252 NITROGEN NUTRITION AND METABOLISM 



tained from Streptomyces sp. (399), and it may be assumed that the 

 destruction of keratin by dermatophyte fungi is carried out by an extra- 

 cellular enzyme (491, 492, 555). A protease of Penicillium sp. converts 

 trypsinogen to trypsin, but is distinct from the enterokinase which 

 carries out this reaction in animal tissues (312). 



Fungal endopeptidases cannot be identified with known exocellular 

 animal enzymes. However, it is reasonable to believe that studies of 

 purified enzymes acting on synthetic substrates will eventually support 

 a rational classification. For the present, a few general observations 

 must suffice: 



1. Crude and purified preparations from a fungus usually have 

 their pH optimum somewhat on the alkaline side of neutrality (142, 

 185, 283, 335, 485), but particular activities may have an optimum as 

 low as 4.5 (115). 



2. Substrate and inhibition studies suggest that endopeptidases of 

 the trypsin type occur in Streptomyces sp. (377), and in Trichophyton 

 gypseum (185) and other fungi (335); a purified enzyme from Strepto- 

 myces proteolyticus acts on pepsin substrates but at a pH very different 

 from that of the animal enzyme (549). 



3. Crude and highly purified preparations alike consist of mixtures 

 of different enzymes (116, 279, 335, 588, 589, 621); enzymes of even 

 closely related strains show differences in rate of attack on different 

 proteins (142). 



Endopeptidases of Aspergillus oryzae have been studied thoroughly 

 (115, 116, 198). Three different enzymes acting on proteins have been 

 separated. The major component probably acts on most proteins, in- 

 cluding those which lack aromatic acids, and has been obtained in a 

 crystalline, although not completely pure, state (114, 198). 



Culture filtrates of Streptomyces spp. lyse bacterial cells. Lysis of heat- 

 killed Gram-negative bacteria has been attributed to a protease (67, 

 285, 376, 519). Lysis of Gram-positive bacterial cells by different 

 species of Streptomyces is effected, however, by an enzyme similar to 

 lysozyme, i.e., a mucopolysaccharase (333, 334), or by the combined 

 action of a ribonuclease and a protease (376, 378). 



The formation of exocellular protein-hydrolyzing enzymes by some 

 fungi and actinomycetes proceeds in synthetic media with inorganic 

 nitrogen sources (131, 185, 352); for other species a more complex 

 medium may be required (131). There is as yet no good evidence of 

 enzyme induction by substrate, although use of protein media may 

 increase yields (53, 530). A study of cultural conditions as they affect 



