Studies on Nitrate Reductase. VII. Reinvestigation on the Identity of the Enzyme with Cytochrome b

Abstract 1. It has been found that thiourea has a marked inhibitory effect on the nitrate reduction by E. coli, but dehydrogenase activity and the aerobic respiration of the same organism are not considerably influenced by this compound. 2. Spectroscopically it has been further clarified that thiourea completely prevents the anaerobic oxidation of the ferrocytochrome b by means of nitrate, whereas the same reagent can not inhibit the aerobic oxidation of the cytochrome. Cyanide prevents both of the oxidation processes. 3. On the basis of these findings the previously suggested identity of nitrate reductase with cytochrome b of E. coli type has been criticized and it has been concluded that the cytochrome b is not identical with nitrate reductase itself, though it is closely associated with the nitrate-reducing mechanism of E. coli. 4. A scheme has been proposed which can account for both the nitrate-reducing and the respiratory mechanisms of E. coli. 5. The chemical nature of nitrate reductase has been discussed and the iron-protein nature of this enzyme has been again emphasized. Addendum I : In a review quoting a previous paper of this series, Wurmser wrote that in the nitrate reduction nitrogen replaces oxygen for the Slater’s respiratory scheme. In spite of the validity of this statement, it should be considered that nitrate, but not nitrogen, replaces it. Addendum II: After this paper had been submitted to the editors, we were informed of the criticism by G. N. Cohen against our earlier hypothesis on the identity of nitrate reductase with the cytochrome b. The nitratereducing mechanism of strict anaerobes and its differences from that of E. coli have been investigated by Mr. Tadao Katsura at the Institute of Infectious Diseases of Tokyo University in connection with us. This material will be published shortly.