MAINTENANCE REQUIREMENTS OF MATTER 315 



On the other hand, they have also shown that lysin is not essential to 

 protein maintenance, they having been able to maintain animals for 

 long periods on rations containing as their sole protein gliadin, which 

 contains no lysin but does contain tryptophan. 



What has been shown regarding the necessity of tryptophan 

 and the dispensability of lysin for maintenance may doubtless 

 prove to be true for other protein constituents, so that ulti- 

 mately it may be possible to estimate the relative maintenance 

 values of proteins on the basis of their chemical constitution. 

 At present, however, this is far from being the case. The 

 constitution of many of the proteins, particularly those of the 

 roughages, is known inadequately or not at all, while the specific 

 amino acid requirements for maintenance have still to be worked 

 out and may conceivably vary as between different species. 



400. Relative values of proteins for maintenance. The 

 facts regarding the variations in the constitution of the different 

 proteins which are recorded in Chapter I (50) render it evident 

 that these substances may be of quite unequal value as sources 

 of amino acids to the organism. Thus, according to the data 

 there tabulated, gliadin and zein would be about three or four 

 times as valuable as legumin as a source of the amino acid 

 prolin, while on the other hand legumin would be 2 \ times as 

 valuable as wheat glutenin as a source of lysin. The cereal 

 proteins, especially those of wheat, are notably rich in glutamic 

 acid and therefore relatively poorer in other constituents. If, 

 then, the protein requirement for maintenance is in reality an 

 amino acid requirement it would seem that these various pro- 

 teins must be of unequal value for that purpose. 



As regards single proteins, the experimental evidence just 

 cited strongly supports this presumption, while Osborne and 

 Mendel l have likewise shown the existence of distinct differences 

 in the values of lactalbumin, casein edestin, gliadin and milk 

 proteins for the maintenance of rats. It must not be forgotten, 

 however, that both man and domestic animals ordinarily con- 

 sume a mixture of proteins, so that it may be presumed that 

 deficiencies or excesses of particular " building stones " com- 

 pensate for each other to a greater or less extent. On the whole 

 the statement seems justified that while distinct differences 



1 Jour. Biol. Chem., 2Z (1915), 241. 



