PRESIDENTIAL ADDRESS. 297 



Argutinsky was losing weight even before the exercise, that he was in a 

 condition of semi -starvation and that therefore, when any extra call for energy 

 was made, it had to be got from the protein of the body, since he was a lean 

 young man. The chief value of his work was in showing that any increased 

 excretion of nitrogen occurs, not on the day of the exercise, but on the two 

 succeeding days. 



Since that time the methods of investigating the processes of metabolism 

 have enormously advanced, and the combination of the study of the respiratory 

 exchange with determinations of the excretion of nitrogen has enabled a definite 

 decision to be obtained as to the utilisation of the three proximate principles 

 during rest and during muscular work. As everyone knows, it has been demon- 

 strated that while carbohydrates must be considered as the most readily avail- 

 able food of the body, it is equally true that the direct or indirect oxidation 

 of the amino acids formed from the proteins of the food or of the tissues on 

 the one hand and of fats on the other are also valuable sources of energy. 



2. Proteins jn Growth and Repair. 



The advance in our knowledge of the way in which protein is used in the 

 construction and repair of tissues shows a less devious course. First came the 

 recognition of changes of the crude protein of the food to more diffusible, and, 

 as was later shown, simpler molecules. Then came the discovery by Kutscher, 

 Seeman and Cohnheim of the more complete breakdown into the constituent 

 amino acids, a recognition of the purpose of the breakdown to yield the con- 

 stituent "building stones" for use as required by each tissue, and lastly the 

 elaborate work upon the special significance and potentialities of each of these. 



Some of these amino acids must be supplied as such, and if certain of them, 

 which may occur only in minute amounts in the body tissues, are withheld, 

 growth is rendered impossible. They become the limiting factor. 



Other amino acids, glycin for example, may be formed in the body. 



Not the least important of the amino acids are the diamine acids, lysin, 

 histidin, and arginin, which are so abundant in the protein which is combined 

 with nucleic acid in the nuclei of cells. That lysin is essential for growth has 

 been for long well established. Histidin and arginin resemble one another 

 closely in the constitution of their molecules, but while histidin has the imina- 

 zole ring, arginin has the guanidin molecule as the end of the chain. 



Experiments recorded by Ackroyd and Hopkins ^ tend to show that in the 

 absence of these two diamino acids growth of young white rats is arrested, and 

 that there is a loss of body weight, but that the addition of both or either of 

 them is sufficient to restore the rate of growth. Such observations seem to 

 indicate that they are among the amino acids which are essential and which must 

 be supplied in the food. 



The fact that the addition of either one or other is sufficient to restore growth 

 led these investigators to suggest that in metabolism each can be converted into 

 the other. The safer conclusion apuears to be that both of them can yield some 

 substance which is necessary for growth and for normal metabolism. 



The same workers further give experiments to show that in the absence of 

 these substances, the excretion of allantoin falls and that it is again increased 

 when one or other or both are added to the diet. They suggest that arginin 

 and histidin probably constitute the most readily available raw material for 

 the synthesis of the purin ring in the animal body. It must be remembered that 

 Abderhalden and Julius Schmidt failed to get evidence of the formation of allan- 

 toin from histidin in the dog. As yet the relationship of these bodies to purin 

 metabolism cannot he considered as definitely established. Investigations upon 

 birds which excrete such large quantities of purin nitrogen should yield more 

 conclusive results. 



The p-ossible source of creatin, methyl-guanidin acetic acid, from histidin 

 has been considered by Ditmann and Welcker on purely theoretical grounds 

 which need not now be considered. 



Arginin is a much more abundant constituent of most proteins than is 

 histidin. The characteristic of the arginin molecule is the presence of guanidin. 

 It is guanidin-a amino S valerianic acid. 



