Urea Formation Is a Complex and Costly Mode of Ammonia Detoxification

Urea Formation Is a Complex and Costly Mode of Ammonia Detoxification 

Urea formation in the liver starts with the multistep conversion of ornithine to arginine (fig. 22.7). This is followed by the breakdown of arginine into ornithine and urea. The cyclic nature of this pathway was first appreciated by Hans Krebs and Henseleit in 1932. In subsequent years the impor- 

The urea cycle is a mechanism for removing unwanted nitrogen. Sources of nitrogens involved in urea formation are shown in red. Five enzymes are used in the urea cycle. Three of these function in the cytosol, and two, as shown, function in the mitochondrial matrix. 

Specific carriers in the inner mitochondrial membrane transport ornithine, citrulline, ammonium ion, and HC03 (C02) into and out of the mitochondrial matrix. 

The mechanism of formation of carbamoyl phosphate. The reaction involves three steps, all of which take place on the same enzyme, carbamoyl phosphate synthase. 

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