is the main mechanism through which mammals transform ammonia to urea. Urea is do in the liver and also excreted in urine. The in its entirety chemical reaction by which 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 bike utilizes five intermediate steps, catalytic analysis by five different enzymes, to transform ammonia to urea, as shown in figure 22.12. The amino mountain L-ornithine gets converted into various intermediates prior to being regenerated in ~ the finish of the urea cycle. Hence, the urea bicycle is additionally referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a an essential step in the urea cycle and also its deficiency have the right to lead to accumulation of toxic levels the ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions happen in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is offered as one indicator of kidney function.

Figure 22.12. The urea bike converts ammonia come urea.

Birds, reptiles, and also most terrestrial arthropods convert toxic ammonia to uric acid or the very closely related link guanine (guano) rather of urea. Mammals also kind some uric acid during breakdown of main point acids. Uric mountain is a compound comparable to purines uncovered in nucleic acids. It is water insoluble and tends to form a white paste or powder; that is excreted through birds, insects, and also reptiles. Counter of ammonia come uric mountain requires more energy and also is much more facility than conversion of ammonia to urea figure 22.13.

Figure 22.13. Nitrogenous garbage is excreted in different forms by various species. These encompass (a) ammonia, (b) urea, and (c) uric acid. (credit a: modification of job-related by Eric Engbretson, USFWS; credit transaction b: alteration of work by B. “Moose” Peterson, USFWS; credit c: alteration of work-related by Dave Menke, USFWS)


Mammals usage uric acid crystals as an antioxidant in your cells. However, too much uric acid has tendency to form kidney stones and also may also cause a painful condition called gout, wherein uric acid crystals accumulate in the joints, as shown in figure 22.14. Food selections that minimize the amount of nitrogenous bases in the diet aid reduce the hazard of gout. Because that example, tea, coffee, and chocolate have actually purine-like compounds, dubbed xanthines, and also should it is in avoided by world with gout and also kidney stones.

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Figure 22.14. Gout causes the inflammation visible in this who left large toe joint. (credit: “Gonzosft”/Wikimedia Commons)


Ammonia is the waste created by metabolism of nitrogen-containing compounds prefer proteins and also nucleic acids. When aquatic pets can quickly excrete ammonia into their watery surroundings, terrestrial animals have developed special mechanisms to eliminate the toxicity ammonia from your systems. Urea is the significant byproduct that ammonia metabolism in vertebrate animals. Uric mountain is the major byproduct of ammonia metabolism in birds, terrestrial arthropods, and reptiles.