342 - Multicellular Animals, Especially Man 



liver, these excess amino acids are deami- 

 nated. Deamination liberates the carbon- 

 hydrogen oxygen residues (CHO residues) of 

 the molecules for oxidation by the body 

 tissues; simultaneously the amino parts of 

 the molecules go into the formation of urea, 

 the main nitrogenous waste in man and 

 other mammals. 



Deamination, as it is accomplished in the 

 mammalian liver, does not occur by the di- 

 rect process that was described previously 

 (p. 138). In the liver, in fact, the amino frac- 

 tions that are discharged from the various 

 amino acids do not appear as free ammonia 

 (NH 3 ), but are transferred to one particular 

 amino acid, namely ornithine, which is thus 

 transformed to arginine: 



NH 2 

 / 

 C=NH 

 \ 

 NH 



I 

 (CH 2 ) 3 

 I 



CHNH 2 

 I 

 COOH 



arginine 



Then arginine decomposes, liberating urea. 

 The importance of arginine, therefore, is 

 that it acts as an intermediary in the forma- 

 tion of urea. Arginine receives amino nitro- 

 gen from the various other amino acids and 

 transfers this nitrogen to urea, a relatively 

 nontoxic waste product (see column 2, top). 



To a small extent the mammalian kidney 

 also engages in the deamination of amino 

 acids. The kidney deaminations are direct, 

 however, and lead to the production of NH 3 

 (p. 378). In fact, the kidney utilizes this am- 

 monia in neutralizing various acici metab- 

 olites, which otherwise would tend to acidify 

 the system unduly (see column 2, bottom). 



Urea is not eliminated by the liver, but 

 passes through the blood stream to the kid- 

 neys. Accordingly, a considerable quantity 



/NH 2 



C=NH 



\NH 



I 



(CH 2 ) 3 +H 2 



I 



CHNH 2 

 I 

 COOH 



arginine 



NH 2 



I 

 /NH 2 (CH 2 ) 3 



c=o + I 



\NH 2 CHNH 2 

 I 

 COOH 



urea ornithine 



of this crystalline end product of amino acid 

 catabolism is present in the urine of man 

 and other mammals. In birds and reptiles, 

 however, much of the urea is converted, also 

 in the liver, into a more complex nitrogen 

 compound, uric acid, and uric acid is the 

 chief nitrogenous waste excreted by these 

 other vertebrates. Some uric acid is also ex- 

 creted by man and other mammals; but in 

 mammals uric acid is derived from the catab- 

 olism of nucleic acids. 



The nonnitrogenous compounds produced 

 by the deamination of the amino acids are 

 all relatively simple organic acids (p. 138). 

 These CHO residues may be sent to other 

 tissues in the body; or they may be retained 

 by the liver for conversion into glucose and 



NH 3 -fr HOH i=t NH4OH 



and 



NH4OH + H 2 C0 3 



NH 4 HC0 3 + HOH 



