260 NITROGEN NUTRITION AND METABOLISM 



interaction accounts for the toxicity to Lactobacillus arabinosus of 

 D-alanine (293). Threonine uptake is possibly inhibited in N. crassa 

 by other amino acids, but this is not yet certain (566). 



Catabolism of Amino Acids. The breakdown of amino acids and 

 amides may be reviewed under two general headings, the processes 

 which lead to removal of nitrogen and the processes which alter the 

 carbon skeleton. These two categories of biochemical change are re- 

 viewed by Cohen (109) and Greenberg (214), respectively. We must 

 restrict this discussion largely to the fungi, and only a very incomplete 

 picture of amino acid metabolism will emerge, inasmuch as many im- 

 portant reactions known in other organisms have not yet been investi- 

 gated in the fungi. 



One approach to amino acid catabolism is to determine what com- 

 pounds are either oxidized or deaminated by resting cells, using mano- 

 metric data or ammonia determinations as criteria. Penicillium 

 chrysogenum, for example, oxidizes glutamic acid, proline, and alanine 

 rapidly, other amino acids more slowly or not at all, as judged by 

 oxygen uptake (603). Washed cells of Streptomyces venezuelae liber- 

 ate ammonia from glutamate, arginine, proline, and other amino acids, 

 not from tyrosine, trytophan, leucine or cysteine; only those amino 

 acids which are attacked can serve as sole sources of nitrogen for growth 

 (212). Leptomitus lacteus oxidizes and deaminates L-alanine, glycine, 

 and L-leucine (463). 



A second avenue of investigation of amino acid catabolism is the 

 study of particular enzymatic activities in cell-free preparations or as 

 reflected in activities of intact cells. Most efforts of this type have been 

 focused on amino acid oxidase (dehydrogenase) systems; scattered 

 information on other reactions is available but difficult to systematize. 



The d- and L-amino acid oxidases are flavoproteins known and 

 studied in animal systems; their general properties are reviewed by 

 Cohen (109) and Meister (355). The usual manometric system, em- 

 ploying crude extracts which contain catalase, may be formulated as 

 follows: 



R— CH(NHo)— COOH + i/ 2 2 -> R— CO— COOH + NH 3 (3) 



Amino acid a-Keto acid 



In the absence of catalase, the keto acid is decarboxylated and the 

 over-all reaction then proceeds: 



R— CH(NHo)— COOH + 2 -* R— COOH + NH 3 + C0 2 (4) 



The D-amino acid oxidase of Neurospora crassa acts at varying rates 

 on a number of amino acids (Figure 7); detailed comparison with the 



