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

摘要

The interaction with nitric oxide (NO) is an important aspect of the biological activity of vitamin B-12 (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 (CblNO(2)) is still questionable. To study this further, spectroscopic and kinetic studies on the reaction of CblNO(2) 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 CblOH(2) and CblNO(2) are reduced to CblCo(II) by ascorbate at pH 7.2, the CblOH(2) complex being two orders of magnitude more reactive than CblNO(2). Clear evidence for redox cycling between CblOH(2)/CblNO(2) 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 CblNO(2) could be found.