238 BACTERIOLOGICAL CHEMISTRY 



deamiiiatioii accompanied by further breakdown with 

 liberation of hydrogen sulphide, the products of the 

 action of Proteus vulgaris on cysteine being hydrogen 

 sulphide, ammonia, carbon dioxide, hydrogen and acetic 

 acid. 



The course of the breakdown of amino-acids can be 

 followed by analysing the resulting solutions for the 

 following fractions : — 



1. Total nitrogen (by Kjeldahl's method). 



2. Amino -nitrogen (by van Slyke's method). 



3. Synthetic nitrogen in proteins and cellular material 



(by precipitation with colloidal iron and Kjeldahl 

 estimation). 



4. Ammonia nitrogen (by vacuum distillation). 



5. Non-amino-nitrogen, that is nitrogen in the ring, 



is given by 1— (2+3+4). 



An increase in the amino -nitrogen figure indicates 

 breakdown of the side chains. A decrease in the non- 

 amino -nitrogen follows a breakdown of the ring structure. 

 An increase in the " synthetic nitrogen " value is usually 

 accompanied by a reduction of the amino-nitrogen and 

 ammonia nitrogen values since the proteins are built up 

 at the expense of such compounds. When an alternative 

 source of carbon, for example glucose or glycerol, is present 

 there is usually an increased utilisation of amino- and 

 ammonia nitrogen and a corresponding increase in the 

 " synthetic nitrogen," which is the probable explanation 

 of the so-called " protein-sparing " action of carbo- 

 hydrates (see p. 226). 



When organisms act on racemic amino-acids both 

 isomers are usually attacked, but the naturally occurring 

 one more readily, so that an optically active mixture 

 results. Three possibilities arise : (a) The natural com- 

 ponent is attacked so rapidly compared with the other 

 one that an almost optically pure amino -acid results ; 



