24 



INTERMEDIARY METABOLISM AND GROWTH 



cytochrome b and c in the heart muscle succinoxidase system, markedly reduces the reduction 

 of nitrate by hydrogen donors. Instead of cytochrome b, as in E. coli, cytochrome c is 

 oxidized during the anaerobic reduction of nitrate in AI. denitrificans. Cytochrome aj may 

 also be involved in the electron transport system in this organism. 



Other microorganisms, such as Neurospora, nitrate adapted Bacillus subtilus, and Azotobacter 

 I'inelandi do not show changes in the absorption spectra of the cytochrome bands during 

 the course of nitrate reduction. These latter organisms are however all strongly aerobic. 

 A third group of microorganisms which utilize nitrate in the electron transport system 

 may be distinguished. These are the obligate anaerobes, such as Clostridium welchii, 

 which lack cytochrome pigments. 



Cytochrome pigments have also been iinplicated in bacterial nitrite reduction. A nitrite 

 reductase has been purified (\'erhoeven and Takeda, 1956) from Pseudomonas aeruginosa. 



In the presence of nitrite and the purified enzyme, bacterial cytochrome c was oxidized. 

 In the obligate anaerobic svilfate bacteria, sulfate may be utilized in place of O2 as an 

 electron and hydrogen acceptor with the production of H2S. Reduced cytochrome c is 

 oxidized by sulfate in the sulfate reducing bacteria. 



7. Qidnone reductase and plant respiration 



Various benzoquinone and napthoquinone derivatives are widely distributed in 

 nature. Oxidases for the phenolic forms of these compounds are also known 

 (polyphenol oxidase, tyrosinase, laccase). Copper rather thaii iron undergoes 

 valency changes in the course of the oxidations catalyzed by the phenol oxidases. 

 It has been suggested that these enzymes function as terminal respiratory catalysts 

 in plants (Mason, 1955). 



In the presence of quinone reductases of animal or plant tissues, pyridine 

 nucleotides can reduce quinones (Kertesz and Albano, 1955; Wosilait et al., 1954; 

 Wosilait and Nason, 1954a, 1954b), (Fig. 10). 



Hydroquinone has been used as an intermediary carrier in coupling experiments 

 with laccase while 1,2-naphthoquinone has been similarly used with polyphenol 

 oxidase (Wosilait et at., 1954). DPNH is rapidly oxidized by the enzymatically 

 produced oxidation products of hydroc|uinone or catechol. 



OH OH 



1) 2 DPNH + 2 



Quinone 



2DPN ■t-2 



OH 



2) 5O2 + 



OH 



H,0 



OH 



3) 2 



OH 



+ O2 



Polyphenol 

 oxidase - 4Cu^ 



+ 2 HjO + 4 Cu"*"— enzyme 

 Fig. 10. Quinone mediated oxidation of DPN^. 



