Purine Degradation (to Uric Acid)

1. 1

   AMP → adenosine

   5'-Nucleotidase (NT5C/NT5E) 3.1.3.5 ST-0214 Irreversible/directional H2O Pi
   In AMP

   Out Adenosine
   › Notes

   Cytosolic 5′-nucleotidase catalyzes AMP + H2O → adenosine + Pi. This hydrolytic dephosphorylation is one route for AMP entry into degradation.

2. 2

   AMP → IMP

   AMP deaminase (adenylate deaminase) 3.5.4.6 ST-0215 Irreversible/directional H2O
   In AMP

   Out 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. 3

   Adenosine → inosine

   Adenosine deaminase (ADA) 3.5.4.4 ST-0216 Irreversible/directional H2O
   In Adenosine

   Out Inosine
   › Notes

   Adenosine deaminase (ADA) catalyzes adenosine + H2O → inosine + NH3. The enzyme requires no external redox cofactor and channels adenosine toward hypoxanthine production.

4. 4

   IMP → inosine

   5'-Nucleotidase (NT5C/NT5E) 3.1.3.5 ST-0217 Irreversible/directional H2O Pi
   In IMP

   Out Inosine
   › Notes

   5′-Nucleotidase catalyzes IMP + H2O → inosine + Pi. Thus AMP that is first deaminated to IMP joins the inosine pathway.

5. 5

   Inosine → hypoxanthine

   Purine nucleoside phosphorylase (PNP) 2.4.2.1 ST-0218 Irreversible/directional Pi
   In Inosine

   Out Hypoxanthine
   › Notes

   PNP catalyzes inosine + Pi ⇌ hypoxanthine + ribose 1-phosphate. In catabolic tissues, the reaction commonly proceeds toward free hypoxanthine.

6. 6

   Hypoxanthine → xanthine

   Xanthine oxidoreductase (XOR / XDH-XO) 1.17.3.2 / 1.17.1.4 ST-0219 Irreversible/directional H2O
   In Hypoxanthine

   Out 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. 7

   Xanthine → uric acid (urate)

   Xanthine oxidoreductase (XOR / XDH-XO) 1.17.3.2 / 1.17.1.4 ST-0220 Irreversible H2O
   In Xanthine

   Out Uric acid
   › 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. 8

   GMP → guanosine

   5'-Nucleotidase (NT5C/NT5E) 3.1.3.5 ST-0221 Irreversible/directional H2O Pi
   In GMP

   Out Guanosine
   › Notes

   5′-Nucleotidase catalyzes GMP + H2O → guanosine + Pi.

9. 9

   Guanosine → guanine

   Purine nucleoside phosphorylase (PNP) 2.4.2.1 ST-0222 Irreversible/directional Pi
   In Guanosine

   Out Guanine
   › Notes

   PNP catalyzes guanosine + Pi ⇌ guanine + ribose 1-phosphate.

10. 10

    Guanine → xanthine

    Guanine deaminase (guanase) 3.5.4.3 ST-0223 Irreversible/directional H2O
    In Guanine

    Out 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.

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.