L. A. Syrtsova

Exchange of cysteamine, thiol ligand in binuclear cationic tetranitrosyl iron complex, for glutathione

This paper describes the comparative study of the decomposition of two iron nitrosyl complexes (NICs) with a cysteamine thiolate ligand {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O (I) and a glutathione (GSH)-ligand, [Fe2(SC10H17N3O6)2(NO)4]SO4·2H2O (II), which spontaneously evolve NO in aqueous medium. NO formation was measured by using a spectrophotometric method by the formation of a hemoglobin (Hb)–NO complex. Spectrophotometry and mass-spectrometry methods have firmly shown that the cysteamine ligands are exchanged for 2 GS− during decomposition of 1.5 × 10−4 M (I) in the presence of 10−3 M GSH, with 77% yield at 68 h. As has been established, such behaviour is caused by the resistance of (II) to decomposition due to the higher affinity of iron towards GSH in the complex. The discovered reaction may impede S-glutathionation of the essential enzyme systems the presence of (I) and is important for metabolism of NICs, connected with their anti-tumor activity.

Reversible Dissociation and Ligand-Glutathione Exchange Reaction in Binuclear Cationic Tetranitrosyl Iron Complex with Penicillamine

This paper describes a comparative study of the decomposition of two nitrosyl iron complexes (NICs) with penicillamine thiolic ligands [Fe2(SC5H11NO2)2(NO)4]SO4 ·5H2O (I) and glutathione- (GSH-) ligands [Fe2(SC10H17N3O6)2(NO)4]SO4 ·2H2O (II), which spontaneously evolve to NO in aqueous medium. NO formation was measured by a sensor electrode and by spectrophotometric methods by measuring the formation of a hemoglobin- (Hb-) NO complex. The NO evolution reaction rate from (I)  k 1 = (4.6 ± 0.1)·10−3 s−1 and the elimination rate constant of the penicillamine ligand k 2 = (1.8 ± 0.2)·10−3 s−1 at 25°C in 0.05 M phosphate buffer,  pH 7.0, was calculated using kinetic modeling based on the experimental data. Both reactions are reversible. Spectrophotometry and mass-spectrometry methods have firmly shown that the penicillamine ligand is exchanged for GS− during decomposition of 1.5·10−4 M (I) in the presence of 10−3 M GSH, with 76% yield in 24 h. As has been established, such behaviour is caused by the resistance of (II) to decomposition due to the higher affinity of iron to GSH in the complex. The discovered reaction may impede S-glutathionylation of the essential enzyme systems in the presence of (I) and is important for metabolism of NIC, connected with its antitumor activity.

Revealing of the cation-binding sites on the surface of hemoglobin in its reaction with the NO donor, the nitrosyl iron complex {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O

Deoxyhemoglobin (Hb) stabilizes the cationic nitrosyl iron complex with cysteamine {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O (CysAm), by slowing down its hydrolysis. In the absence of Hb, the electrochemical detection of NO release in the course of the hydrolysis using a sensor electrode gave the rate constant of (5.2±0.2)·10−5 s−1. The release of NO is a reversible process, and the amount of released NO is 1.4% of the CysAm concentration. In the presence of Hb, NO is released much more slowly, and the reaction is more intense than that in the absence of Hb. The adsorption of CysAm by an Hb molecule results in NO release from the CysAm-Hb complex with a rate constant of 1·10−8 s−1. The analysis of the Hb surface revealed the possible location of the cation-binding sites, which reversibly bind the cationic CysAm complex. The kinetic parameters of NO release from CysAm in the absence and in the presence of Hb were studied by the kinetic modeling.

Two decomposition mechanisms of nitrosyl iron complexes [Fe 2 (μ-SR)(NO) 4 ]

Two decomposition mechanisms of nitrosyl iron complexes (NICs) [Fe2(μ-SR)(NO)4] in aqueous medium are known. One mechanism (for instance, in the case of complex [Fe2(μ-SC4H3N2)2(NO)4]) involves irreversible and rapid hydrolysis of NIC with the NO release accompanied with the formation of the products of further NO transformations. In the other mechanism (for instance, in the case of complexes [Fe2(μ-S(CH2)2NH3)2(NO)4]SO4• •2.5H2O and [Fe2(μ-SC5H11NO2)2(NO)4]SO4•5H2O), no hydrolysis occurs but NICs reversibly dissociate to release both NO and thiolate ligand into the medium. In the present work, the difference in the mechanisms of the NIC decomposition is explained by the difference in the NIC redox potentials. The experimental evidences of this fact are given.

Study on the decomposition of iron nitrosyl complex of μ-N–C–S type and its reaction with GSH in aqueous solution

Decomposition of binuclear neutral iron nitrosyl complex [Fe2(S2C7H4N)2(NO)4]0 (I) of μ-N–C–S structural type in aqueous solution has been studied. Effect of glutathione GSH on the decomposition of complex I has been studied.

A rapid method to evaluate potential vasodilatory activity of organic nitrates

The kinetics of interaction between organic nitrates (3,3-bis(nitroxymethyl)oxetane) and cysteine were evaluated by the rate of nitrite ion formation at various concentrations of reagents and pH. The activities of natural reducing agents, including cysteine, glutathione, and NADH, in generating the nitrite ion from organic nitrates (3,3-bis(nitroxymethyl)oxetane) were compared. Cysteine was shown to be the most potent reducing agent. Studying the effectiveness of nitrates (trinitroglycerol, 3,3-bis(nitroxymethyl)oxetane, and nicorandil) at a concentration of 3 mM showed that the rate of nitrite ion accumulation in the reaction with 10 mM cysteine is 1.66, 0.37, and 0.02 μM/min, respectively. The reaction of organic nitrate with cysteine (Cys) is used as a test system for analyzing the effectiveness of nitrates in nitrite ion formation, which correlates with vasodilatory activity of these compounds (dilation of blood vessels).

Electron transfer coupled with ATP hydrolysis in nitrogenase

Nitrogenase, which is not a membrane protein in vivo, performs energy coupling: the transfer of an electron coupled with ATP hydrolysis from one protein component of nitrogenase, Fe protein (Av2), to another its protein component, MoFe protein (Av1), to form the so-called “super-reduced state” of the active site responsible for the reduction of the substrates, FeMo cofactor (FeMoco) containing Fe, Mo, S, and homocitrate. The review discusses recent publications on studying the electron transfer coupled with ATP hydrolysis in nitrogenase and evaluates a possible value of the redox potential of the super-reduced FeMoco.