Can nitrocobalamin be reduced by ascorbic acid to nitroxylcobalamin? Some surprising mechanistic findings.

Despite detailed studies on nitroxylcobalamin (CblNO) formation, the possible intracellular generation of CblNO via reduction of nitrocobalamin (CblNO) remains questionable. To study this further, spectroscopic studies on the reaction of CblNO with the intracellular antioxidant ascorbic acid (HAsc) were performed in aqueous solution at pH < 5.0. It was found that nitroxylcobalamin is the final product of this interaction, which is not just a simple reaction but a rather complex chemical process. We clearly show that an excess of nitrite suppresses the formation of CblNO, from which it follows that ascorbic acid cannot reduce coordinated nitrite. We propose that under the influence of ascorbic acid, nitrocobalamin is reduced to Cbl(II) and nitric oxide (NO), which can subsequently react rapidly to form CblNO. It was further shown that this system requires anaerobic conditions as a result of the rapid oxidation of both Cbl(II) and CblNO.