1921] on Quality of Protein in Nutrition 347 



tryptophan was added ; on the 13th day the tryptophan was omitted 

 and included once more on the 35th day. There was growth during 

 the first period, decline in weight during the second period followed 

 by growth on inclusion once more of the tryptophan. 



Similar experiments have been made by Osborne and Mendel in 

 America. They used gHadin of wheat as protein. This protein is 

 a complete one, but it contains very little of certain amino acids, 

 especially lysine. Adult rats were maintained for quite loug periods 

 — as long as 500 days — but young rats capable of growth, though 

 maintained for long periods, failed to grow. 



AVe may here notice that though the growth of the animal may 

 be suppressed and it reaches maturity in age, the capacity to grow is 

 not lost. Osborne and Mendel illustrated this by a photograph of a 

 rat which had failed to grow for 273 days, but resumed growth on 

 being given a suitable diet. The small amount of lysine in gliadin 

 led the authors to regard this unit as essential for growth. In a 

 later experiment they added lysine at intervals ; growth occurred 

 with the lysine, but not without it. 



The effect of lysine on growth was again demonstrated by 

 Buckner, Nollau and Kastle in the case of chickens living under the 

 natural conditions of a poultry farm. The birds were fed upon 

 grain mixtures of high and low lysine content. As their photographs 

 showed, more rapid growth took place on the mixture of high lysine 

 content. 



The element sulphur is present in proteins in the form of cystine, 

 though it is possible another sulphur-containing unit is present. 

 Little or no cystine in a protein has also an effect upon the growth of 

 rats. This has been most clearly demonstrated in the case of the 

 protein, phaseolin, of the navy bean. There was slow growth with 

 this protein alone, but normal growth if the protein were supple- 

 mented with :2 per cent of its amount of cystine. Casein is deficient 

 in cystine. Less casein is required in a diet for producing normal 

 growth, if extra cystine be included. Fifteen per cent casein was 

 required by itself, but only 9 per cent if cystine were added . 



The amino acids containing aromatic nuclei are probably essential 

 units of the protein, but it is difficult to carry out a decisive experi- 

 ment, since all proteins contain phenylalanine, though they may lack 

 tyrosine. There is plenty of evidence that phenylalanine can be 

 transformed in the body by oxidation ; both tyrosine and phenyla- 

 lanine yield homogentisic acid in cases of alkaptonuria. Totani has 

 shown that the almost complete removal of tyrosine from the 

 mixture of units yielded by casein made no difference to the 

 growth of rats. There was evidently enough phenylalanine for all 

 purposes. 



The two hexone bases — arginine and histidine — as shown by 

 Ackroyd and Hopkins, are inter-related in nutrition. Absence of 

 both causes loss of weight : absence of either alone lessens the rate 

 Yol. XXIII. (No. 115) 2 b 



