Purine Degradation (to Uric Acid)
- Compartment
- Cytosol
- Main tissue
- Liver, intestine
- Rate-limiting
- Not flagged in the source
- Steps
- 10
Reaction steps
In source order, 10 total
-
1
AMP → adenosine
› Notes
Cytosolic 5′-nucleotidase catalyzes AMP + H2O → adenosine + Pi. This hydrolytic dephosphorylation is one route for AMP entry into degradation.
-
2
AMP → IMP
› Notes
AMP deaminase (adenylate deaminase) catalyzes AMP + H2O → IMP + NH3. This route is prominent in exercising skeletal muscle and in the purine nucleotide cycle; it is not the universal route of AMP degradation.
-
3
Adenosine → inosine
› Notes
Adenosine deaminase (ADA) catalyzes adenosine + H2O → inosine + NH3. The enzyme requires no external redox cofactor and channels adenosine toward hypoxanthine production.
-
4
IMP → inosine
› Notes
5′-Nucleotidase catalyzes IMP + H2O → inosine + Pi. Thus AMP that is first deaminated to IMP joins the inosine pathway.
-
5
Inosine → hypoxanthine
In InosineOut Hypoxanthine› Notes
PNP catalyzes inosine + Pi ⇌ hypoxanthine + ribose 1-phosphate. In catabolic tissues, the reaction commonly proceeds toward free hypoxanthine.
-
6
Hypoxanthine → xanthine
In HypoxanthineOut Xanthine› Notes
Xanthine oxidoreductase (XOR) catalyzes hypoxanthine + H2O + oxidized electron acceptor → xanthine + reduced electron acceptor. As xanthine dehydrogenase, XOR can transfer electrons to NAD+; as xanthine oxidase, it transfers electrons to O2, generating superoxide and/or H2O2. The molybdenum cofactor, FAD, and iron-sulfur centers are required.
-
7
Xanthine → uric acid (urate)
› Notes
XOR catalyzes xanthine + H2O + oxidized electron acceptor → uric acid + reduced electron acceptor. This is the terminal oxidation in humans and is effectively irreversible; uric acid is largely deprotonated to monosodium urate in extracellular fluid.
-
8
GMP → guanosine
› Notes
5′-Nucleotidase catalyzes GMP + H2O → guanosine + Pi.
-
9
Guanosine → guanine
› Notes
PNP catalyzes guanosine + Pi ⇌ guanine + ribose 1-phosphate.
-
10
Guanine → xanthine
› Notes
Guanine deaminase (guanase) catalyzes guanine + H2O → xanthine + NH3. Xanthine then enters step 7 and is oxidized to uric acid.
Showing all 10 steps.
Regulation
What speeds each enzyme up and what slows it down
AMP deaminase
High AMP, especially during ATP depletion in muscle
ATP, GTP, and phosphate-dependent energy sufficiency signals (isoform dependent)
No principal direct endocrine control; enzyme activity rises functionally during intense exercise/energy stress.
Adenosine deaminase
Adenosine availability
No major physiologic small-molecule allosteric inhibitor
No established direct hormonal regulation.
Xanthine oxidoreductase
Hypoxanthine/xanthine availability; conversion of dehydrogenase to oxidase can occur with oxidation/proteolysis
Allopurinol (via oxypurinol), febuxostat, topiroxostat
No dominant acute hormonal regulation; expression can rise with inflammatory and hypoxic/ischemic stress.
Renal urate transport (URAT1/GLUT9; not an enzyme)
Volume depletion and organic anion exchange can favor urate reabsorption
Uricosuric drugs such as probenecid/lesinurad inhibit relevant transport
Insulin resistance and hyperinsulinemia are associated with increased renal urate reabsorption.
Overview
Humans degrade purine nucleotides to uric acid (present predominantly as urate at physiologic pH), which is excreted mainly by the kidney and, to a lesser extent, the intestine. Unlike most mammals, humans lack functional uricase, so urate is the terminal product rather than allantoin. Purine catabolism is integrated with salvage because free hypoxanthine and guanine can either be recycled or diverted to oxidation.
Cellular location
Nucleotidases, adenosine deaminase, PNP, and deaminases are cytosolic and widely distributed, with substantial activity in liver, intestine, kidney, erythrocytes, and immune cells. Xanthine oxidoreductase (xanthine dehydrogenase/xanthine oxidase) is particularly abundant in liver and intestinal mucosa and is largely cytosolic in hepatocytes and intestinal mucosal cells. Uric acid is released to blood and handled by renal proximal tubular transporters, including URAT1 and GLUT9, which influence serum urate concentration.
Net energetics
The core degradative sequence does not consume ATP; nucleotide dephosphorylation releases inorganic phosphate and phosphorolysis conserves the ribose moiety as ribose 1-phosphate. Two XOR-mediated oxidations convert hypoxanthine to urate and transfer reducing equivalents to NAD+ (dehydrogenase form) or, often in vivo, to O2 (oxidase form), generating reactive oxygen species. Deamination reactions release ammonia, which is detoxified predominantly by hepatic urea synthesis at an additional whole-body energetic cost.
Clinical significance
Urate can act as a plasma antioxidant, but its limited solubility makes sustained hyperuricemia a risk for monosodium urate crystal deposition, gout, nephrolithiasis, and urate nephropathy. Allopurinol and febuxostat lower urate by inhibiting XOR; rasburicase or pegloticase supplies a uricase activity absent from humans and converts urate to the more soluble allantoin. ADA deficiency causes accumulation of adenosine/deoxyadenosine and toxic dATP elevation, leading to severe combined immunodeficiency (SCID). Tumor lysis syndrome can cause acute hyperuricemia when large amounts of nucleic acid are rapidly degraded.
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