593 METABOLISM, NUTRITION AND DIETETICS 



waste of the tissues, which in nitrogen equilibrium is exactly com- 

 pensated for by a corresponding release of amino-acids or their 

 equivalent from the cell-proteins, are indeed speedily deaminated 

 and the nitrogen of the' amino-groups excreted as urea (with 

 ammonia compounds and kreatinin). But, as we have seen, only a 

 small proportion of the chemical energy of the amino-acids and only 

 a small fraction of their carbon are liberated in this process. The 

 carbon-containing residues are katabolized only to the extent re- 

 quired by the momentary needs of the tissues, any balance being 

 stored as part of the reserve of carbo-hydrate or of fat. The body 

 does not possess the means of storing surplus amino-acids as such 

 or even in the form of proteins, except to the small extent corre- 

 sponding to any increase which may occur in the body-protein 

 when the food-protein is increased beyond the minimum required 

 for nitrogen equilibrium. Why the organism has not developed 

 the capacity to store large quantities of protein is, of course, an 

 interesting question, but it need scarcely be discussed here. One 

 obvious reason is that protein is not a suitable source, nor are amino- 

 acids apparently a suitable source of energy for the tissues until 

 they have been deaminated and have probably undergone further 

 decomposition and transformation. Therefore they are decomposed 

 at once and their available residue stored, if it is in any case to be 

 stored, in the more available form of carbo-hydrate (or fat). 

 Where the food-proteins differ greatly from the body-proteins in 

 the proportions of the various amino-acids, there would be no object 

 in storing a great surplus of those which are most plentiful in the 

 food, if they were at the same time the scarcest in the tissues, or, 

 in the case of gland-cells, the scarcest in the proteins which they 

 manufacture for their secretions. 



At any moment the magnitude of this non-utilizable surplus will 

 depend upon the quantity of that one of the indispensable amino- 

 acids which is present in the smallest amount. For the proper 

 proportion must be preserved between the different ' stones ' out of 

 which the molecule is built. When a single amino-acid is intro- 

 duced into the body, it is at once changed into urea and excreted, 

 since it cannot be utilized by itself for building up protein. 



When the cells have once culled from the mixture circulating in 

 the blood the amino-acids, a full supply of which they have most 

 difficulty in obtaining, a residue, large or small, according to the 

 quantity and quality of the protein intake, will be left, and this can 

 only be utilized to supply energy or to add to the fat and carbo- 

 hydrate stores. For these uses removal of the amino-group is an 

 essential preliminary. The question whether the deamination of a 

 large part of the amino-acids coming from the intestine takes place 

 in the liver, so that the surplus nitrogen is shunted out of the main 

 metabolic current at its very source, has been already touched upon 



