M3 The Urea Cycle

Birds and terrestrial reptiles excrete the ammonia in the form of uric acid, while most terrestrial vertebrates convert the ammonia into urea before excretion. These three classes of organisms are called: ammonotelic, uricotelic and ureotelic, respectively. Urea is synthesized in the liver by the urea cycle. It is then secreted into the bloodstream and taken up by the kidneys for excretion in the urine. The urea cycle was the first cyclic metabolic pathway to be discovered by Hans Krebs and Kurt Henseleit in 1932. One of the nitrogen atoms of urea comes from ammonia, the other is transferred from the amino acid aspartate, while the carbon atom comes from CO₂. The urea cycle is also the starting point for the synthesis of another important metabolite creatine phosphate. Uric acid 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.