278 NITROGEN NUTRITION AND METABOLISM 



Urease, the enzyme hydrolyzing urea, is well known from higher 

 plants and bacteria (511). It is evidently of very general occurrence 

 among fungi, including Aspergillus spp. (34, 35, 36, 84, 115, 161, 392, 

 181, 522), Penicilliurn spp. (134, 371, 547), Pythiacystis citrophthora 

 (296), several dermatophytic fungi (524), and most of the fleshy basid- 

 iomycetes which have been studied in this connection (70, 189, 209, 

 357). Both direct and indirect evidence indicate that urease is formed 

 by at least some actinomycetes (10, 15, 240, 485). 



The urease of Aspergillus niger is most active at pH 7.6 (34), al- 

 though its production is favored to a marked degree by low pH (371). 

 It is formed by A. niger on various media and appears in the medium 

 after autolysis (34). In basidiomycete sporophores the enzyme is most 

 abundant in the hymenium (209). 



Urea is formed in the fruit bodies of the higher basidiomycetes (209, 

 268, 269, 579, 615), and by many saprophytic fungi and actinomycetes 

 in pure culture (102, 194, 217, 266, 267, 579). Oxygen and exogenous 

 ammonia increase urea formation by sporophores (272, 273). The 

 function of urea — in some fungi very large amounts are reported (266, 

 268) — is presumably as a nitrogen reserve. 



The biochemical origin of urea in fungi has not been definitely 

 proved, and there are several possible processes. First among these is 

 the breakdown of arginine in the ornithine cycle (p. 269). Urea bio- 

 genesis by this route is indicated by the finding that exogenous arginine 

 increases urea accumulation in fungi (267, 522), and by the demon- 

 stration of urea formation from arginine by cell-free preparations from 

 basidiomycete sporophores (273, 615). 



Apart from the specific action of arginase on arginine, it appears 

 that urea is also formed by one or more enzymes which act on guani- 

 dine and derivatives of it to form urea and ammonia; this activity is 

 found in Streptomyces griseus (540) and in several saprophytic fungi 

 (103,270). 



Formation of urea from calcium cyanamide by cell-free preparations 

 of Aspergillus niger has been reported (314). 



An origin of urea from purines is also possible in the fungi, by the 

 general route: 



Xanthine -^ uric acid —> allantoin -» 



allantoic acid — > glyoxylic acid -f urea (19) 



Evidence for this pathway is still incomplete, but may be summarized 

 briefly: • 



1. Xanthine oxidase and uricase occur in several fungi (518). Uri- 

 case is an inducible enzyme in Torulopsis utilis (450). 



