alsberg: biochemical analysis of nutrition 273 



with the carboyxl group of another. One of the simplest com- 

 pounds of this type is glycylleucine : 



NH,CH 2 — CO ' NH-CH-COOH 



Giycyi I I 



Leucine 



These compounds are known as peptides and were first made 

 artificially. They were later discovered among the decom- 

 position products of proteins. A peptide with a molecular 

 weight of more than one thousand, composed of eighteen mole- 

 cules of amino-acids, has been made. Substances with still 

 larger molecular weights could be made, were it worth while. 

 Thus substances which are believed to have a structure similar 

 to that of proteins, and which have a molecule approaching in 

 size that qf some proteins, have actually been made. 



After this digression in explanation of the chemical structure 

 of proteins let us return to the problem of the nutrition of ani- 

 mals with amino-acids. Loewi found that animals could be kept 

 for a time without loss in weight upon a mixture of completely 

 digested protein. It was not at that time known that such 

 mixtures contained peptides, which, as I have explained, have 

 certain resemblances to proteins. It was therefore a most 

 important discovery when it was later determined that an animal 

 could be maintained for a time upon an artificial mixture of the 

 pure crystalline seventeen or eighteen amino-acids found in 

 proteins free from peptides. If these observations are correct, 

 it is theoretically possible to supply the so-called protein needs 

 of animals by wholly artificial substances. 



Certain investigators have very recently gone still further 

 by endeavoring to show that some of the nitrogenous needs 

 of the animal organism can be supplied by simple salts of am- 

 monia such as are used in fertilizers. It has hitherto been be- 

 lieved that only plants are capable of utilizing ammonia. The 

 matter has not been settled; but there is good reason to believe, 

 as will appear later, that even if it be shown that animals can 

 utilize ammonia, it is impossible to support animal life upon 

 ammonia as the sole source of nitrogen. 



