Answer to NcM-01: (a)
During the catabolism of purine bases Xanthine Oxidase catalyzes the conversion of Hypoxanthine to Xanthine and the conversion of Xanthine to uric acid.
Adenosine—>Inosine—>Hipoxanthine—>Xanthine<—Guanine<—Guanosine
X.O.
Xanthine —> Uric Acid
Allopurinol is a structural analog of Hipoxanthine:
Hipoxanthine structure
Allopurinol structure
Allopurinol can bind to Xanthine Oxidase and be converted to alloxanthine (oxypurinol), the metabolite responsible of most of the inhibitory effects of the administration of allopurinol.
“ The action of oral allopurinol differs from that of uricosuric agents, which lower the serum uric acid level by increasing urinary excretion of uric acid. Allopurinol reduces both the serum and urinary uric acid levels by inhibiting the formation of uric acid. The use of allopurinol to block the formation of urates avoids the hazard of increased renal excretion of uric acid posed by uricosuric drugs. (Source: U.S. Food and Drug Administration • Center for Drug Evaluation and Research. FDA Oncology Tools Product Label Details for Prescribing allopurinol)
The solubility of Xanthine and Hypoxanthine is higher than the solubility of Uric acid and the renal clearance is at least 10 times higher than the clearance for uric acid.
For more information about Allopurinol and Gout, follow these links:
If you already studied Pharmacology, try to answer the following questions:
Is it indicated to use Allopurinol during an accute attack of Gout?
Which other uses does Allopurinol have in Medicine?
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