Humans urea synthesis

Figure 8.32 Urea salvage in humans. Urea diffuses into the colon where it is converted to ammonia in those microorganisms that possess the enzyme urease. The ammonia is used by most microorganisms in the colon to synthesise their protein. Upon death in the colon, these organisms are degraded and the protein is hydrolysed to amino acids, some of which are absorbed by the host to be used for protein synthesis, etc.

In comparison, the vampire bat has a capacity for urea synthesis approximately 1000-fold greater than that of a human. A bat consumes one half of its weight in blood in 10-15 minutes so that the massive amount of protein that is metabolised produces a massive amount of ammonia which must be removed as quickly as possible. Indeed, it also ingests sufficient fluid so that it is too heavy to fly. Hence, it urinates very quickly. 

Figure 19.4 Protein secretion (shade bars) and urea synthesis (full bars) of human hepatocytes cultured in the galactosylated membrane bioreactor for 21 days. The values are the mean of six experiments standard deviation.

Consideration of other plasma amino acids also informs the diagnosis of inborn errors of urea synthesis. The plasma concentrations of glutamine and alanine are often elevated in parallel with or prior to the ammonium concentration as they act as a nitrogen buffer. Plasma arginine concentrations are low since the only synthetic route for arginine in humans is via the urea cycle. In contrast, the arginine concentration is elevated in ARG-1 deficiency. Hyperornithinemia and homocitrullinuria are the characteristic features of the hyperammonemia, hyperornithinemia, and homocitrullinuria (HHH) syndrome caused by a defect in the ornithine transporter (ORNT-1). 

The term organic chemistry was first used by the Swedish chemist Berzelius in 1807 (Larsson, 1981). He coined the name to describe the chemistry of substances derived from living matter. Berzelius was a staunch believer in the vis vitalis theory, which held that such substances were endowed with a mystical vital force that precluded their synthesis in the laboratory from materials of mineral origin. Ironically, it was a student of Berzelius, Wohler, who heralded the demise of vitalism with his synthesis of urea from ammonium cyanate (Wohler, 1928). In a letter to Berzelius in 1828, Wohler wrote I must tell you that I can make urea without requiring kidneys, or even an animal, whether a human being or a dog . 

Carbamoyl phosphate synthase contributes to two processes (a) the initial enzyme in the biosynthesis of pyrimidines and (b) a component in the synthesis of arginine biosynthesis or the urea cycle. In bacteria both of these processes occur within the same compartment. In human beings the carbamoyl phosphate synthase involved in the urea cycle is contained in... 

Wenzel AG, Jacobsen EN (2002) Asymmetric catalytic Mannich reactions catalyzed by urea derivatives enantioselective synthesis of beta-aryl-beta-amino acids. J Am Chem Soc 124 12964-12965 Wertz PW, Miethke MC, Long SA, Stauss JS, Downing DT (1985) The composition of the ceramides from human stratum comeum and from comedones. J Invest Dermatol 84 410—412... 

Cao P, Ding H, Ge HM et al. (2007) Synthesis and cytotoxic evaluation of substituted urea derivatives as inhibitors of human-leukemia K562 cells. Chem Biodivers 4 881-186... 

Many biologically active compounds contain cyclic ureas, including inhibitors of human immunodeficiency virus (HIV) protease and HIV replication [70]. Kim et al. [71] presented an illustration of the synthesis of oligomeric cyclic ureas as nonnatural biopolymers. Applying the libraries from libraries [72] concept, triamines [65] such as those described earlier were used as templates for the generation of different heterocyclic compounds such as cyclic ureas, cyclic thioureas, and bicyclic guanidines [65]. The cyclizations to obtain the five-membered ring cyclic ureas and cyclic thioureas were... 

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