Ureotelic organisms

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. 

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. 

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

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... 

The enzyme is present in high concentration in the liver of all ureotelic animals, and is also present, but in much lower amounts, in kidney of all animals (451, 453, 454). The enzyme is present in most organs of the shark... 

In terms of amino nitrogen excretion the animal kingdom can be classified into ammonotelic (ammonia), ureotelic (urea) and uricotelic (uric acid) organisms depending on the nature of the discharged substance. 

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