806 PHYSIOLOGY 



intermediate products of metabolism may be excreted still attached to the 

 benzene nucleus. In the a-amino-acids the point where disintegration 

 first occurs is the a-group. Deamination Knoop finds most usually asso- 

 ciated with oxidation. The primary product is therefore an a-keto-acid. 



Further oxidation affects the CO group, so that carbon dioxide is eliminated 



and the next lower acid in the fatty acid series is produced. Thus from 

 alanine the body would produce pyruvic acid, CH 3 .CO.COOH, and this 

 on further oxidation would form acetic acid, CH 3 .COOH, and carbon dioxide. 

 On the other hand, these keto-acids may undergo reduction to an oxy-acid, or 

 even a step further, to a fatty acid, though the conditions which determine 

 whether oxidation or reduction shall take place have not yet been fully 

 studied. 



This loss of nitrogen diminishes little, if at all, the energy value of the 

 amino-acids of the body. The following table shows the heat equivalents 

 of some of the amino-acids and their corresponding fat and oxy-acids : 



S bstance 



Leucine ..... 855 



Isobutylacetic acid . . . 837 



Alanine ..... 389 



Propionic acid .... 367 



Lactic acid ...... 329 



Pyruvic acid .... not determined 



These heat equivalents represent the heat evolved on the total oxidation 

 of the substances in question. In the case of the amino-acids, part of the 

 molecule is not oxidised, the nitrogen leaving the body not as free nitrogen 

 but as urea. To obtain the total possible heat value of an amino-acid to 

 the body, we must subtract from its heat equivalent half the heat equivalent 

 of urea, (an amino-acid contains 1 atom of nitrogen, while urea contains 

 2 atoms, so that one molecule of urea is produced from 2 molecules of an 

 amino-acid.) The heat equivalent of urea being 80, the physiological 

 heat equivalent of leucine will be not 855 but 815, while the physiological 

 heat equivalent of alanine will be 349, as against 329 for lactic aeid. Thus 

 even in the case of the smallest molecule, the loss of energy attendant on 

 simple deamination and conversion into the corresponding oxy-acid amounts 

 to little more than 5 per cent., and the proportion will be much smaller in 

 the case of the larger molecules. We are accustomed to regard the urea 

 excretion as an index to protein metabolism. In truth it is an index only 

 of the deamination of the protein constituents, and it tells us nothing what- 

 ever about the fate of that part of the protein, the non-nitrogenous part, 

 which contains 95 per cent, or more of the total energy of the protein food. 

 The rise in the rate of excretion of urea after a protein meal was regarded 

 both by Voit and Pfliiger as a sign that the cells of the body prefer to use 

 protein for all their requirements, if this substance were available. We 

 see now that the rapid output of urea after a protein meal affords no basis 



