It is not surprising to see that biochemical pathways like the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved.
Arid conditions probably led to the evolution of the uric acid pathway as a means of conserving water. Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or the closely related compound guanine guano instead of urea. Mammals also form some uric acid during breakdown of nucleic acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder; it is excreted by birds, insects, and reptiles.
Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea Figure 2. Figure 2. Nitrogenous waste is excreted in different forms by different species. These include a ammonia, b urea, and c uric acid. Mammals use uric acid crystals as an antioxidant in their cells. However, too much uric acid tends to form kidney stones and may also cause a painful condition called gout, where uric acid crystals accumulate in the joints, as illustrated in Figure 3.
Food choices that reduce the amount of nitrogenous bases in the diet help reduce the risk of gout. For example, tea, coffee, and chocolate have purine-like compounds, called xanthines, and should be avoided by people with gout and kidney stones. Figure 3. Ammonia is the waste produced by metabolism of nitrogen-containing compounds like proteins and nucleic acids.
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Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or the closely related compound guanine guano instead of urea. Mammals also form some uric acid during breakdown of nucleic acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder; it is excreted by birds, insects, and reptiles.
Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea [link]. Everyday Connection Gout Mammals use uric acid crystals as an antioxidant in their cells. However, too much uric acid tends to form kidney stones and may also cause a painful condition called gout, where uric acid crystals accumulate in the joints, as illustrated in [link].
Food choices that reduce the amount of nitrogenous bases in the diet help reduce the risk of gout. For example, tea, coffee, and chocolate have purine-like compounds, called xanthines, and should be avoided by people with gout and kidney stones.
Section Summary Ammonia is the waste produced by metabolism of nitrogen-containing compounds like proteins and nucleic acids. While aquatic animals can easily excrete ammonia into their watery surroundings, terrestrial animals have evolved special mechanisms to eliminate the toxic ammonia from their systems.
Urea is the major byproduct of ammonia metabolism in vertebrate animals. Uric acid is the major byproduct of ammonia metabolism in birds, terrestrial arthropods, and reptiles. It is believed that the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved. The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea.
Birds, reptiles, and insects, on the other hand, convert toxic ammonia to uric acid instead of urea. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea. Skip to content Osmotic Regulation and Excretion.
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