Lesch-Nyhan syndrome (complete HGPRT deficiency) causes severe neurological symptoms — self-injurious behavior, dystonia, intellectual disability — not just gout. What best explains why the brain is so specifically affected?
AHGPRT is expressed exclusively in neurons, so only brain tissue is affected by its deficiency
BExcess uric acid from purine degradation is specifically neurotoxic and accumulates in brain tissue
CCertain brain cells depend almost exclusively on salvage for purine nucleotides and cannot upregulate de novo synthesis to compensate
DHGPRT deficiency blocks the blood-brain barrier, preventing nucleotides synthesized elsewhere from reaching the brain
The neurological devastation of Lesch-Nyhan syndrome cannot be explained by uric acid alone — gout is painful but not sufficient to cause the severe neurological phenotype. The deeper explanation is tissue-specific dependency: certain neurons (especially in the basal ganglia) rely almost entirely on salvage to maintain their purine nucleotide pools. Unlike liver or other tissues with high de novo capacity, these cells cannot compensate when HGPRT is absent. This reveals that salvage pathways are not merely energy-efficient alternatives — they are irreplaceable supply routes for specific tissues.
Question 2 Multiple Choice
What role does PRPP (phosphoribosyl pyrophosphate) play in purine salvage reactions?
AIt donates a phosphate group to energize the reaction, similar to ATP in kinase reactions
BIt provides the ribose-phosphate backbone that converts a free base into a metabolically active nucleotide
CIt allosterically activates HGPRT and APRT to increase their reaction rates
DIt serves as the immediate precursor to the purine ring structure in both salvage and de novo synthesis
PRPP (5-phosphoribosyl-1-pyrophosphate) acts as a universal adapter in purine salvage: it donates its phosphoribosyl group to a free purine base, producing a nucleoside monophosphate and releasing pyrophosphate. Without PRPP, the free base — whether hypoxanthine, guanine, or adenine — has no ribose-phosphate handle and cannot be used metabolically. PRPP plays the same role in de novo purine synthesis (as the initial carbon-nitrogen acceptor), making it a central hub connecting both biosynthetic routes. Kinases, which are used in pyrimidine salvage, work differently: they add phosphate to an existing nucleoside.
Question 3 True / False
Tissues that depend primarily on salvage pathways for nucleotide supply may be devastated by HGPRT deficiency even if de novo synthesis remains fully intact in those same cells.
TTrue
FFalse
Answer: True
De novo synthesis is not automatically upregulated to compensate when salvage fails. Some tissues — particularly certain neurons — have low intrinsic de novo synthetic capacity and are physiologically configured to rely on salvage. When HGPRT is absent, these cells cannot simply switch to making purines from scratch; the biosynthetic machinery isn't present at sufficient capacity. This tissue-specific dependency is why neurological symptoms occur in Lesch-Nyhan syndrome even though de novo synthesis continues normally in the liver and other tissues with high biosynthetic capacity.
Question 4 True / False
Pyrimidine salvage works by the same phosphoribosyltransferase mechanism as purine salvage — free bases are converted to nucleotides by attaching PRPP.
TTrue
FFalse
Answer: False
Purine and pyrimidine salvage use fundamentally different chemistry. Purine salvage uses phosphoribosyltransferases (HGPRT, APRT) that transfer a phosphoribosyl group from PRPP to a free purine base. Pyrimidine salvage, by contrast, relies on kinases — enzymes that phosphorylate an existing nucleoside (which already has its ribose). Thymidine kinase, for example, adds a phosphate group to thymidine (a nucleoside) to produce thymidine monophosphate. The distinction matters clinically and pharmacologically: the selectivity of antiviral drugs often depends on these enzymatic differences.
Question 5 Short Answer
Why does the antiviral drug acyclovir selectively target virus-infected cells rather than healthy cells, and how does this depend on nucleotide salvage pathways?
Think about your answer, then reveal below.
Model answer: Acyclovir is a nucleoside analog — a modified form of guanosine — that must be phosphorylated to become active (acyclovir triphosphate, which inhibits viral DNA polymerase). The initial phosphorylation step is performed by viral thymidine kinase, an enzyme that herpes viruses encode and express in infected cells. Human cells do not express the same thymidine kinase and cannot efficiently phosphorylate acyclovir. Therefore, acyclovir accumulates as the active triphosphate only in infected cells, sparing uninfected tissue. This selectivity exploits the difference between viral and cellular salvage enzyme repertoires.
This pharmacological principle — designing prodrugs that are activated by pathogen-specific salvage enzymes — extends throughout antiviral and anticancer pharmacology. Many nucleotide analogs are selectively toxic because they depend on enzymes that are differentially expressed in diseased cells. Understanding salvage enzyme distributions across tissues and pathogens is therefore essential for rational drug design.