1923] 



KLOTZ — NITROGEN METABOLISM IN FUNGI 



311 



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a 



plastic equivalent of the carbon independently of the nature of the 

 carbon as well as of that of the nitrogen. 



"2. The nitrogen number, by which is meant the nitrogen per 100 

 parts of weight of assimilated carbon, lowers with time; for a mature 

 mould it is about 2 (glucose or levulose as source of carbon). 



3. The metabolism of nitrogen has much resemblance to that of 

 the carbon. 



"a. An accumulation of carbon is combined with a high nitrogen 

 number; inversely the mature mould has a low nitrogen number. 



"b. The nature of the metabolism of the nitrogen does not 

 change under the influence of many factors; neither is this the case 

 with the carbon. 



"c. The velocity of the metabolism is subject to great changes, 

 "d. The same factors that accelerate the metabolism of the 



carbon also furthers that of the nitrogen. 



e. Substitution of rubidium for potassium is of little influence 



on the metabolism of nitrogen. 



"4. The nature of the metabolism of the nitrogen is independent 

 of the source of nitrogen. At first the nitrogen number is high, then 

 it decreases whilst the freed nitrogen returns into the nutrient solution 

 as ammonia. This is proved for the cases when ammonium nitrate, 

 ammonium chlorid, or potassium nitrate, is given as N food. Asper* 

 gillus niger, thus, reduces nitrates to ammonia but not to free nitrogen. 

 Only in the culture tubes with a deficiency of nitrogen as to the quan- 

 tity of carbon, no ammonia can return into the solution as it is directly 

 used for the production of new cells. 



" 5. In cases of a deficiency of N no fixation of atmospheric nitrogen 

 could be observed." 



Zaleski and Israilsky ('14), working with yeasts, found that 

 single amino acids stood below NH< salts in point of assimil- 

 ability. This, they explained, was comprehensible when one 

 considered that yeast cannot build protein directly out of a 

 single amino acid, but must first deamidize it to obtain N for 

 other groups of the protein molecule. Asparagin was superior 

 to NH 4 salts because the acid amide part of the molecule is readily 

 deamidized and the N group thus obtained is first used in the 

 formation of amino acids and then coupled directly with the 

 liberated aspartic acid to form the protein chain. Aspartic acid 

 and NH< salts mixed gave as good growth as asparagin; but, as 

 shown by the fact that the amide group of the asparagin pro- 

 tected added NH 4 salts, the amide group was found superior 

 to NH 4 . The best source of N was that found in autolyzed 

 yeast because, they explained, the suitable amino acids were 

 linked directly to form protein. Where asparagin was added to 



