262 NITROGEN NUTRITION AND METABOLISM 



kcto analogue, which exists in a cyclic form as, probably, A'-dehydro- 

 pipecolic acid; this compound may be cither reduced to pipecolic acid 

 or reconverted to lysine (476). 



Although several other fungi show L-amino acid oxidase activity (51, 

 87, 99, 224, 299), it is not yet certain that the enzyme is identical in all 

 forms. The formation of a-keto acids by growing or resting cells sup- 

 plied with the corresponding L-amino acid is often observed, and may 

 be taken as presumptive evidence for an L-amino acid oxidase (551, 

 552, 602). The formation of isoamyl alcohol from leucine (422) prob- 

 ably reflects oxidative conversion of the amino acid to a keto acid, 

 followed by decarboxylation and reduction. 



It should be stressed that chemical attack on an L-amino acid does 

 not, by itself, prove that an L-amino acid oxidase is present. A com- 

 bination of transaminase and glutamic dehydrogenase activities could 

 — and probably does in mammalian tissues (355) — bring about oxi- 

 dative deamination without the participation of an oxidase, and the 

 over-all stoichiometry would be identical with Equation 3. 



Glutamic dehydrogenase has a wide distribution in nature, and has 

 been found in extracts of Neurospora crassa mycelium (152). Unlike 

 the oxidases, it is specific to L-glutamic acid and requires the partici- 

 pation of a pyridine nucleotide. The reaction may be formulated: 



HOOC— (CHo)j- CH(NH 2 )COOH + TPN + 



L-Glutamic acid 



— HOOC— (CH 2 ) 2 — CO— COOH 



a-Ketoglutaric acid 



+ TPNH + H+ + NH 3 (5) 



Glutamic dehydrogenase is probably the key to synthesis of amino 

 acids (see below) and to the entrance of ammonia into organic com- 

 bination; its possible role in deamination of other amino acids has 

 just been mentioned. 



Non-oxidative deamination of L-serine and L-threonine is effected 

 by serine dehydrase of Neurospora crassa (435, 618); the products are, 

 respectively, pyruvate and a-ketobutyrate. Another pyridoxal phos- 

 phate enzyme of N. crassa deaminates D-serine (617). Neither D-serine 

 nor D-threonine is attacked by the JV. crassa D-amino acid oxidase (47). 



Aspartase, converting aspartate non-oxidatively to fumaric acid and 

 ammonia, is also reported to be formed by Neurospora crassa (480) 

 and Penicillium notatum (547). 



Transamination results, of course, in the deamination of the donor 

 amino acid; the process is considered later, in connection with amino 

 acid biosynthesis. 



