PROTEIN REQUIREMENTS 73 



nitrogen in his own diet by 93 per cent, of gelatin nitrogen plus 4 

 per cent, tyrosine nitrogen, 2 per cent, cystine nitrogen, and I per 

 cent, tryptophane nitrogen, the mixture almost sufficed to prevent 

 protein waste (no trace of the amino acids was found in the urine). 

 In the case of two dogs which he also tested, he found that from 

 one-half to one-third gelatin nitrogen plus 4 per cent, tyrosine and 

 2-5 per cent, tryptophane partially prevented loss of tissue pro- 

 tein. Murlin (296 A) held that, quite apart from the addition of 

 these missing amino acids, the retention of nitrogen during gelatin 

 feeding depended to a large extent on the nature of the remainder 

 of the diet. He believed that the presence of carbohydrate in large 

 amount was of paramount importance. If carbohydrate were given 

 with the pure gelatin it was possible to replace, for a short period 

 at least, 63 per cent, of the total nitrogen, and even to obtain a small 

 retention of nitrogen. Rona and Miiller (340), on the other hand, 

 came to the conclusion that the capacity of gelatin to replace protein 

 was not enhanced by the addition of 4 per cent, tyrosine and 2-5 per 

 cent, tryptophane. In only one of their experiments was there the 

 slightest evidence of increased sparing of protein due to this addition. 

 About two-fifths of the protein nitrogen could be replaced by gelatin with 

 the addition of the amino acids. Abderhalden and Bloch (24) found that, 

 in the case of alkaptonuria, if the amino acids (tryptophane, cystine, 

 tyrosine, phenylalanine, leucine, alanine, glutamic and aspartic acids) 

 were added to the gelatin to make up the deficiencies, about one- 

 half of the protein could be replaced. The experiments of Hopkins 

 and Miss Willcock (199) on the protein-replacing power of zein, a pro- 

 tein which contains no tryptophane in its molecule, yielded evidence 

 similar to that obtained from the gelatin-feeding experiments. These 

 observers fed mice on a diet of zein, carbohydrate and fat They 

 found that 77 per cent, of the mice died within twenty days. After 

 the addition of tyrosine to the food 70 per cent, died within the same 

 period, but after the addition of the missing tryptophane only 20 per 

 cent, died in the twenty days, and nearly 70 per cent of the remaining 

 animals lived over thirty days. Not only did the mice fed with the 

 zein and tryptophane live longer, but their physical condition was 

 much better than that of the other two series of animals. 



