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Kinetic studies on the reaction between dicyanocobinamide and hypochlorous acid.

Maitra D, Ali I, Abdulridha RM, Shaeib F, Khan SN, Saed GM, Pennathur S, Abu-Soud HM - PLoS ONE (2014)

Bottom Line: The formation of (OCl)(CN)-Cbi and its conversion to (OCl)2-Cbi was fitted to a first order rate equation with second order rate constants of 0.002 and 0.0002 µM(-1) s(-1), respectively.Plots of the apparent rate constants as a function of HOCl concentration for all the three steps were linear with Y-intercepts close to zero, indicating that HOCl binds in an irreversible one-step mechanism.Collectively, these results illustrate functional differences in the corrin ring environments toward binding of diatomic ligands.

View Article: PubMed Central - PubMed

Affiliation: Department of Obstetrics and Gynecology, The C.S. Mott Center for Human Growth and Development, Wayne State University School of Medicine, Detroit, MI, United States of America.

ABSTRACT
Hypochlorous acid (HOCl) is a potent oxidant generated by myeloperoxidase (MPO), which is an abundant enzyme used for defense against microbes. We examined the potential role of HOCl in corrin ring destruction and subsequent formation of cyanogen chloride (CNCl) from dicyanocobinamide ((CN)2-Cbi). Stopped-flow analysis revealed that the reaction consists of at least three observable steps, including at least two sequential transient intermediates prior to corrin ring destruction. The first two steps were attributed to sequential replacement of the two cyanide ligands with hypochlorite, while the third step was the destruction of the corrin ring. The formation of (OCl)(CN)-Cbi and its conversion to (OCl)2-Cbi was fitted to a first order rate equation with second order rate constants of 0.002 and 0.0002 µM(-1) s(-1), respectively. The significantly lower rate of the second step compared to the first suggests that the replacement of the first cyanide molecule by hypochlorite causes an alteration in the ligand trans effects changing the affinity and/or accessibility of Co toward hypochlorite. Plots of the apparent rate constants as a function of HOCl concentration for all the three steps were linear with Y-intercepts close to zero, indicating that HOCl binds in an irreversible one-step mechanism. Collectively, these results illustrate functional differences in the corrin ring environments toward binding of diatomic ligands.

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Structures of cyanocobalamin (Cbl) (left panel) and dicyanocobinamide ((CN)2-Cbi)) (right panel).
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pone-0110595-g001: Structures of cyanocobalamin (Cbl) (left panel) and dicyanocobinamide ((CN)2-Cbi)) (right panel).

Mentions: The structure of the Cbl and dicyanocobinamide ((CN)2-Cbi), a naturally occurring intermediate of vitamin B12 synthesis, are based on a corrin ring. In the corrin ring four of the six coordination sites of the cobalt (Co) atom are the pyrrole nitrogen atoms, which are provided by the corrin ring (Figure 1). In Cbl, the fifth position of the Co atom (the lower or α- axial ligand) is taken by one of the nitrogens of the 5,6- dimethylbenzimidazole group [9]. The other nitrogen of the 5,6-dimethylbenzimidazole is connected to a five-carbon sugar, which in turn links to a phosphate group, and then back to the corrin ring via one of the seven-amide groups at the periphery of the corrin ring. Finally, the sixth position (the upper/β-axial ligand, also called the site of reactivity, is occupied by a cyano group (-CN) (Figure 1A). Cobinamides (Cbi) are vitamin B12 derivatives that lack the dimethylbenzimidazole group at the α-axial ligand. The solubility, stability, and the CN- binding ability of cobalamin derivatives depend on the type of the β-axial ligand and the presence or lack of 5,6-dimethylbenzimidazole at the α-axial ligand [10]–[16] The presence of this group in Cbi causes the increased binding ability to CN- (100 times greater than Cbl), explaining the difference of Cbi as compared to Cbl in CN- detoxification of massive CN- poisoning [17], [18]. Dicyanocobinamide plays an important role, both in vitro and in intact cells, as a soluble guanylate cyclase (sGC) co-activator by targeting the N-terminal regulatory regions, an action which resembles the effect of forskolin on adenylyl cyclases [19]. It increases intracellular cGMP levels and displays vasorelaxant activity in phenylephrine-constricted rat aortic rings in an endothelium-independent manner. Both effects are synergistically potentiated by BAY41-2272, (an NO-independent soluble guanylate cyclase stimulator consisting of structurally diverse benzylindazole/pyrazolopyridine and acrylamide derivatives) [19]. Recently, it has been shown that (CN)2-Cbi and related vitamin B12 derivatives may serve not only as nitric oxide (NO) scavengers [20], but also as NO synthase (NOS) inhibitors [21]. They have also been shown to serve as potent inhibitors of HIV-1 integrase and may prove useful as anti-viral treatments [22].


