DISCUSSION OF THE RESULTS AND THEIR BEARINGS. 85 



again, failure to purify carefully a protein like casein will vitiate the 

 study of a problem like the synthesis of amino-acids. Pure casein is 

 glycocoll-f ree ; and the continued feeding of such a product as the 

 sole protein of the dietary enables one to make deductions respecting 

 the synthesis of glycocoll. The use of crude commercial protein 

 preparations can never satisfy the requirements of refined study in 

 this domain, where small effects continued over long periods are of 

 great importance. We believe, therefore, that such considerations 

 justify the energy and expense which have been put into the work. 



In relation to the much-discussed problem of the relative value 

 of organic vs. inorganic phosphorus in nutrition, our data after feed- 

 ing phosphorus-free edestin to growing rats (cf . Charts LXXV and 

 LXXVI) show a success quite as great as that with phosphorus-con- 

 taining casein (cf. Charts LVI, LVII, LVIII, UX, and LX). The 

 animals must here have synthesized their phosphorus compounds 

 from inorganic phosphorus. Whether milk production and other 

 functions calling for such synthetic reactions will continue adequately 

 is open to investigation. It is also noteworthy that all of our animals 

 grow on a dietary that is purine-free, or essentially so. Here the ques- 

 tion of purine synthesis suggests itself. It is apparent, e. g., in the 

 case of gliadin, that the grown as well as ungrown rats may be main- 

 tained for long periods on single proteins. 



With such an ideal non-protein dietary component at hand 

 amino-acid substitutions can be attempted in the adult as well as in 

 the growing animal. The protein minimum (or minima) is also open 

 to accurate investigation. With a method of feeding devised which 

 will permit a differentiation between growth and maintenance, which 

 furnishes an energy-yielding protein-free component that is appro- 

 priate, and leaves the protein as the sole variable in the dietary, we 

 believe that further contributions can be made to the problems of 

 nutrition. 



In the preparation of the large quantities of carefully purified 

 proteins required for these experiments, we have been assisted by 

 Mr. Charles S. Leavenworth, Mr. Owen Nolan, Mr. Leigh I. Hol- 

 dredge, and Mr. Lawrence Nolan, whose valuable cooperation we 

 are glad to acknowledge here. 



