SPECIAL METABOLISMS 423 



fed on the amino-acid products of a meat digest. If, however, it be 

 fed on selected amino-acids, it is found that on some it can still live, 

 on others it gradually starves. For example, the monamino -acids 

 by themselves do not support life, neither do the diamino-acids. On 

 the other hand, the addition of ringed amino-aeids, such as phenyl- 

 alanin, tyrosin, and tryptophan, has been found to support life. 

 So, too, proteins which do not contain these last amino-acids, such 

 as gelatin and zein, fail to keep an animal alive. The addition of 

 ringed amino-acids to such proteins renders them more life-supporting. 

 From this point of view, it is interesting to note that Nature provides 

 the young growing animal with a protein caseinogen especially 

 rich in both tyrosin and tryptophan. 



The two hypotheses held in regard to the metabolism of protein 

 differ (1) as to the place of selection of the building -stones the 

 amino-acids; (2) as to the form in which the material for reconstruc- 

 tion is presented to the tissues. 



According to one view, the amino-acids pass as such into the portal 

 blood. It is claimed that their presence there can.be demonstrated 

 by special indicators, such as /2-naphtha-sulphonic acid. The gut of 

 the octopus is naturally suspended in a bath of blood, and amino- 

 acids are said to appear in this blood when protein is digested in the 

 gut. The absorbed amino-acids are then taken in the portal blood 

 to the liver, which controls their passage into the general circulation 

 according to the needs of the body. Each amino-acid has its own 

 special metabolism. 



It has been shown that, if such bodies as glycin, alanin, arginin, 

 be perfused through the isolated liver, the urea content of the blood 

 leaving the liver is increased. These amino-acids are only of partial 

 value to the body. The nitrogenous moiety contained in them may 

 perhaps be regarded as valueless, for it is rapidly excreted. These 

 amino-acids are first deaminized, the ammonia split off being 

 converted into urea, while the non-nitrogenous moiety remains. 

 There is reason to suppose that this is first converted into a lower 

 fatty acid, which then becomes converted into dextrose, a conversion 

 of great importance in carbohydrate metabolism. In the case of 



alanin, for example, the process may be represented as follows : 

 - 



CH 3 CH.NH 2 COOH + KjO -- CH 3 CHOH.COOH + NH 3 



Alanin Lactic acid 



2CH 3 CHOH.COOH =C 6 H 12 6 



(C 3 H 6 O 3 ) Dextrose 



The fate of the monamino-dicarboxylic acids, such as aspartic and 

 glutamic acids, is probably the same. The diamino-acids, such as 

 arginin and lysin, together with the closely allied histidin, are probably 

 also broken down into nitrogenous and non-nitrogenous moieties, 

 the nitrogenous being mainly excreted from the body in the form of 

 urea, the non-nitrogenous part being possibly converted into dextrose. 



It is not yet known sufficiently well what exactly happens to 



