NITROGEN METABOLISM 227 



teolytic enzymes, that good aeration favours production 

 of the enzymes, and that on synthetic media, even 

 though good growth may occur in their absence, no 

 proteolytic enzymes are formed unless small quantities 

 (0-025 per cent.) of calcium and magnesium are present. 



Another possible explanation of apparent protein 

 sparing action is that the methods of estimating protein 

 utilisation are fallacious. Normally the extent of protein 

 degradation is estimated by the amount of ammonia 

 and amino -nitrogen appearing in the medium as a result 

 of the breakdown of the protein. In the presence of sugar 

 less of these breakdown products appear than is the case 

 when little or no sugar occurs in the medium. Now the 

 amount of ammonia and amino -nitrogen found in the 

 medium is that which is left over after the organism has 

 taken what it needs to build up its protoplasm and other 

 nitrogenous constituents. The more rapid and profuse 

 the growth the more of such raw materials will it require 

 and the less wall be left over in the medium. One of the 

 chief effects of a plentiful supply of easily assimilated 

 carbon, such as glucose or other sugar, is to increase the 

 growth of the organism in such a medium. That is, in 

 presence of glucose more growth occurs, more amino - 

 nitrogen is used and less remains in the medium ; in other 

 words, there appears to be less production of amino - 

 nitrogen and consequently less breakdown of protein, 

 whereas in reality there may be just as much protein 

 breakdow^i, or even more, in presence of sugars. 



Amino-acid Degradation. — The amino-acids resulting 

 from hydrolj^sis of the proteins find their way into the 

 cell where they are acted upon by the endo -enzymes to 

 give a variety of products, the nature of which depends 

 on the amino-acid, the organism and the condition of 

 the medium. The earliest work on this subject was of 

 little scientific value since it was done with mixed cultures 

 of putrefactive organisms on mixtures of proteins, Tliis 

 period was followed by one in which the action of mixed 



