Here's why:
* Proteins are made of amino acids. Amino acids contain nitrogen.
* When proteins are broken down (catabolized) for energy or other purposes, the nitrogen-containing amino groups are removed. This process generates ammonia (NH3), which is toxic to animals.
* Animals have evolved different mechanisms to convert ammonia into less toxic forms:
* Fish excrete ammonia directly into the water.
* Mammals convert ammonia into urea in the liver.
* Birds and reptiles convert ammonia into uric acid.
While other molecules like nucleic acids also contain nitrogen, their catabolism contributes a much smaller amount to the overall nitrogenous waste production compared to proteins.
