Autoinhibition by Iodide Ion in the Methionine-Iodine Reaction.

Li Xu,Attila K Horváth,György Csekő

doi:10.1021/acs.jpca.0c04271

Li Xu, Attila K Horváth + Show 1 more

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https://doi.org/10.1021/acs.jpca.0c04271

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Abstract

The methionine–iodine reaction was reinvestigated spectrophotometrically in detail monitoring the absorbance belonging to the isosbestic point of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered acidic medium. The stoichiometric ratio of the reactants was determined to be 1:1 producing methionine sulfoxide as the lone sulfur-containing product. The direct reaction between methionine and iodine was found to be relatively rapid in the absence of initially added iodide ion, and it can conveniently be followed by the stopped-flow technique. Reduction of iodine eventually leads to the formation of iodide ion that inhibits the reaction making the whole system autoinhibitory with respect to the halide ion. We have also shown that this inhibitory effect appears quite prominently, and addition of iodide ion in the millimole concentration range may result in a rate law where the formal kinetic order of this species becomes −2. In contrast to this, hydrogen ion has just a mildly inhibitory effect giving rise to the fact that iodine is the kinetically active species in the system but not hypoiodous acid. The surprisingly complex kinetics of this simple reaction may readily be interpreted via the initiating rapidly established iodonium-transfer process between the reactants followed by the subsequent hydrolytic decomposition of the short-lived iodinated methionine. A seven-step kinetic model to be able to describe the most important characteristics of the measured kinetic curves is established and discussed in detail.

Highlights

Sulfur-containing amino acids are important sources of sulfur nutrients in humans and mammals[1] and play an important role in various biochemical processes such as transformation and synthesis of proteins,[2] immunity,[3,4] as well as oxidative stress resistance.[5−7] It is generally considered that redox reaction balance of many sulfur-containing amino acids is closely associated with cardiovascular disease, neuropsychiatric disorder, neurogenic diseases, renalischemia, liver failure, diabetes, cancer, and aging.[1,6,8−12] The sulfurcontaining amino acids supplementation may readily scavenge biological reactive oxidants in vivo through their redox reactions to protect the stability and activity of other essential amino acids in proteins reducing the damage to cell membrane and DNA caused by oxidative stress and, in turn, incidence of these diseases
It is found that Met in proteins is highly susceptible to oxidation under oxidative stress.[15−19] As an antioxidant defense system, Met oxidation participates in regulation of protein stability[20] and cellular viability[21] through its own oxidation: Met is oxidized to form methionine sulfoxide(MetSO) by adding oxygen to its sulfur atom in the physiological environment
Any reliable kinetic studies should begin with unraveling the stoichiometry of the given process at the experimental condition studied

Summary

Introduction

Sulfur-containing amino acids are important sources of sulfur nutrients in humans and mammals[1] and play an important role in various biochemical processes such as transformation and synthesis of proteins,[2] immunity,[3,4] as well as oxidative stress resistance.[5−7] It is generally considered that redox reaction balance of many sulfur-containing amino acids is closely associated with cardiovascular disease, neuropsychiatric disorder, neurogenic diseases, renalischemia, liver failure, diabetes, cancer, and aging.[1,6,8−12] The sulfurcontaining amino acids supplementation may readily scavenge biological reactive oxidants in vivo through their redox reactions to protect the stability and activity of other essential amino acids in proteins reducing the damage to cell membrane and DNA caused by oxidative stress and, in turn, incidence of these diseases.Methionine (Met), as one of two essential sulfur-containing amino acids, may be obtained only from the human diet. It is found that Met in proteins is highly susceptible to oxidation under oxidative stress.[15−19] As an antioxidant defense system, Met oxidation participates in regulation of protein stability[20] and cellular viability[21] through its own oxidation: Met is oxidized to form methionine sulfoxide(MetSO) by adding oxygen to its sulfur atom in the physiological environment. In order to understand the relationship between its oxidation process and biological regulation function, it is crucial to perform further investigations on the mechanism of methionine oxidation

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