Reactions of Cytochrome b₁ and Nitrate Reductase in a Preparation Solubilized from Escherichia coli

抄録

The reactions of cytochrome b₁ and nitrate reductase in an enzyme preparation solubiliz-ed and partially purified from the particulate fraction of Escherichia coli were investigated. The cytochrome b₁ reduced by the addition of formate and vitamin K₃ was found to be reoxidizable by oxygen, nitrate and chlorate. While the oxidation by oxygen was insensitive to cyanide and azide (the autoxidizability of the cytochrome), these reagents strongly inhi-bited the reaction by nitrate and chlorate suggesting the intervention of nitrate reduc-tase. With chlorate as the oxidant, the de-formation of the absorption spectrum of the cytochrome was observed. The nitrate reduc-tion by formate in the solubilized system was found to require the addition of vitamin K₃, but flavins had no effects on this reaction. Since the vitamin is required also for the
reduction of cytochrome b₁ in this prepara-tion, the involvement of the cytochrome in the nitrate-reducing mechanism was further substantiated. The formate-nitrate reaction was strongly inhibited by 2-heptyl-4-hydroxy-quinoline-N-oxide (HOQNO), but this inhibi-tion was not competitive with respect to vitamin K₃. It was spectrophotometrically revealed that HOQNO inhibits the reoxidation of cytochrome bt by nitrate, but not the re-duction by formate plus vitamin K₃. In the particulate fraction, the formate-nitrate reduc-tase activity was inhibited much more remark-ably than the formate oxidase activity. Some of the implications of these results are dis-cussed.
We wish to thank Dr. S. Taniguchi for a gener-ous gift of 2-heptyl-4-hydoxyquinoline-N-oxide.