62 THE PHYSIOLOGY OF PROTEIN METABOLISM 



more than 50 grms. of tissue protein be formed by the conversion into 

 leucine of other amino acids which are present in excess in the protein 

 fed? In the case of certain of the amino acids which resemble one 

 another very closely it is more than probable that such an interchange 

 can take place. 



This question has been investigated by Magnus-Levy (274) and 

 others by the study of the ratio of the amount of glycine nitrogen to 

 total nitrogen, hippuric acid being used as the product for the estimation 

 of the glycine. With the exception of gelatin the body proteins contain 

 on an average 4 per cent, of glycine. 1 Now as protein contains some 16 

 per cent, of nitrogen, and glycine 187 per cent there is present in 100 

 grms. protein-nitrogen rather less than 5 per cent, glycine-nitrogen. 

 As Magnus-Levy points out if in the benzoic acid experiments a 

 higher value for the quotient (glycine-nitrogen : total nitrogen) be 

 found than this figure (5 per cent), then there is evidence of the pro- 

 duction of glycine from a source other than preformed glycine. 

 Magnus-Levy found that the quotient was larger, therefore a synthesis 

 of glycine had taken place somewhere in the tissues. He has also 

 pointed out as further evidence in favour of such a synthesis the fact 

 that the protein food of a suckling animal was remarkably poor in pre- 

 formed glycine he estimated that milk proteins, at the outside, contain 

 only from 'I to -3 per cent of glycine, and yet it is found that from 

 100 grms. of this protein a suckling calf can in a short period build up 

 78 grms. of tissue protein containing about 2*5 grms. of glycine. It is 

 suggested that in the formation of this glycine two courses are open, 

 either (i) the in vivo breakdown of protein is identical with that in 

 vitro, and that the other amino acids formed are converted by oxidative 

 processes into glycine, or (2) that the in vivo breakdown of protein is not 

 the same as that observed in vitro, in that a greater amount of glycine 

 is formed. Magnus- Levy rather inclines to the first hypothesis. As 

 an example of his experiments the following may be cited. A rabbit 

 weighing 1500 grms., containing thus some 200 grms. protein with 

 about 6'6 grms. of glycine, excreted after treatment with benzoic acid 

 8 grms. of glycine. During the course of the experiment the weight 

 of the animal had fallen to about 1250 grms.; a loss of 250 grms. 

 which (even supposing the total loss were due to protein utilized) 

 would have only yielded a little over I grm. of glycine. Magnus-Levy 

 later suggested that the glycine which was excreted in the previous ex- 



1 This value is probably less than the true value, for according to Osborne the esterifica- 

 tion method by which the amino acid content of tissues has been estimated gives too low 

 results. 



