Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. The Urea Cycle
The excretion of amino nitrogen from the organism presents yet another complex biochemical challenge.

How is excess amino nitrogen excreted from the body? Comparative biochemical studies across various animal species have demonstrated that amino nitrogen is excreted in three primary forms: free ammonia, urea, and uric acid. Most aquatic animals, such as teleost fish, excrete amino nitrogen as ammonia; these organisms are termed ammonotelic. In most terrestrial animals, amino nitrogen is excreted as urea; they are called ureotelic. In birds, lizards, and snakes, it is excreted as uric acid, and such organisms are referred to as uricotelic (Fig. 19-15).

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Fig. 19-15. Various forms in which amino nitrogen is excreted by different animal species.

These differences are driven by the Anatomical and physiological distinctions among these animal groups, which are shaped by their respective habitats. In teleost fish, amino nitrogen is transported in the Blood as glutamine, but it is excreted through the gills as ammonia. This occurs because the gills contain glutaminase, an enzyme that catalyzes the Hydrolysis of glutamine to yield glutamate and ammonia. Because ammonia is highly Water-soluble, it is rapidly diluted and washed away by the abundant stream of water flowing over the gills. Consequently, teleost fish do not require a sophisticated renal system to excrete ammonia.

However, as certain aquatic animals began adapting to a terrestrial lifestyle during the course of evolution, excreting amino nitrogen as ammonia through the gills became impossible. Over time, land animals developed alternative mechanisms for excreting amino nitrogen. Such organisms require Kidneys and a bladder to eliminate water-soluble nitrogenous waste products. Yet, excreting large quantities of free NH₃ directly into the urine—given its ability to readily cross Introduction/36.html">Biological Membranes—could lead to its reabsorption, thereby returning it to the bloodstream. Furthermore, there is another drawback: because ammonia in the blood exists predominantly as the NH₄⁺ ion, its excretion would require the simultaneous elimination of equivalent amounts of counter-ions, such as chloride or phosphate. To circumvent these complications, most terrestrial animals evolved The ability to excrete amino nitrogen as urea—a neutral, highly water-soluble, and nontoxic compound. Nevertheless, this capacity to synthesize and excrete urea does not come without a cost: as we will see later, it requires the expenditure of significant amounts of energy in the form of ATP.

For birds, body weight is a critical factor. However, excreting urea necessitates the elimination of substantial amounts of water along with it. Therefore, birds evolved an alternative mechanism for eliminating amino nitrogen that does not require significant water loss. In avian species, amino nitrogen is converted into uric acid, which has relatively low water solubility. Consequently, bird urine is a semisolid paste consisting of uric acid crystals and a minimal amount of water (Fig. 19-15). The advantage of excreting amino nitrogen as solid uric acid is offset by increased metabolic costs, as The Biosynthesis of uric acid is a complex, energy-requiring process.

The crucial role of habitat in determining the mode of nitrogen excretion is well illustrated by tadpoles, which undergo a shift in their excretion pathway during metamorphosis. Aquatic tadpoles excrete amino nitrogen as ammonia through their gills. The tadpole Liver initially lacks the Enzymes required to produce urea; however, these enzymes appear during metamorphosis, and the Organism simultaneously loses its ability to excrete ammonia. In the adult frog, which spends most of its time on land, amino nitrogen is excreted almost entirely as urea.