URINE 



555 



H.,— 







II 



.H..N— C— NH., 



+ 2H.0 



When a condition of acidosis (p. 482) develops, a certain 

 proportion of the ammonia is not dehydrated, but is used to 

 neutralise the acid and to decrease the Ch of the blood 

 (p. 482), and the proportion of NH3 to CO(NH,), is thus 

 raised. When alkalies are given the proportion of ammonia is 

 decreased. 



In the diamino acids, such as lysin, arginin, histidin (p. 17), 

 the amidooren is dealt with as in the mou-amino acids. But in 

 the case of arginin, where the guanidin nucleus — 



NH 



II 

 H.,N-C-NH, 



is present, this may in part, at least, escape complete oxidation 

 to urea, may be methylated and then linked to acetic acid to 

 form creatin (p. 209), from which creatinin may be formed by 

 dehydration. 



The amino acids linked to the benzene ring, e.g. tyrosin, 

 are deaminised and the amidogen changed to urea, while the 

 propionic acid chain is oxidised from its free end with the 

 formation of homogentisic acid (alkapton) then hydroquinone, 

 which is finally oxidised to CO., and H._.0, the former of which 

 is largely excreted by the lungs. 



In one abnormality of metabolism the oxidation stops at 

 alkapton, and this is excreted in the urine. It oxidises to a 

 black pigment. 



In tryptophan not only is the propionic acid chain oxidised, 

 but the pyrrol ring is split off and the molecule then undergoes 

 the same changes as tyrosin. 



When, as a result of bacterial putrefaction in the intestme 

 (p. 329), the tryptophan molecule has had the propionic acid 

 oxidised without the removal of the pyrrol ring, skatol and 

 indol are formed. These are hydrated to skatoxyl or indoxyl, 