Kinetic studies on the reaction between dicyanocobinamide and hypochlorous acid.

Maitra D, Ali I, Abdulridha RM, Shaeib F, Khan SN, Saed GM, Pennathur S, Abu-Soud HM - PLoS ONE (2014)

Structures of cyanocobalamin (Cbl) (left panel) and dicyanocobinamide ((CN)2-Cbi)) (right panel).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4222763&req=5

pone-0110595-g001: Structures of cyanocobalamin (Cbl) (left panel) and dicyanocobinamide ((CN)2-Cbi)) (right panel).
Mentions: The structure of the Cbl and dicyanocobinamide ((CN)2-Cbi), a naturally occurring intermediate of vitamin B12 synthesis, are based on a corrin ring. In the corrin ring four of the six coordination sites of the cobalt (Co) atom are the pyrrole nitrogen atoms, which are provided by the corrin ring (Figure 1). In Cbl, the fifth position of the Co atom (the lower or α- axial ligand) is taken by one of the nitrogens of the 5,6- dimethylbenzimidazole group [9]. The other nitrogen of the 5,6-dimethylbenzimidazole is connected to a five-carbon sugar, which in turn links to a phosphate group, and then back to the corrin ring via one of the seven-amide groups at the periphery of the corrin ring. Finally, the sixth position (the upper/β-axial ligand, also called the site of reactivity, is occupied by a cyano group (-CN) (Figure 1A). Cobinamides (Cbi) are vitamin B12 derivatives that lack the dimethylbenzimidazole group at the α-axial ligand. The solubility, stability, and the CN- binding ability of cobalamin derivatives depend on the type of the β-axial ligand and the presence or lack of 5,6-dimethylbenzimidazole at the α-axial ligand [10]–[16] The presence of this group in Cbi causes the increased binding ability to CN- (100 times greater than Cbl), explaining the difference of Cbi as compared to Cbl in CN- detoxification of massive CN- poisoning [17], [18]. Dicyanocobinamide plays an important role, both in vitro and in intact cells, as a soluble guanylate cyclase (sGC) co-activator by targeting the N-terminal regulatory regions, an action which resembles the effect of forskolin on adenylyl cyclases [19]. It increases intracellular cGMP levels and displays vasorelaxant activity in phenylephrine-constricted rat aortic rings in an endothelium-independent manner. Both effects are synergistically potentiated by BAY41-2272, (an NO-independent soluble guanylate cyclase stimulator consisting of structurally diverse benzylindazole/pyrazolopyridine and acrylamide derivatives) [19]. Recently, it has been shown that (CN)2-Cbi and related vitamin B12 derivatives may serve not only as nitric oxide (NO) scavengers [20], but also as NO synthase (NOS) inhibitors [21]. They have also been shown to serve as potent inhibitors of HIV-1 integrase and may prove useful as anti-viral treatments [22].

Bottom Line: The formation of (OCl)(CN)-Cbi and its conversion to (OCl)2-Cbi was fitted to a first order rate equation with second order rate constants of 0.002 and 0.0002 µM(-1) s(-1), respectively.Plots of the apparent rate constants as a function of HOCl concentration for all the three steps were linear with Y-intercepts close to zero, indicating that HOCl binds in an irreversible one-step mechanism.Collectively, these results illustrate functional differences in the corrin ring environments toward binding of diatomic ligands.

View Article: PubMed Central - PubMed

Affiliation: Department of Obstetrics and Gynecology, The C.S. Mott Center for Human Growth and Development, Wayne State University School of Medicine, Detroit, MI, United States of America.

ABSTRACT
Hypochlorous acid (HOCl) is a potent oxidant generated by myeloperoxidase (MPO), which is an abundant enzyme used for defense against microbes. We examined the potential role of HOCl in corrin ring destruction and subsequent formation of cyanogen chloride (CNCl) from dicyanocobinamide ((CN)2-Cbi). Stopped-flow analysis revealed that the reaction consists of at least three observable steps, including at least two sequential transient intermediates prior to corrin ring destruction. The first two steps were attributed to sequential replacement of the two cyanide ligands with hypochlorite, while the third step was the destruction of the corrin ring. The formation of (OCl)(CN)-Cbi and its conversion to (OCl)2-Cbi was fitted to a first order rate equation with second order rate constants of 0.002 and 0.0002 µM(-1) s(-1), respectively. The significantly lower rate of the second step compared to the first suggests that the replacement of the first cyanide molecule by hypochlorite causes an alteration in the ligand trans effects changing the affinity and/or accessibility of Co toward hypochlorite. Plots of the apparent rate constants as a function of HOCl concentration for all the three steps were linear with Y-intercepts close to zero, indicating that HOCl binds in an irreversible one-step mechanism. Collectively, these results illustrate functional differences in the corrin ring environments toward binding of diatomic ligands.

Show MeSH