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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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Cyanocobalamin and (CN)2-Cbi destruction mediated by HOCl causes the liberation of CNCl.Equal concentrations of Cbl and (CN)2-Cbi (110 µM) were treated with 50-fold molar excess of HOCl and CNCl generation were assayed colorimetrically as detailed under Materials and methods. The data are representative of three independent experiments with the error bars representing the standard error measurements.
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pone-0110595-g006: Cyanocobalamin and (CN)2-Cbi destruction mediated by HOCl causes the liberation of CNCl.Equal concentrations of Cbl and (CN)2-Cbi (110 µM) were treated with 50-fold molar excess of HOCl and CNCl generation were assayed colorimetrically as detailed under Materials and methods. The data are representative of three independent experiments with the error bars representing the standard error measurements.

Mentions: We next investigated whether the CN- released from the (CN)2-Cbi molecule reacts with the excess HOCl in the reaction mixture to form cyanogen chloride (CNCl). Dicyanocobinamide (110 µM) was treated with 50-fold molar excess of HOCl and the accumulation of CNCl was measured using the pyridine-1,3 dimethyl barbituric acid colorimetric assay as previously reported. As shown in Figure 6, the amount of CNCl generated from the reaction of (CN)2-Cbi with HOCl was approximately 1.8 fold the amount generated from the same reaction of Cbl with HOCl. Thus, the destruction of (CN)2-Cbi mediated by HOCl is more toxic than that of Cbl.


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)

Cyanocobalamin and (CN)2-Cbi destruction mediated by HOCl causes the liberation of CNCl.Equal concentrations of Cbl and (CN)2-Cbi (110 µM) were treated with 50-fold molar excess of HOCl and CNCl generation were assayed colorimetrically as detailed under Materials and methods. The data are representative of three independent experiments with the error bars representing the standard error measurements.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0110595-g006: Cyanocobalamin and (CN)2-Cbi destruction mediated by HOCl causes the liberation of CNCl.Equal concentrations of Cbl and (CN)2-Cbi (110 µM) were treated with 50-fold molar excess of HOCl and CNCl generation were assayed colorimetrically as detailed under Materials and methods. The data are representative of three independent experiments with the error bars representing the standard error measurements.
Mentions: We next investigated whether the CN- released from the (CN)2-Cbi molecule reacts with the excess HOCl in the reaction mixture to form cyanogen chloride (CNCl). Dicyanocobinamide (110 µM) was treated with 50-fold molar excess of HOCl and the accumulation of CNCl was measured using the pyridine-1,3 dimethyl barbituric acid colorimetric assay as previously reported. As shown in Figure 6, the amount of CNCl generated from the reaction of (CN)2-Cbi with HOCl was approximately 1.8 fold the amount generated from the same reaction of Cbl with HOCl. Thus, the destruction of (CN)2-Cbi mediated by HOCl is more toxic than that of Cbl.

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