UTILIZATION OF AMINO ACIDS AND AMIDES 263 



Catabolism of the carbon chain of amino acids by fungi can be 

 considered at present only on the basis of fragments of information, 

 many of which, however, are suggestive of breakdown pathways known 

 in other organisms. 



Tyrosine in at least some fungi and actinomycetes is attacked by 

 tyrosinase, a polyphenol oxidase (Chapter 6). The sequence of reac- 

 tions in Neurospora crassa (248) appears to be at least in general the 

 same as in mammalian systems (324): tyrosine is oxidized via 2,4-di- 

 hydroxyphenylalanine ("dopa") through a red hallochrome pigment 

 to an insoluble black pigment, a melanin. The pigment of Strepto- 

 myces scabies probably— although evidence is incomplete — is formed 

 by this same pathway (15, 133, 242, 243), and it is usually assumed 

 that black insoluble pigments as a group are products of tyrosinase 

 action. 



In mammalian liver, tyrosine is oxidized to acetoacetic acid and 

 fumaric acid by the following sequence of reactions (214): 



Tyrosine — > p-hydroxyphenylpyruvic acid -^ 

 2,5-dihydroxyphenylpyruvic acid — » homogentisic 



acid — » acetoacetate + fumarate (6) 



The last reaction — actually three reactions (300) — is not known in 

 fungi. Homogentisic acid can be isolated as a product of the tyrosine 

 metabolism of Aspergillus niger and Penicillium spp. (261, 298, 554); 

 inhibition data implicate p-hydroxyphenylpyruvate in the tyrosine 

 metabolism of Neurospora crassa (125). 



The isolation of tyramine from Aspergillus niger cultures (552) sug- 

 gests another type of tyrosine breakdown, by decarboxylation, as in 

 bacterial systems. 



Tryptophan catabolism in fungi has been studied principally in rela- 

 tion to nicotinic acid biogenesis (Chapter 10). Other pathways are 

 known in bacteria (14, 121, 214, 493) involving indole, catechol, 

 kynurenic acid, or 5-hydroxytryptophan. A preparation from Neuro- 

 spora crassa converts 3-hydroxykynurenine, an intermediate in nico- 

 tinic acid biosynthesis, to xanthurenic acid (353). 



Indoleacetic acid (auxin), a growth regulator for higher plants, is 

 produced by several fungi, including Aspergillus niger, Phy corny ces 

 nitens, and Rhizopus suinus, and by soil actinomycetes and bacteria 

 (72, 73, 236, 302, 394, 446, 448). Its obvious structural relation to 

 tryptophan and the effect of the amino acid in stimulating its forma- 

 tion by fungi indicate that the auxin is formed from tryptophan (537, 

 605). Production by Rhizopus suinus is increased by aeration (538). 

 Detailed enzymatic studies of the synthesis have not appeared, but it 



