626 GENERAL METABOLISM [CH. XLT. 



relative tyrosine is another; for when they are injected into the 

 blood-stream they do not reappear as urea in the urine. We also 

 know that proteins which yield no tyrosine, such as gelatin, are of 

 inferior value as food. Gelatin is also destitute of the tryptophane 

 radical, arid tryptophane is specially useful too. Zein, the protein 

 of maize, lacks tryptophane, and if tryptophane is added to a zein 

 diet, animals fed on the mixture thrive better than those whose sole 

 nitrogenous food is zein. Histidine and pyrrolidine have been sug- 

 gested as being in the same category, but here we must await further 

 information. 



Growth and Maintenance. Synthesis in the Body. 



The different members of the protein family have thus unequal 

 powers in repairing the body waste. Still more important is their 

 difference in promoting growth in young animals. Eecent work by 

 Osborne, Mendel, and others has shown that it is possible to keep 

 animals alive and in health when they receive their nitrogen in the 

 form of one protein only, mixed of course with the proper supply of 

 non-nitrogenous food. But if these animals are growing, the choice 

 of the protein given is a matter of great importance. Eepair pro- 

 cesses in the adult are of a different character from those of growth 

 in the young animal; in the adult cell, katabolism and repair do not 

 involve the destruction and resynthesis of entire protein-molecules, 

 such as obviously must be the case when growth is taking place. 

 This work has also brought to light the fact that, in the animal body, 

 there is a power of synthesising comparatively simple food material 

 into more complex substances far in excess of what was previously 

 considered to be the case. Thus lipoids can be built from inorganic 

 phosphorus compounds, and proteins constructed from amino-acids. 

 Some even have asserted that ammonium salts given by the mouth 

 can be utilised for protein construction, but this has not yet been 

 conclusively proved. The body also possesses the power of building 

 up amino-acids which are absent from the food, and of converting one 

 ammo-acid into others. This ability is most marked in the adult 

 animal ; it is in the young animal that the actual administration in 

 the food of the necessary building stones is indispensable. 



Let us take one or two instances which illustrate this. Eats were 

 fed for long periods on food mixtures which contained only a single 

 protein. When gliadin, edestin, or caseinogen, proteins of very 

 different composition were administered, the animals thrived and 

 showed neither loss of weight nor any other metabolic disorder. 

 Glycine is absent from caseinogen, lysine and glycine from gliadin, 

 phosphorus from gliadin and edestin, and purines were absent from 



