Mechanistic Studies on the Reaction of Nitroxylcobalamin with Dioxygen: Evidence for Formation of a Peroxynitritocob(III)alamin Intermediate

Abstract

Studies by others suggest that the reduced vitamin B12 complex, cob(II)alamin, scavenges nitric oxide to form air-sensitive nitroxylcobalamin (NO(-)-Cbl(III); NOCbl) in vivo. The fate of newly formed NOCbl is not known. A detailed mechanistic investigation of the oxidation of NOCbl by oxygen is presented. Only base-on NOCbl reacts with O2, and the reaction proceeds via an associative mechanism involving a peroxynitritocob(III)alamin intermediate, Co(III)-N(O)OO(-). The intermediate undergoes O-O bond homolysis and ligand isomerization to ultimately yield NO2Cbl and H2OCbl(+)/HOCbl, respectively. Ligand isomerization may potentially occur independent of O-O bond homolysis. Formation of (•)OH and (•)NO2 intermediates from O-O bond homolysis is demonstrated using phenol and tyrosine radical traps and the characterization of small amounts of a corrinoid product with minor modifications to the corrin ring.

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

“…But at the same time, oxygen will be reduced to form superoxide, peroxide and OH · , which are stronger oxidants and can account for more redox cycling to occur during which ascorbic acid will be depleted, reactions ( 10 )–( 12 ). Finally, CblNO can also react with dioxygen to form CblNO 2 and CblOH 2 in a complex process involving OH · and · NO 2 intermediates [ 15 ]. …”

“…However, the oxidation of CblNO is not just a simple reaction [ 14 ]. According to Brasch et al, the mechanism of CblNO oxidation under certain conditions is rather complex, with multiple products that can be formed [ 15 ]. Oxidation of CblNO is a reversible process in which CblNO 2 can be reduced by strong reducing agents to reform CblNO.…”

Accumulation of NO2‐cobalamin in nutrient‐stressed ammonia‐oxidizing archaea and in the oxygen deficient zone of the eastern tropical North Pacific

“…Gaseous NO is an intermediate of ammonia oxidation in both AOA and anammox bacteria (Kartal et al, 2011;Martens-Habbena et al, 2015;Kozlowski et al, 2016) and may diffuse into the cell to react with the reduced cobalamin that forms during cellular processing, forming nitrosylcobalamin (NO-cobalamin). This rapidly formed NO-cobalamin is readily oxidized to NO 2cobalamin after exposure to O 2 (within cells or during sample processing) (Subedi and Brasch, 2013).…”

Studies on the Reaction of Reduced Vitamin B12 Derivatives with the Nitrosyl ­Hydride (HNO) Donor Angeli's Salt: HNO Oxidizes the Transition‐Metal Center of Cob(I)alamin

“…Stock AS solutions were prepared by dissolving solid AS in NaOH (0.01 M ). Stock solutions were stored in the freezer (–24 °C) inside the glove box and used within 24 h. The major reaction product in the reaction of Cbl(II) with AS under different pH conditions is the extremely air‐sensitive nitroxylcobalamin (NOCbl) 43…”