The participation of cytochromes in the process of nitrate respiration in Klebsiella (Aerobacter) aerogenes

1. The participation of cytochromes in the membrane-bound, nitrate and oxygen respiratory systems of Klebsiella (Aerobacter) aerogenes has been investigated. The membrane preparations contained the NADH, succinate, lactate and formate oxidase systems, and in addition a high respiratory nitrate reductase activity.2. Difference spectra indicated the presence of cytochromes b, a₁, d, and o. Cytochromes of the c-type could not be detected in these membranes. Both cytochrome b content and respiratory nitrate reductase activity were the highest in bacteria grown anaerobically in the presence of nitrate.3. Cytochrome b was the only cytochrome which, after being reduced by NADH, could be partially reoxidized anaerobically in the presence of nitrate. Furthermore, nitrate caused a lower aerobic steady state reduction only of cytochrome b.4. NADH oxidase and NADH-linked respiratory nitrate reductase activities were both inhibited by antimycin A, 2-n-heptyl-4-hydroxyquinoline-N-oxide and KCN. NADH oxidase activity was selectively inhibited by CO, while azide was found to inhibit only the respiratory nitrate reductase. In the presence of azide, nitrate did not affect the level of reduction of cytochrome b.5. The evidence presented suggests that cytochrome b is a carrier in the electron transport systems to both nitrate and oxygen; from cytochrome b branching occurs, with one branch linked to the respiratory nitrate reductase and one branch linked to oxidase systems, containing the cytochromes a₁, d and o.