Electron Transfer in Nitrate and Sulphate Respiration 



395 



fraction was found to be devoid of not only dehydrogenase activity (using 

 methylene blue (Mb) for assay) for lactate, succinate, />-hydroxybutyrate and 

 glucose, but also hydrogenase (production of H., from reduced methyl- 

 viologen, MVH), and of reduced tri- and di-phosphopyridine nucleotide 

 (TPNH and DPNH) oxidase activity. Contrary to the very high activity of 



+0.3r 432 



[Reduced] formate " Coxidized] 



10 min.- preincubation 



[Reduced]-[oxidized] 

 formaie+KN03 (IOMmoles,solid) 

 after 1 minute 



^0.l 



-0.1'- 412 



Fig. 1 . Difference spectra of particulate preparation. Tlie solution in Thunberg 



tube-type cuvette contained large particles (0-6 mg N/ml), phosphate buffer 



50/imoles, pH 7-1 and formate 50/imoles in 3-0 ml under anaerobic conditions 



at 30°C. 



formate dehydrogenase and oxidation of formate by nitrate, aerobic oxida- 

 tion of formate was found to be weak. Thus, the particle seems to be highly 

 characterized by the well-developed formate dehydrogenase — cytochrome b^ 

 — nitrate reductase system, i.e. the enzyme system of nitrate respkation type. 

 Nitrate reduction by the particulate nitrate reductase with various electron 

 donors is summarized in Table 3. 



Cytochrome Zj^ in the particulate fraction was rapidly reduced with DPNH 

 or even more rapidly with formate under anaerobic conditions. TPNH was 

 quite inactive as an electron donor. Cytochrome b^ thus reduced was rapidly 

 reoxidized with nitrate (Fig. 1). 



In this case nitrate reduction was strongly inhibited by 2-heptyl-4-hydroxy- 

 quinoline-N-oxide (HOQNO), a specific inliibitor for cytochrome b^. On 

 the contrary, nitrate reduction by reduced dyes such as MVH was not 

 inhibited by HOQNO (Table 4). 



