Mechanistic information on the nitrite-controlled reduction of aquacob(III)alamin by ascorbate at physiological pH.

The interaction with nitric oxide (NO) is an important aspect of the biological activity of vitamin B12 (Cbl). Whereas the formation of nitroxylcobalamin (CblNO) via the binding of NO to reduced CblCo(II) has been studied in detail before, the possible intracellular formation of CblNO via reduction of nitrocobalamin (CblNO2) is still questionable. To study this further, spectroscopic and kinetic studies on the reaction of CblNO2 with the intracellular antioxidant ascorbic acid (Asc) were performed in aqueous solution at the physiological pH of 7.2. It was found that the redox pathway of this reaction requires anaerobic conditions as a result of the rapid re-oxidation of reduced CblCo(II). In the studied system, both CblOH2 and CblNO2 are reduced to CblCo(II) by ascorbate at pH 7.2, the CblOH2 complex being two orders of magnitude more reactive than CblNO2. Clear evidence for redox cycling between CblOH2/CblNO2 and CblCo(II) under aerobic conditions was observed as an induction period during which all oxygen was used prior to the formation of CblCo(II) in the presence of an excess of ascorbate. No evidence for the intermediate formation of CblNO or NO radicals during the reduction of CblNO2 could be found. Nitrocob(III)alamin can be reduced by ascorbic acid under physiological conditions. The products of the reaction are cob(II)alamin and nitrite ion. This reaction is ca. 200 times slower than the one involving aquacob(III)alamin.