Reductive nitrosylation of ferric cyanide horse heart myoglobin is limited by cyanide dissociation.

The research article investigates a method to measure cyanide’s dissociation rate constant, k(off), from ferric heme-proteins using ferric myoglobin and the irreversible reductive nitrosylation reaction.

Objective of the Research

• The aim of the research was to establish a technique for estimating the dissociation rate constant (k(off)) of cyanide from ferric heme-proteins. This is crucial as cyanide is known to bind to ferric heme-proteins with high affinity, translating to a very low k(off) value.

Methodology and Experimentation

• The researchers used an irreversible reductive nitrosylation reaction as a means to measure the k(off), focusing primarily on ferric myoglobin (Mb(III)).
• The k(off) for cyanide dissociation from ferric cyanide horse heart myoglobin (Mb(III)-cyanide) was determined at a pH level of 9.2 at a temperature of 20.0 degrees Celsius.
• The researchers introduced nitric oxide (NO) solutions into the Mb(III)-cyanide mix, which led to absorption spectral changes that indicated the disappearance of Mb(III)-cyanide and the formation of ferrous nitrosylated Myoglobin.

Results and Findings

• The research found that the kinetics of reductive nitrosylation of Mb(III) is much quicker than the dissociation of Mb(III)-cyanide. Thus, the k(off) value, which symbolises the rate-limiting step, could be determined directly.
• The experimentally derived k(off) value closely corresponds with the one calculated from the second-order rate constant (k(on)) and the dissociation equilibrium constant (K) for cyanide binding to Mb(III).

Study Implications

• The results of this study can provide valuable insights into the kinetics of cyanide dissociation from ferric heme-proteins and help improve our understanding of these chemical processes. Being able to estimate the k(off) value could also influence the development of better designed strategies aimed at counteracting cyanide poisoning.