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 K
3 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 K
3, 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 K
3. It was spectrophotometrically revealed that HOQNO inhibits the reoxidation of cytochrome bt by nitrate, but not the re-duction by formate plus vitamin K
3. 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.
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