NUTRITION 281 



is any special form particularly adapted for the purpose of storage is at present 

 doubtful, although certain observers believe that they have evidence of some such 

 substance (see Cathcart's monograph, 1912, pp. 58 and 79). 



Berg (1914), and Berg and Cahn -Brenner (1914) describe the presence of granules of 

 protein material (not protoplasm) in the liver cells of well-fed animals, particularly after 

 feeding with amino-acids. These are regarded as storage of nitrogenous substance. 



Noel Paton (1910) has brought evidence, depending on the proportion of 

 creatine to total nitrogen excreted, to show that, in fasting, after an abundant 

 protein diet, it is mainly non-muscle protein that is first used up. The value of 

 the evidence depends, of course, on the constant percentage of creatine contained 

 in muscle. 



Other investigators hold that there may be a storage of nitrogen in some form which 

 lias a simpler constitution than protein. In making this distinction between protein nitrogen, 

 and that in simpler form ("extractives"), itr may be noted that we assume that any nitrogen 

 stored otherwise than as cell protoplasm does not form an integral part of a "giant" proto- 

 plasmic molecule, or " biogen," as a purely chemical system. 



It was mentioned above that van Slyke and Mayer found that amino acids, 

 introduced into the blood, disappeared rapidly therefrom. In a further paper 

 (1913, 1), they show that these substances are taken up by the tissues in a form 

 such that they can readily be washed out again, even by cold water. There is, 

 moreover, a definite equilibrium established, so that the blood, even in starvation, 

 still contains 3-8 mg. per 100 c.c. This points to an adsorption process, especially 

 since the alternative hypothesis mentioned by the authors, that compounds like 

 those supposed to be prepared by Pfeiffer and Modelski between amino-acids and 

 neutral salts, is improbable, on account of the fact that there is no evidence for 

 the existence of such compounds, as I have shown (Communication to Biochem. 

 Society, not yet published). 



A word may be said here with respect to the experiments of Grafe and 

 Schlitpfer (1912), already referred to, in which, on feeding animals with a diet 

 containing ammonium salts as the only source of nitrogen, a diminution of nitrogen 

 output occurred. The conclusion drawn seems to be that protein can be synthesised 

 from carbohydrate and ammonia. We have seen, indeed, that, from pyruvic acid 

 and ammonia, alanine can be formed in the organism, but evidence is wanting 

 as to other necessary amino-acids. Admitting the possibility of the requisite a- 

 ketonic acids being formed from carbohydrate, the retention of nitrogen from 

 ammonium salts, if utilised for synthesis of protein, should not be observed if. 

 the diet is free from carbohydrate. Accordingly, Taylor and Ringer (1913) made 

 the experiment. They found that, even in these conditions, nitrogen was retained 

 from ammonia and they are of the opinion that the evidence points to the reversal 

 of the process of de-amination as the explanation of the phenomenon. This reaction, 

 if an oxidation, may be presented thus : 



^ _ (ketonic acid) x (NH 3 ) 

 (amino-acid) x (oxygen) 



where K is the equilibrium constant and the factors in brackets are the concentra- 

 tions of the four components of the system. Increase of ammonia leads to 

 diminution of ketonic acid and this again involves increase in amino-acid, which 

 may be utilised in the organism. Alanine, for example, obtained in this way, may 

 be used for the same purposes as that derived from proteins, so that the latter are 

 spared from breaking up. 



Support is given to this view by the later experiments of Grafe (1913), in which it is shown 

 that retention of nitrogen can be obtained when urea is given as food. It is only necessary 

 to assume that the reaction by which ammonia is converted into urea is also reversible, and 

 we see that excess of urea involves increase of ammonia, and we have the same phenomenon 

 as when ammonia itself is given. The fact that when either ammonia or urea is injected 

 subcutaneously, it is entirely excreted by the kidneys, without giving rise to any retention 

 of nitrogen, suggests that the concentration in which it arrives at the de-aminating tissues is 

 too small to result in any perceptible mass action. 



