The energy‐conserving nitric‐oxide‐reductase system in Paracoccus denitrificans
Authors: CARR Grant J. , PAGE M. Dudley , FERGUSON Stuart J. 
Published: March 2005

1. A Clark‐type electrode that responds to nitric oxide has been used to show that cytoplasmic membrane vesicles of Paracoccus denitrificans have a nitric‐oxide reductase activity. Nitrous oxide is the reaction product. NADH, succinate or isoascorbate plus 2,3,5,6‐tetramethyl‐1,4‐phenylene diamine can act as reductants. The NADH‐dependent activity is resistant to freezing of the vesicles and thus the NADH: nitric‐oxide oxidoreductase activity of stored frozen vesicles provides a method for calibrating the electrode by titration of dissolved nitric oxide with NADH. The periplasmic nitrite reductase and nitrous‐oxide reductase enzymes are absent from the vesicles which indicates that nitric‐oxide reductase is a discrete enzyme associated with the denitrification process. This conclusion was supported by the finding that nitric‐oxide reductase activity was absent from both membranes prepared from aerobically grown P. denitrificans and bovine heart submitochondrial particles.

2. The NADH: nitric‐oxide oxidoreductase activity was inhibited by concentrations of antimycin or myxothiazol that were just sufficient to inhibit the cytochrome bc1 complex of the ubiquinol – cytochrome‐c oxidoreductase. The activity was deduced to be proton translocating by the observations of: (a) up to 3.5‐fold stimulation upon addition of an uncoupler; and (b) ATP synthesis with a P: 2e ratio of 0.75.

3. Nitrite reductase of cytochrome cd1 type was highly purified from P. denitrificans in a new, high‐yield, rapid two‐ or three‐step procedure. This enzyme catalysed stoichiometric synthesis of nitric oxide. This observation, taken together with the finding that the maximum rate of NADH: nitric‐oxide oxidoreductase activity catalysed by the vesicles was comparable with that of NADH: nitrate‐oxidoreductase, is consistent with a role for nitric‐oxide reductase in the physiological conversion of nitrate or nitrite to dinitrogen gas.

4. Intact cells of P. denitrificans also reduced nitric oxide in an antimycin‐ or myxothiazol‐sensitive manner. However, nitric oxide was not detected by the electrode during the reduction of nitrate. Nitric‐oxide synthesis from nitrate could be detected with cells in the presence of very low concentrations of Triton X‐100 which selectively inhibits nitric‐oxide reductase activity.

5. Nitric oxide was detected as an intermediate in denitrification by including haemoglobin with an anaerobic suspension of cells that was reducing nitrate. The characteristic spectrum of the nitric oxide derivative of haemoglobin was observed.

6. A role for nitric oxide as an intermediate was also indicated by the demonstration of oxidation of NADH by cytoplasmic membrane vesicles that were incubated under anaerobic conditions in the presence of intact cells of P. denitrificans and nitrite. Since such vesicles cannot reduce either nitrite or nitrous oxide and intact cells cannot oxidise exogenous NADH, the oxidation of NADH can only be attributed to the supply of nitric oxide to the vesicles following reduction of nitrite by the cells.

7. Explanations are considered for the previous view that nitric oxide is not an intermediate in denitrification, particularly in P. denitrificans.