Reactions of Nitric Oxide with Vitamin B₁₂and Its Precursor, Cobinamide

Abstract

Despite early claims that nitric oxide does not react with cobalamin under any circumstances, it is now accepted that NO has a high affinity for cobalamin in the 2+ oxidation state [Cbl(II)]. However, it is still the consensus that NO does not react with Cbl(III). We confirmed that NO coordinates to Cbl(II) at all pH values and that Cbl(III) does not react with NO at neutral pH. At low pH, however, Cbl(III) does react with NO by way of a two-step process that also reduces Cbl(III) to Cbl(II). To account for the pH dependence, and because of its intrinsic interest, we also studied reactions of NO with cobinamide [Cbi] in the 2+ and 3+ oxidation states. Both Cbi(II) and Cbi(III) react readily with NO at all pH values. Again, Cbi(III) is reduced during the process of coordinating NO. Compared to cobalamin, cobinamide lacks the tethered 5,6-dimethylbenzamidazolyl moiety bound to the cobalt ion. It may, therefore, be considered a "base-off" form of Cbl. To explain the reaction of Cbl(III) at low pH, we infer that the base-off form of Cbl(III) exists in trace amounts that are rapidly reduced to Cbl(II), which then binds NO efficiently. Base dissociation, we postulate, is the rate-limiting step. Interestingly, Cbi(II) has 100 times greater affinity for NO than does Cbl(II), proving that there is a strong trans effect due to the tethered base in nitrosyl derivatives of both Cbl(II) and Cbl(III). The affinity of Cbi(II) for NO is so high that it is a very efficient NO trap and, consequently, may have important biomedical uses.

Nitric Oxide Inhibits Mammalian Methylmalonyl-CoA Mutase

“…Because succinic acid is a completely symmetrical molecule, the radioactive labels from [1,[4][5][6][7][8][9][10][11][12][13][14] C]succinic acid will be incorporated into amino acids similarly to the radioactive label from [1][2][3][4][5][6][7][8][9][10][11][12][13][14] C]propionic acid, allowing the former compound to control for effects of the experimental conditions on steps distal to the methylmalonyl-CoA mutase reaction. To control for potential changes by the experimental conditions on propionyl-CoA conversion to methylmalonyl-CoA, we performed some experiments in which we incubated cells with [methyl- 14 C]methylmalonic acid. Although the radioactive label will be in carbon 2 of succinic acid rather than in carbons 1 or 4, this should not affect incorporation into amino acids because the full 4-carbon skeleton of succinic acid is converted to aspartate, the major amino acid derived from succinate.…”

“…First, the reactivity of NO with Cbl(II) is much greater than the reactivity of oxygen with Cbl(II), with removal of NO from cobalamin requiring the use of an NO trap, whereas oxygen can be removed from cobalamin simply by passing argon over the complex (14,53). In this respect, Cbl(II) is similar to heme that has a much higher affinity for NO than for oxygen (54).…”

“…Therefore, at low concentrations (i.e. Ͻ10 M), NO might not be able to compete with such a high reaction rate because its rate of reaction with Cbl(II) is somewhat lower at 0.5 ϫ 10 8 M Ϫ1 s Ϫ1 (14). Similarly, for NO to react with the deoxyadenosyl radical, it has to compete with cyclization of the radical to 8,5Ј-anhydro-5-deoxyadenosine, the predominant species formed in the absence of NO (42,43).…”

Gestation linked radical oxygen species fluxes and vitamins and trace mineral deficiencies in the ruminant

“…The Co content of the conserved forage grown and harvested after periods of heavy rains can also be decreased under such circumstances. In addition, direct loss of vitamins and mineral trace elements from the organism of the animals can occur during exposure to an external stress (such as for example, a climatic stress), since MS-Co(II), an inactivated form of vitamin B12 produced by oxidation of MS-Co(I) in the course of enzymatic methionine synthesis, can be oxidized by ROS [110,115]. Indeed, inactivation of vitamin B12 by • NO [115] has been reported and is involved in pernicious anemia in humans, after disturbing both methionine synthesis and carbon flow through the folate pathway [116].…”

Comparison of cobinamide to hydroxocobalamin in reversing cyanide physiologic effects in rabbits using diffuse optical spectroscopy monitoring

“…Cobinamide is a novel agent we are investigating for treating cyanide poisoning 27-29 that appears to have some advantages over current treatments, including hydroxocobalamin. Cobinamide is the penultimate precursor in the biosynthesis of cobalamin, lacking the dimethyl-benzimidazole ribonucleotide tail coordinated to the cobalt atom in the lower axial position ͑Fig.…”