264 PRINCIPLES OF GENERAL PHYSIOLOGY 



Folin and Denis (1912) came to the same conclusions and could find no 

 evidence whatever of protein synthesis in the intestine. Abel (1913), again, 

 by the ingenious method of dialysis of the living blood, referred to above 

 (page 83), has been able to collect as much as 20 g. of amino-acids from the 

 blood of three or four dogs, so that it is possible to separate them and find 

 out which are present. This is indeed being done. The fact that they have 

 now been detected in the blood is due to the improved methods devised for 

 their determination and we may conclude that they do actually, as such, 

 reach the tissue cells. Confirmatory evidence is afforded by the experiments 

 of Buglia (1912, p. 184) who found that sufficient nitrogen food to meet 

 requirements can be injected, in the form of amino-acids, into the veins slowly 

 without disturbance. 



We have already seen evidence that the actual amount of nitrogen required 

 for repair is very small and further details will be given later. How then is the 

 remaining nitrogen of a considerable protein diet dealt with ? If we start from 

 the other end, as it were, we find by experiment that the amino-acid nitrogen not 

 needed for growth or repair appears in the urine as urea, while the rest of the 

 molecule is ultimately converted into carbon dioxide and water. 



From the chemical standpoint, the most obvious stages between an amino-acid 

 and urea are, first, de-amination, by which ammonia is split off and some derivative 

 of a fatty acid formed, and, secondly, the ammonia is converted into urea, while 

 the hydrocarbon acid is burnt up for the purpose of affording energy. 



This view is, in fact, a part of the theory of protein metabolism associated with 

 the name of Folin (1905). We have to inquire what evidence there is of such 

 reactions occurring in the living organism, and, if so, the further question arises as 

 to the organs in which they take place. 



With regard to the first step in the process, it has been shown by Dakin and 

 Dudley (1913, 3) that an a-amino-acid in solution in water undergoes spontaneous 

 dissociation into the corresponding a-ketonic aldehyde and ammonia and this fact 

 makes it probable that the process is accelerated in the organism by an enzyme. 

 Alanine becomes in this way pyruvic aldehyde and ammonia. The ketonic 

 aldehyde may undergo further change in three modes, oxidation, hydrolysis, or 

 reduction, thus : 



CH :! 



+ O > CO 



COOH (pyruvic acid) 



CH. { CH, ' CH S 



CH.NH., >- NH :t + CO + H a O > CHOH (lactic acid) 



COOH CHO COOH 



(alanine) (pyruvic\ 



aldehyde)\ 



\ CH 3 



\ I 



+ H 2 > CH., (propionic acid) 



COOH 



Incidental^', it may be noted that, as we shall see later, there are three series of enzymes, 

 known to be present in cells, capable of effecting these three processes of oxidation, hydrolysis, 

 or reduction, respectively. 



De-amiiiation. The whole of the blood from the intestines, containing amino- 

 acids, in the mammal, passes through the liver before reaching the various other 

 organs and tissues. In other vertebrates, a part of it goes this way. The liver 

 has the power of converting ammonium salts into urea, as was first definitely 

 proved by Schroder (1882 and 1885). We might expect, then, that the main 

 body of the amino-acids would be first de-aminated in the liver, the resulting 

 ammonia converted to urea, while the fatty acid remainders would be sent on to 



