616 METABOLISM 



OH ONH, ONH 4 NH, 



CO + 2NH 3 ?CO -H 2 O?CO -H 2 O CO 



\ \ \ \ 



OH ONH 4 NH 2 NH 2 



(carbonic (ammo- (ammonium 1 (ammonium (urea) 



acid) nia) carbonate) carbamate) 



Some of the urea may come from metabolic processes of an entirely 

 different type. One of these at least is known ; namely, the splitting-off 

 of urea from arginine, which it will be remembered is guanidine-amino- 

 valerianic acid (see page 605). An enzyme called arginase, having this 

 action, has been isolated from various organs and tissues. The diamino- 

 valerianic acid, or ornithine, which remains after the urea is split off, 

 may be further used in protein metabolism. The reaction is shown in 

 the following equation: 



NH 2 - C - NH - CH 2 - CH 2 - CH 2 - CHNH 2 - COOH + H 2 O ' 

 I! 

 NH (arginine) 



= NH 2 -CO 



| + NH 2 -CH 2 -CH 2 -CH 2 -CHNH 2 -COOH 

 NH 2 

 (urea)' (ornithine) 



On an ordinary diet, as we have seen, a man excretes somewhat more 

 than 90 per cent of his total nitrogen as urea and about 3 per cent as 

 ammonia, the remainder of the nitrogen appearing in the other nitrog- 

 enous metabolites. 



Influence of Acidosis on Ammonia-Urea Ratio. It sometimes happens 

 that a large proportion of the ammonia is not converted into urea, but 

 is used for the purpose of neutralizing abnormal acids present in the 

 organism. "When mineral acids are given to an animal, or when acids 

 are produced in the organism itself by some faulty type of metabolism, 

 the ammonia excretion by the urine immediately rises. In diabetes, for 

 example, where considerable quantities of /3-oxybutyric acid are pro- 

 duced (see page 683), a decided increase in the ammonia excretion by 

 the urine is observed. A milder type of acidosis may also be induced 

 in normal persons by withholding carbohydrates from the diet, and 

 here again the ammonia excretion is relatively increased. 



In such cases it is quite evident that ammonia is used as an alkaline 

 reserve of the body; that is, as a substance which is capable of prevent- 

 ing acidosis by neutralizing the acids. It does not appear, however, 

 that all types of acidosis entail the utilization of ammonia as reserve 

 alkali, and an increase in the relative amount of ammonia in the urine 

 does not necessarily indicate a condition of acidosis. In the pernicious 



