RELATIVE VALUES OE PROTEINS. 



101 



long ago showed that gelatin in whatever amount fed is completely katabolized 

 in the body, at least so far as its nitrogen is concerned, although it may some- 

 what diminish the waste of protein tissue. Subsequent investigations by Kirch- 

 mann 1 and by Krummacher 2 showed that when gelatin is fed alone an amount 

 equivalent to the fasting nitrogen katabolism may reduce the loss of nitrogen 

 from the body by something over 20 per cent, while, on the other hand, even 

 very large quantities can effect a reduction of only about 35 per cent. Murlin 8 

 finds that in the mixed diet of men about two-thirds of the protein may be 

 replaced by gelatin without disturbing existing nitrogen equilibrium. That the 

 inferior value of gelatin is due to the absence of certain groupings in its mole- 

 cule seems to have been shown by Kaufmann, 4 who found that gelatin with the 

 addition of proper quantities of tyrosin, cystin, and tryptophan was able to 

 maintain nitrogen equilibrium at least several days. 



Investigations by Wilcock and Hopkins 5 upon zein, which, as already noted, 

 lacks lysin and tryptophan, approach the subject from a slightly different angle. 

 They found that a diet containing zein as its only protein material was unable 

 to maintain growth in young mice. The addition of tryptophan approximately 

 doubled the survival period and added markedly to the well-being of the ani- 

 mals, but was unable to maintain life indefinitely. On the zein diet the animals 

 became torpid early in the experiment and almost comatose before death en- 

 sued, while with the addition of tryptophan no such symptoms were observed. 

 The authors interpret this result as showing that tryptophan has some specific 

 function in the body aside from the mere maintenance of nitrogen equilibrium. 

 The results recently obtained by Osborne and Mendel, 6 however, show that 

 great caution is necessary in the interpretation of such survival experiments, 

 while they also indicate that growth is largely dependent on some other factor 

 than the protein supply. 



Experiments on rats by Henriques 7 gave a similar result as regards zein, 

 with which it was found impossible to obtain nitrogen equilibrium in short 

 experiments. On the other hand, however, an abundant supply of gliadin main- 

 tained nitrogen equilibrium for some days, notwithstanding the fact that it 

 lacks both lysin and glycocol. 



PROPORTIONS OF CONSTITUENTS. 



Still further, even when all the constituents of the body protein are present 

 in the feed protein, their proportions may be widely different. Thus, a mixture 

 of equal parts of glutenin and gliadin would contain about 30 per cent of glu- 

 tamic acid as compared with about half that amount in ox muscle, while the 

 latter yields over 11.5 per cent of leucin as compared with less than 6 per cent 

 from the former. In such a case it would seem that the tissues in which the 

 synthesis takes place must make a selection from the material supplied by the 

 digestive tract, reproportioning the various constituents, while the excess of cer- 

 tain ones would be attacked by the deamidizing enzyms of the body, their nitro- 

 gen being finally excreted as urea. Accordingly, it might be anticipated that the 

 more nearly the feed protein resembled in its make-up the average of the body 



1 Zeitschrift fur Biologie, vol. 40, p. 54. 

 2 Zeitschrift fur Biologie, vol. 42, p. 242. 



3 American Journal of Physiology, vol. 19, p. 285; vol. 20, p. 234. 



4 Archiv fur die Gesammte Physiologie des Menschen und der Thiere (Pfluger), vol. 109, 

 p. 440. 



5 Journal of Physiology (London), vol. 35, p. 88. 



Carnegie Institute of Washington Publication No. 156. 



7 Zeitschrift fur Physiologische Chemie, vol. 60, p. 105. 



