Nitrogen Excretion and the Urea Cycle:

Nitrogen Excretion and the Urea Cycle:
Excess amino Nitrogen from amino acids is removed as ammonia, which is toxic to the human body. Some ammonia is excreted in urine, but nearly 90% of it is utilized by the liver to form urea, which is highly soluble and is passed in to circulation for being excreted by the kidneys. Daily excretion of urea amounts to about 30g with a protein intake of nearly 100g in the food.

DISPOSAL/DETOXIFICATION OF AMMONIA
1. First line of Defense (Trapping of ammonia)
Being highly toxic, ammonia should be eliminated or detoxified, as and when it is formed. Even very minute quantity of ammonia may produce toxicity in central nervous system. But, ammonia is always produced by almost all cells, including neurons. The intracellular ammonia is immediately trapped by glutamic acid to form glutamine, especially in brain cells (Fig. 2). The glutamine is then transported to liver, where the reaction is reversed by the enzyme glutaminase (Fig. 2). The ammonia thus generated is immediately detoxified into urea .Aspartic acid may also undergo similar reaction to form asparagine.