Question

Why the amphibians, birds and reptiles have uric acid and guanine as end products of purine metabolism?

• A. Because they lack Aldehyde dehydrogenase
• B. Because they lack Xanthine oxidase
• C. Because they lack Xanthine dehydrogenase
• D. Because they lack uricase

Hint:

Humans also lack uricase, which is why we excrete uric acid.
Elevated levels of uric acid can lead to the crystallization of urate in joints, a clinical condition known as gout.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Purine catabolism involves the oxidation of adenine and guanine bases to xanthine, which is then oxidized to uric acid.
The final excretory product of this pathway varies among animal species depending on the presence of downstream catabolic enzymes.

Step 2: Detailed Explanation:
In many mammals (such as dogs and cats), the enzyme uricase (urate oxidase) oxidizes insoluble uric acid into a highly soluble compound called allantoin.
Allantoin can be further degraded or excreted easily in urine.
However, humans, higher primates, birds, terrestrial reptiles, and some amphibians lack a functional uricase enzyme.
Because they lack uricase, they cannot convert uric acid to allantoin.
As a result, their purine breakdown stops at uric acid.
For birds and reptiles, excreting nitrogenous waste as insoluble uric acid or guanine is an adaptation to conserve water.
These compounds can be excreted as a semi-solid paste without requiring large volumes of water, preventing dehydration in terrestrial and egg-laying environments.

Step 3: Final Answer:
Amphibians, birds, and reptiles excrete uric acid and guanine because they lack the enzyme uricase.