is the primary mechanism by which mammals transform ammonia to urea. Urea is made in the liver and also excreted in urine. The as a whole chemical reactivity whereby ammonia is converted to urea is 2 NH3 (ammonia) + CO2 + 3 ATP + H2O → H2N-CO-NH2 (urea) + 2 ADP + 4 Pi + AMP.

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The urea cycle uses five intermediate measures, catalyzed by five various enzymes, to transform ammonia to urea, as shown in Figure 22.12. The amino acid L-ornithine gets converted right into different intermediates before being reproduced at the finish of the urea cycle. Hence, the urea cycle is also described as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes an essential step in the urea cycle and its deficiency have the right to bring about accumulation of toxic levels of ammonia in the body. The initially 2 reactions happen in the mitochondria and also the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is offered as an indicator of kidney function.

Figure 22.12. The urea cycle converts ammonia to urea.

Birds, reptiles, and many terrestrial arthropods transform toxic ammonia to uric acid or the very closely connected compound guanine (guano) instead of urea. Mammals likewise form some uric acid throughout breakdvery own of nucleic acids. Uric acid is a compound equivalent to purines discovered in nucleic acids. It is water insoluble and has a tendency to form a white paste or powder; it is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid needs more power and also is a lot even more complex than conversion of ammonia to urea Figure 22.13.

Figure 22.13. Nitrogenous waste is excreted in different develops by different species. These encompass (a) ammonia, (b) urea, and (c) uric acid. (crmodify a: change of job-related by Eric Engbretson, USFWS; credit b: alteration of job-related by B. “Moose” Peterboy, USFWS; crmodify c: change of work-related by Dave Menke, USFWS)


Mammals use uric acid crystals as an antioxidant in their cells. However, as well a lot uric acid has a tendency to form kidney stones and may also reason a painful problem dubbed gout, where uric acid crystals accumulate in the joints, as illustrated in Figure 22.14. Food selections that reduce the amount of nitrogenous bases in the diet aid mitigate the hazard of gout. For instance, tea, coffee, and also chocolate have purine-like compounds, referred to as xanthines, and also have to be avoided by civilization with gout and kidney stones.

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Figure 22.14. Gout reasons the inflammation visible in this person’s left big toe joint. (credit: “Gonzosft”/Wikimedia Commons)


Ammonia is the waste created by metabolism of nitrogen-containing compounds favor proteins and nucleic acids. While aquatic animals have the right to easily excrete ammonia into their watery surroundings, terrestrial pets have progressed special mechanisms to eliminate the toxic ammonia from their units. Urea is the major byproduct of ammonia metabolism in vertebrate pets. Uric acid is the major byproduct of ammonia metabolism in birds, terrestrial arthropods, and also reptiles.