156 PHYSIOLOGY 



In the same way ornithine derived from the breakdown of arginine is con- 

 verted by putrefactive bacteria into tetra-methylene diamine or putrescine. 

 Other examples of this process of decarboxylation are : 



Isoamylamine from leucine, (CH 3 ) 2 .CH.CH 2 .CH 2 .NH 2 . 



P phenylethylamine from phenylalanine, C 6 H 5 .CH 2 .CH 2 .NH 2 . 



Para-oxyphenylethylamine from tyrosine, OH.C 6 H 4 .CH 2 .CH 2 .NH 2 . 



A similar process has been supposed to take place as a step in the suc- 

 cessive oxidation of the carbon atoms in the long chain fatty acids- or carbo- 

 hydrates, but a thorough study of this process as it occurs in the higher 

 animals is still wanting, and its very existence is indeed still hypothetical. 

 In the case of the fats the oxidation takes place chiefly or entirely in the 

 P position. On the other hand, decarboxylation certainly takes place in 

 substances such as the a ammo-acids, where the first oxidation change occurs 

 in the a group, and probably closely follows this oxidation change. The 

 reverse reaction, namely, the insertion of the group CO.O at the end of the 

 long carbon chain, is not known to take place, but would furnish a means 

 by which the organism with apparent simplicity could build up long carbon 

 chains and so imitate the process which in the laboratory is generally effected 

 by attaching a ON group to the end of the molecule. In the case of the 

 fats the building up, like the oxidative breakdown, appears to occur by 

 two carbon atoms at a time ; hence all the fatty acids met with in the body 

 have an even number of carbon atoms in their chain. 



It is worthy of note that all the changes which we have been considering 

 changes which not only account for the greater part of the chemical re- 

 actions of the living body, but may lead to the production of the most 

 complex substances known are performed with little expenditure or evolu- 

 tion of energy. This is evident if we examine the heat evolved by the 

 total combustion of one gramme molecule of the initial and final substances 

 in a number of typical reactions. In the following Table these are given 

 for the substances involved in typical instances of the three classes of chemical 

 change that we have just been considering : 



(1) HYDROLYSIS 



T .4.- i 



Heat of com- 



Maltose . . . .1350 

 Glucose .... 677 

 Ilippuric acid . . 1013 



Heat 



Final of 



substance combustion 



2 Glucose . . . 1354 

 2 Lactic acid 659 



(Glycine . . 235 \ 



\Benzoic acid . 773 J 



1008 



(2) DEAMINATION 



Initial Heat of 



substance combustion 



Alanine . . . . 389-2 



Leucine . . . . 855 



Aspartic acid . . . 386 



Final Heat of 



substance combustion 



Lactic acid . . 329-5 



Caproic acid . . 837 



Succinic acid . 354 



