ESSENTIAL NONMETALLIC ELEMENTS 105 



These authors suggest that different pathways of carbohydrate iitihzation 

 may be followed, depending upon whether nitrate or ammonium nitrogen 

 is present. 



Organic sources of nitrogen. Of the vast number of organic com- 

 pounds which contain nitrogen the ones of interest in fungus nutrition are 

 those which occur naturally. A few exceptions will be noted later. In 

 practice, this means proteins and the products of protein hydrolysis. 

 The following steps in protein hydrolysis have been recognized : protein -^ 

 metaprotein — > proteoses — > peptones — > peptides — > amino acids. Pep- 

 tone, which is a complex mixture of peptides and amino acids, is frequently 

 used as a nitrogen source in media. According to Gortner (1929), 

 peptones are neither coagulated by heat nor precipitated by saturating a 

 solution with ammonium sulfate, properties which distinguish peptones 

 from proteins, metaproteins, and proteoses. Since peptides having some 

 11 amino-acid residues are precipitated by ammonium sulfate, it may be 

 deduced that the peptides in peptone have on the average 10 or less 

 amino-acid residues. Peptone is a useful source of nitrogen when it is 

 desired to culture a large number of species upon a single medium. A 

 part of its virtue may be ascribed to its complex nature, for a mixture of 

 nitrogen sources may be better utilized than a single source. Peptone 

 also contains most of the water-soluble vitamins (Stokes et al., 1944). 



Most of the amino acids which have been isolated from proteins are 

 listed in Table 20. In addition, the amides of aspartic and glutamic 

 acids are included. These compounds are found free in many plants and 

 are thus available to the fungi in nature. 



These amino acids are not of equal value in fungus nutrition. The 

 relative value of 24 amino acids for 14 fungi was tested by Leonian and 

 Lilly (1938) who found no one amino acid w^as best for all these species. 

 Steinberg (1942) made an extensive study of growth of Aspergillus niger 

 on 22 amino acids. Seven were excellent sources of nitrogen for A. niger: 

 alanine, arginine, aspartic and glutamic acids, glycine, proline, and 

 hydroxyproline. Steinberg expressed the opinion that the seven amino 

 acids which supported the most growth of A. niger are those which are 

 synthesized first (primary amino acids) by this fungus and from which the 

 other amino acids (secondary amino acids) are normally formed. It is 

 assumed that the "primary" amino acids enter directly into the metabolic 

 pathways, while the "secondary" amino acids must undergo preliminary 

 deamination before use. The primary amino acids are probably not 

 the same for all fungi. Lilly and Leonian (1942) investigated the effect 

 of nitrogen source on the growth of 10 strains of Saccharomyces cerevisiae. 

 The data in Table 21 show clearly that different amino acids vary in 

 effectiveness, and that different strains of the same organism respond 

 differently to the same source of nitrogen. 



