72 THE PHYSIOLOGY OF PROTEIN METABOLISM 



repair of a certain protein tissue x amount of amino acid K, it does not 

 matter apparently whether the body in getting this amount of K has 

 to discard ten times x amount of say amino acid B, and thirty times x 

 amount of amino acid M. As Zisterer (425) has pointed out (p. 59) 

 supposing syntonin is the substance to be formed and caseinogen be 

 fed, then in order to obtain the necessary amount of alanine required in 

 this synthesis between four and five times more caseinogen is required 

 than if syntonin itself be given or, in other words to yield the necessary 

 amount of preformed alanine for 100 grms. syntonin, 444 grms. casein- 

 ogen must be consumed. Zisterer, of course, in his calculation 

 assumes that the amino acids required are present in the proteins and 

 are not synthetized. Obviously, it is not so much the quantity but 

 the quality of the protein given which is of prime importance, and it 

 follows that on. the whole it is safer to give a relatively large intake of 

 protein than a small one. There is the further consideration that in 

 order to obtain the requisite daily caloric intake protein must be 

 taken in fair amount. There is a definite limit to the intake of 

 both carbohydrate and fat on account of the difficulties of digestion at- 

 tendant on too full meals of these substances. At most carbohydrate 

 and fat supply only about four-fifths of the caloric requirements, and 

 the balance must be made up by the addition of protein. 



Feeding Experiments with "Abnormal" Proteins. 



Attempts have been made to maintain life by replacing the ordinary- 

 protein of the diet by other proteins, which may be termed abnormal 

 forms of protein, e.g. gelatin. It is, of course, a well-known fact that 

 the members of the aromatic group of amino acids and cystine are 

 practically completely absent from the gelatin molecule. Feeding 

 experiments have been carried out with gelatin, and with gelatin plus 

 the missing amino acids, but complete replacement of protein has 

 not yet been achieved. Whether this be due simply to the amino 

 acids not being added in proper amount, or in proper combination, 

 or whether it be due to the absence of some other essential sub- 

 stance is not clear. Kaufmann (215) carried out very complete ex- 

 periments on himself and dogs. He found, like many others, that 

 only a comparatively small amount of the protein can be replaced by 

 pure gelatin (in dogs between one-fifth and one-fourth of the protein 

 of the diet). He stated, however, that if he replaced the caseinogen 



