Nitric oxide interactions with cobalamins: biochemical and functional consequences

Abstract

Nitric oxide (NO) is a paramagnetic gas that has been implicated in a wide range of biologic functions. The common pathway to evoke the functional response frequently involves the formation of an iron- nitrosyl complex in a target (heme) protein. In this study, we report on the interactions between NO and cobalt-containing vitamin B12 derivatives. Absorption spectroscopy showed that of the four Co(III) derivatives (cyanocobalamin [CN-Cbl], aquocobalamin [H2O-Cbl], adenosylcobalamin [Ado-Cbl], and methylcobalamin [MeCbl]), only the H2O- Cbl combined with NO. In addition, electron paramagnetic resonance spectroscopy of H2O-Cbl preparations showed the presence of a small amount of Cob-(II)alamin that was capable of combining with NO. The Co(III)-NO complex was very stable, but could transfer its NO moiety to hemoglobin (Hb). The transfer was accompanied by a reduction of the Co(III) to Co(II), indicating that NO+ (nitrosonium) was the leaving group. In accordance with this, the NO did not combine with the Hb Fe(II)-heme, but most likely with the Hb cysteine-thiolate. Similarly, the Co(III)-NO complex was capable of transferring its NO to glutathione. Ado-Cbl and Me-Cbl were susceptible to photolysis, but CN- Cbl and H2O-Cbl were not. The homolytic cleavage of the Co(III)-Ado or Co(III)-Me bond resulted in the reduction of the metal. When photolysis was performed in the presence of NO, formation of NO-Co(II) was observed. Co(II)-nitrosyl oxidized slowly to form Co(III)-nitrosyl. The capability of aquocobalamin to combine with NO had functional consequences. We found that nitrosylcobalamin had diminished ability to serve as a cofactor for the enzyme methionine synthase, and that aquocobalamin could quench NO-mediated inhibition of cell proliferation. Our in vitro studies therefore suggest that interactions between NO and cobalamins may have important consequences in vivo.

Biological nitric oxide signalling: chemistry and terminology

“…In contrast to sGC, nitrosylation of most enzymes leads to regulation of function by reducing activity. For example, nitrosylation of the Co(II)(H 2 O) form of cobalamin (vitamin B 12 derivative) results in a diminished ability for this complex to serve as a cofactor for methionine synthase (Brouwer et al ., ).…”

Nitroxylcob(III)alamin: Synthesis and X‐ray Structural Characterization

“…Mammalian vitamin B 12 ‐dependent enzyme reactions, methylcobalamin‐dependent methionine synthase, and adenosylcobalamin‐dependent methylmalonyl‐CoA mutase are inhibited by nitric oxide or nitric oxide donors2 in vitro and in vivo 3. 4 Importantly, it is postulated that the inhibition of these enzymes is a consequence of NOCbl formation 5. Evidence supporting formation of an NO derivative of vitamin B 12 includes observations that hydroxocobalamin suppresses NO‐induced relaxation of smooth muscle in rodents,6 NO‐induced vasodilation,7 and NO‐mediated inhibition of cell proliferation 5.…”

“…4 Importantly, it is postulated that the inhibition of these enzymes is a consequence of NOCbl formation 5. Evidence supporting formation of an NO derivative of vitamin B 12 includes observations that hydroxocobalamin suppresses NO‐induced relaxation of smooth muscle in rodents,6 NO‐induced vasodilation,7 and NO‐mediated inhibition of cell proliferation 5. Vitamin B 12 can also reverse NO‐induced neural tube defects 8.…”

High‐Dose Vitamin B12 in Vasodilatory Shock: A Narrative Review

“…Specifically, NO oxidizes the cobalt (Co) atom in hydroxocobalamin . The Co‐NO complex could transfer its NO to hemoglobin, reducing the circulating NO level . Less circulating NO and H 2 S may stabilize the capillary membrane and restore vascular tone .…”