TERMINAL RESPIRATION 231 



It is of some interest whether the cytochrome system is responsible 

 for the entire terminal respiration, or whether other systems may also 

 be important. Elsewhere (Chapter 6) it has been noted that a role 

 of the polyphenol oxidases or of quinone reductases is not yet estab- 

 lished. The other alternative is a flavin system, and this possibility 

 has received some attention. 



The cytochrome system is poisoned by cyanide, azide, and carbon 

 monoxide. In general, the aerobic respiration of intact cells of fungi 

 is also inhibited by these poisons (31, 108, 189, 194, 324). However, 

 both in Myrothecium verrucaria (71) and Ustilago sphaerogena (117) 

 cell respiration is not affected by cyanide or carbon monoxide at con- 

 centrations which inhibit cytochrome oxidase. This suggests — al- 

 though there are other possibilities — that some system other than the 

 cytochrome is operative in intact cells. Cells of Gelasinospora tetra- 

 sperma, on the other hand, are as sensitive to these poisons as is the 

 extracted oxidase (31); in this and in some other fungi (32, 57) the 

 activity of the cytochrome system is sufficient to explain mycelial 

 respiration. 



There is some possibility that slow-growing strains of Neurospora 

 crassa possess a terminal respiratory system other than the cytochromes. 

 The "poky" strain, the distinctive features of which are inherited 

 cytoplasmically, is relatively insensitive to cytochrome inhibitors, more 

 responsive than the normal strain to oxygen pressure, and has a high 

 content of flavin adenine dinucleotide (122, 301). A similar but gene- 

 determined strain lacks cytochrome oxidase activity and is insensitive 

 to azide and cyanide (300). Finally, although the "poky" character is 

 suppressed by a nuclear gene, the cytochrome system is not increased 

 along with the return to normal growth rate; this suggests that the 

 effect of the gene is to open up some pathway alternative to the 

 cytochrome route of terminal respiration (196). These observations 

 all point toward a terminal system involving flavin enzymes. 



Tamiya (291) summarizes the Japanese literature which suggests 

 that in Aspergillus oryzae the respiration of mycelium from sub- 

 merged (aerated or shaken) culture is mediated by the cytochromes, 

 that of mycelium from still culture by some other, possibly flavin, 

 system. This perhaps merits re-examination with the improved 

 methods now available, although there are some contradictions (313). 



Catalase, an iron-containing enzyme of the hydroperoxidase group 

 (296), is present in all fungi which have been examined for it. Catalyz- 

 ing the breakdown of hydrogen peroxide, the enzyme may serve only 

 to protect the organism against hydrogen peroxide or may have a 

 broader function in the oxidation of organic molecules (54, 157). The 



