- Normally, RNR catalyzes the conversion of ribonucleoside diphosphates (NDFs) into deoxy derivatives (dNDFs) using reduced thioredoxin. When the enzyme is blocked, the synthesis of other dNDFs ceases.
- An acute shortage of building substrates for DNA replication arises. The cell loses its ability to divide, which has the most detrimental effect on the lymphocyte population.
Severe Combined Immunodeficiency (SCID)
This disease is based on a deficiency of the enzyme adenosine deaminase (ADA). This enzyme catalyzes crucial deamination reactions:
- In the presence of water, adenosine is converted into inosine with the release of ammonia.
- Deoxyadenosine is transformed in a similar manner into deoxyinosine.
When ADA is inactive, deoxyadenosine is not degraded, but is phosphorylated by intracellular kinases into dATP (deoxyadenosine triphosphate). Accumulating in giant concentrations, dATP binds to ribonucleotide reductase and completely shuts down the synthesis of all other deoxyribonucleotides (dNTPs).
As a result, the proliferation and maturation of two immune lineages at once—T-lymphocytes and B-lymphocytes—suffer. Profound cellular and humoral immunodeficiency develops. Without timely therapy, children born with this pathology rapidly die from incessant infectious diseases.
Purine Nucleoside Phosphorylase Deficiency
The second type of nucleotide-related immunodeficiency is associated with a defect in the enzyme purine nucleoside phosphorylase.
In this case, the biochemical culprit of the pathology is an excess of dGTP (deoxyguanosine triphosphate), which accumulates in precursor cells. As in SCID, dGTP blocks ribonucleotide reductase, thereby inhibiting the production of pyrimidine deoxyribonucleotides.
Features of this disease:
- Unlike the accumulation of dATP, the inhibitory effect of dGTP is significantly weaker.
- The pathological process halts the maturation predominantly of T-lymphocytes, which is why patients suffer primarily from cellular immunity impairment.
- The clinical picture is characterized by frequent and prolonged infections progressing into a chronic form.
Briefly: answers to topical questions
What chemical reaction does purine nucleoside phosphorylase normally catalyze?
Normally, purine nucleoside phosphorylase catalyzes the phosphorolytic cleavage of nucleosides, converting them into nitrogenous bases. The reaction is a phosphorolysis of the N-glycosidic bond, in which ribose-1-phosphate is cleaved off and the base is released.
Specific catalyzed transformations:
- Inosine is cleaved to hypoxanthine.
- Deoxyinosine is also converted to hypoxanthine.
- Guanosine is converted to guanine.
By means of which specific kinases is uncleaved deoxyadenosine phosphorylated to dATP?
In the cited sources, only deoxycytidine kinase is specifically named: it has broad substrate specificity and phosphorylates not only deoxycytidine, but also deoxyguanosine and deoxyadenosine.
The sources also indicate that excess deoxyadenosine is phosphorylated by kinases to dATP, but other specific kinases for this conversion are not named.
SCID (ADA deficiency) affects all — both T- and B-lymphocytes, whereas PNP deficiency affects only T-lymphocytes.
Class and exam questions
Why do lymphocytes specifically suffer in these biochemical defects?
Lymphocytes are rapidly dividing cells. Their multiplication and maturation require ongoing DNA synthesis, which comes to a complete halt in the event of a deoxyribonucleotide deficiency.
Which enzyme is inhibited upon the accumulation of dATP and dGTP?
Ribonucleotide reductase (RNR) is allosterically blocked — this is the key enzyme that converts NDFs into dNDFs.
What accumulates in adenosine deaminase deficiency?
Excess uncleaved deoxyadenosine is phosphorylated by kinases, leading to a critical intracellular accumulation of dATP.
What else to study on this topic
- Mechanism of the ribonucleotide reductase reaction involving reduced thioredoxin
- Deoxyadenosine phosphorylation pathways by cellular kinases
- Allosteric regulation of ribonucleotide reductase by various nucleotides
- Final stages of purine catabolism and uric acid formation
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «Nucleotide metabolism-related immunodeficiencies» and 3,616 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the "Nucleotide Metabolism" section
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.