Ureotelism

Ammonia (NH3) is a relatively strong base, and at physiological pH values it is mainly present in the form of the ammonium ion NH4 (see p. 30). NH3 and NH4 are toxic, and at higher concentrations cause brain damage in particular. Ammonia therefore has to be effectively inactivated and excreted. This can be carried out in various ways. Aquatic animals can excrete NH4 directly. For example, fish excrete NH4 via the gills (ammonotelic animals). Terrestrial vertebrates, including humans, hardly excrete any NH3, and instead, most ammonia is converted into urea before excretion ureotelic animals). Birds and reptiles, by contrast, form uric acid, which is mainly excreted as a solid in order to save water uricotelic animals). 

Ureotelic animals many terrestrial vertebrates also sharks... 

In ureotelic organisms, the ammonia deposited in the mitochondria of hepatocytes is converted to urea in the urea cycle. This pathway was discovered in 1932 by Hans Krebs (who later also discovered the citric acid cycle) and a medical student associate, Kurt Henseleit. Urea production occurs almost exclusively in the liver and is the fate of most of the ammonia channeled there. The urea passes into the bloodstream and thus to the kidneys and is excreted into the urine. The production of urea now becomes the focus of our discussion. 

Excess nitrogen is excreted as ammonia. Ammonotelic organisms excrete ammonia directly, uricotelic organisms excrete it as uric acid, and ureotelic organisms excrete it as urea. 

Uric acid (Fig. 6) is the main nitrogenous waste product of uricotelic organisms (reptiles, birds and insects), but is also formed in ureotelic organisms from the breakdown of the purine bases from DNA and RNA (see Topics FI and Gl). Some individuals have a high serum level of sodium urate (the predominant form of uric acid at neutral pH) which can lead to crystals of this compound being deposited in the joints and kidneys, a condition known as gout, a type of arthritis characterized by extremely painful joints. 

Arginase is a hydrolase that is in the liver of urea-producing organisms (ureoteles). It catalyzes the reaction ... 

Since urea is quite soluble and it doesn t make you sick We each excrete (4 times) as a ureotelic (repeat). 

As discussed earlier, most terrestrial vertebrates are ureotelic they excrete excess nitrogen as urea. However, urea is not the only excretable form of nitrogen. Ammoniotelic organisms, such as aquatic vertebrates and invertebrates, release... 

In ureotelic organisms the urea cycle disposes of approximately 90% of surplus nitrogen. As shown in Figure 15.1, urea is formed from ammonia, C02, and aspartate in a cyclic pathway referred to as the urea cycle. Because the urea cycle was discovered by Hans Krebs and Kurt Henseleit, it is often referred to as the Krebs urea cycle or the Krebs-Henseleit cycle. 

Both ureotelic and uricotelic animals form uric add, but in the former it is only derived fi-om purine catabolism. In uricotelic animals, such as birds, uric add formation assumes much greater importance. 

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