3 14 NUTRITION OF FARM ANIMALS 



katabolized. According to the second hypothesis, it might 

 be presumed that only certain of the atomic groups contained 

 in protein, such as tryptophan, e.g., would need to be supplied. 

 Moreover, it appears not unlikely that both hypotheses may 

 be true and that body protein is katabolized both as a whole 

 and at times as a means of obtaining certain amino acids. If 

 such is the case, substantially all the " building stones " of the 

 proteins, so far as they cannot be manufactured in the body, 

 must be supplied in the feed, but relatively more of certain 

 particular ones might be required than would be indicated by 

 the make-up of the body proteins. Finally, it seems to be fairly 

 well established that at least some of the amino acids can be 

 manufactured in the body. This is almost certainly true of 

 glycin and perhaps of prolin and arginin. If such be the 

 case, it becomes even more clear that the protein requirement, 

 so called, is really an amino acid requirement. 



399. Amino acids required for maintenance. Our actual 

 knowledge of the amino acid requirement for maintenance is 

 still meager, but it has been shown that a supply of tryptophan 

 and probably of tyrosin is necessary for the maintenance of 

 nitrogen equilibrium, while lysin is dispensable. 



Willcock and Hopkins, 1 for example, found that the zein of 

 maize, which contains neither tryptophan nor lysin, was capable of 

 supporting neither growth nor maintenance in mice, while the addi- 

 tion of tryptophan diminished although it did not altogether stop 

 the loss of nitrogen from, the body. Henriques 2 obtained similar 

 although less striking results in experiments with rats. It is to the 

 work of Osborne and Mendel 3 that we owe the most conclusive demon- 

 stration of the necessity of tryptophan for maintenance. They 

 showed conclusively that zein as the sole source of protein was in- 

 capable of maintaining rats, while with the addition of tryptophan 

 much better results were obtained and in two cases complete main- 

 tenance for a long time was secured. Miss Wheeler 4 has reported 

 similar results on mice. Furthermore, Osborne and Mendel have shown 

 that the deficiencies of zein may be compensated for by the addition 

 to the ration of other proteins containing the lacking amino acids. 



1 Jour. Physiol. (London), 35 (1906), 88. 



2 Ztschr. Physiol. Chem., 60 (1909), 108. 



3 Carnegie Institution of Washington, Publication No. 156 (1911); Jour. Biol. 

 Chem., 13 (1912), 233; 17 (1914), 325. 



4 Jour. Exp. Zoology, 15 (1913), 209. 



