20.5 Nitrogenous Wastes

Mary Ann Clark; Jung Choi; Matthew Douglas

20.5 Nitrogenous Wastes

Mary Ann Clark; Jung Choi; and Matthew Douglas

Learning Objectives

By the end of this section, you will be able to do the following:

Compare and contrast the ways in which aquatic animals and terrestrial animals can eliminate toxic ammonia from their systems
Compare the major byproduct of ammonia metabolism in vertebrate animals to that of birds, insects, and reptiles

Of the four major macromolecules in biological systems, both proteins and nucleic acids contain nitrogen. During the catabolism, or breakdown, of nitrogen-containing macromolecules, carbon, hydrogen, and oxygen are extracted and stored in the form of carbohydrates and fats. Excess nitrogen is excreted from the body. Nitrogenous wastes tend to form toxic ammonia, which raises the pH of body fluids. The formation of ammonia itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system. Animals that live in aquatic environments tend to release ammonia into the water. Animals that excrete ammonia are said to be ammonotelic. Terrestrial organisms have evolved other mechanisms to excrete nitrogenous wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric acid. Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce uric acid. Animals that secrete urea as the primary nitrogenous waste material are called ureotelic animals.

Nitrogenous Waste in Terrestrial Animals: The Urea Cycle

The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The overall chemical reaction by which ammonia is converted to urea is 2 NH₃ (ammonia) + CO₂ + 3 ATP + H₂O → H₂N-CO-NH₂ (urea) + 2 ADP + 4 P_i + AMP.

The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea, as shown in the figure below. The amino acid L-ornithine gets converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function.

The urea cycle begins in the mitochondrion, where bicarbonate, shown as upper case H upper case C upper case O subscript 3 baseline is combined with ammonia, shown as upper case N upper case H subscript 3 baseline to make carbamoyl phosphate. Two A T P are used in the process. Ornithine transcarbamylase adds the carbamoyl phosphate to a five-carbon amino acid called ornithine to make L citrulline. L citrulline leaves the mitochondrion, and an enzyme called arginosuccinate synthetase adds a four carbon amino acid called L aspartate to it to make arginosuccinate. In the process, one A T P is converted to A M P and upper case P upper case P lower case i. Arginosuccinate lyase removes a four carbon fumarate molecule from the arginosuccinate, forming the six carbon amino acid L arginine. Arginase 1 removes a urea molecule from the L arginine, forming ornithine in the process. Urea has a single carbon double bonded to an oxygen and single bonded to two ammonia groups. Ornithine enters the mitochondrion, completing the cycle. — The figure illustrates how the urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea. The urea cycle converts ammonia to urea.

Evolution Connection: Excretion of Nitrogenous Waste

The theory of evolution proposes that life started in an aquatic environment. 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.

Nitrogenous Waste in Birds and Reptiles: Uric Acid

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 (see the figure below).

Part A shows a photo of a freshwater fish and states that many invertebrates and aquatic species excrete ammonia. The chemical structure of ammonia is upper case N upper case H subscript 3 baseline. Part B shows a photo of a wood rat and states that mammals, many adult amphibians, and some marine species excrete urea. The chemical structure of urea is shown. Urea has one upper N upper H subscript 2 baseline group, and one upper N upper H subscript 2 baseline group attached to a central carbon. An oxygen is also double-bonded to this central carbon. Part C shows a photo of a pigeon and states that insects, land snails, birds, and many reptiles excrete uric acid. The chemical structure of uric acid is shown. Uric acid has a six-membered carbon ring attached to a five-membered ring. Each ring has two upper N upper H groups embedded in it. An oxygen is double-bonded to each ring. — The figure illustrates how nitrogenous waste is excreted in different forms by different species. These include (a) ammonia, (b) urea, and (c) uric acid. (credit a: modification of work by Eric Engbretson, USFWS; credit b: modification of work by B. “Moose” Peterson, USFWS; credit c: modification of work by Dave Menke, USFWS)

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 the figure below. 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.

Photo shows a person's feet. One foot is swollen and red, while the other appears normal. — THe figure illustrates how 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. Gout causes the inflammation visible in this person’s left big toe joint. (credit: “Gonzosft”/Wikimedia Commons)

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.

Review Questions

Critical Thinking Questions

Glossary

ammonia: compound made of one nitrogen atom and three hydrogen atoms

ammonotelic: describes an animal that excretes ammonia as the primary waste material

antioxidant: agent that prevents cell destruction by reactive oxygen species

blood urea nitrogen (BUN): estimate of urea in the blood and an indicator of kidney function

urea cycle: pathway by which ammonia is converted to urea

ureotelic: describes animals that secrete urea as the primary nitrogenous waste material

uric acid: byproduct of ammonia metabolism in birds, insects, and reptiles

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