Purines are vital nitrogen-containing compounds that form the building blocks of nucleic acids—DNA and RNA—and participate in numerous cellular functions. However, like all biomolecules, purines undergo catabolism to maintain metabolic balance. The final catabolic product of purines varies among species, with humans uniquely producing uric acid as the end product. This article delves into the purine catabolic pathway, emphasizing uric acid formation, its physiological significance, and related health implications.

Overview of Purine Catabolism

Purine catabolism involves the breakdown of purine nucleotides—adenine and guanine—through sequential enzymatic reactions:

1. Dephosphorylation and deamination: Purine nucleotides are converted to nucleosides and then to free bases.

2. Conversion of adenine and guanine bases:

   • Adenine is deaminated to hypoxanthine.

   • Guanine is deaminated to xanthine.

3. Oxidation to uric acid:

   • Hypoxanthine is oxidized to xanthine by xanthine oxidase.

   • Xanthine is further oxidized to uric acid, the final product in humans.

Why Uric Acid Is the Final Product in Humans

• Unlike many other mammals, humans lack the enzyme uricase (urate oxidase), which converts uric acid into the more soluble allantoin.

• As a result, uric acid accumulates and is excreted directly in urine.

• This evolutionary difference leads to higher serum uric acid levels in humans compared to other animals.

Other Catabolic Products in Different Species

• Allantoin: In most mammals, uric acid is further converted by uricase into allantoin, which is more water-soluble and easily excreted.

• Urea: While urea is a major nitrogenous waste product in mammals, it is not the end product of purine catabolism.

• Xyloric acid: This is not a recognized purine catabolic product.

Physiological and Clinical Significance of Uric Acid

• Normal excretion: Uric acid is primarily produced in the liver and excreted via the kidneys.

• Hyperuricemia: Elevated uric acid levels in the blood can lead to the formation of urate crystals.

• Gout: Deposition of uric acid crystals in joints causes inflammation, pain, and arthritis known as gout.

• Kidney stones: Uric acid can precipitate in the urinary tract, leading to stone formation.

Summary Table of Purine Catabolic End Products

Conclusion

In humans, the catabolic end product of purines is uric acid, produced through the oxidation of xanthine by xanthine oxidase. This contrasts with most other mammals, which convert uric acid to allantoin via uricase. The accumulation and excretion of uric acid have significant physiological and pathological implications, notably in conditions like gout and kidney stone disease.

Correct answer: (4) Uric acid