NO Scavenging through Reductive Nitrosylation of Ferric Mycobacterium tuberculosis and Homo sapiens Nitrobindins

Ferric nitrobindins (Nbs) selectively bind NO and catalyze the conversion of peroxynitrite to nitrate. In this study, we show that NO scavenging occurs through the reductive nitrosylation of ferric Mycobacterium tuberculosis and Homo sapiens nitrobindins (Mt-Nb(III) and Hs-Nb(III), respectively). The conversion of Mt-Nb(III) and Hs-Nb(III) to Mt-Nb(II)-NO and Hs-Nb(II)-NO, respectively, is a monophasic process, suggesting that over the explored NO concentration range (between 2.5 × 10⁻⁵ and 1.0 × 10⁻³ M), NO binding is lost in the mixing time (i.e., ^NOk_on ≥ 1.0 × 10⁶ M⁻¹ s⁻¹). The pseudo-first-order rate constant for the reductive nitrosylation of Mt-Nb(III) and Hs-Nb(III) (i.e., k) is not linearly dependent on the NO concentration but tends to level off, with a rate-limiting step (i.e., k_lim) whose values increase linearly with [OH⁻]. This indicates that the conversion of Mt-Nb(III) and Hs-Nb(III) to Mt-Nb(II)-NO and Hs-Nb(II)-NO, respectively, is limited by the OH⁻-based catalysis. From the dependence of k_lim on [OH⁻], the values of the second-order rate constant k_OH− for the reductive nitrosylation of Mt-Nb(III)-NO and Hs-Nb(III)-NO were obtained (4.9 (±0.5) × 10³ M⁻¹ s⁻¹ and 6.9 (±0.8) × 10³ M⁻¹ s⁻¹, respectively). This process leads to the inactivation of two NO molecules: one being converted to HNO₂ and another being tightly bound to the ferrous heme-Fe(II) atom.

Ferric nitrosylated myoglobin catalyzes peroxynitrite scavenging

Myoglobin (Mb), generally taken as the molecular model of monomeric globular heme-proteins, is devoted: (i) to act as an intracellular oxygen reservoir, (ii) to transport oxygen from the sarcolemma to the mitochondria of vertebrate heart and red muscle cells, and (iii) to act as a scavenger of nitrogen and oxygen reactive species protecting mitochondrial respiration. Here, the first evidence of ^·NO inhibition of ferric Mb- (Mb(III)) mediated detoxification of peroxynitrite is reported, at pH 7.2 and 20.0 °C. ^·NO binds to Mb(III) with a simple equilibrium; the value of the second-order rate constant for Mb(III) nitrosylation (i.e., ^·NOk_on) is (6.8 ± 0.7) × 10⁴ M^-1 s^-1 and the value of the first-order rate constant for Mb(III)-NO denitrosylation (i.e., ^·NOk_off) is 3.1 ± 0.3 s^-1. The calculated value of the dissociation equilibrium constant for Mb(III)-NO complex formation (i.e., ^·NOk_off/^·NOk_on = (4.6 ± 0.7) × 10^-5 M) is virtually the same as that directly measured (i.e., ^·NOK = (3.8 ± 0.5) × 10^-5 M). In the absence of ^·NO, Mb(III) catalyzes the conversion of peroxynitrite to NO₃^-, the value of the second-order rate constant (i.e., ^Pk_on) being (1.9 ± 0.2) × 10⁴ M^-1 s^-1. However, in the presence of ^·NO, Mb(III)-mediated detoxification of peroxynitrite is only partially inhibited, underlying the possibility that also Mb(III)-NO is able to catalyze the peroxynitrite isomerization, though with a reduced rate (^Pk_on* = (2.8 ± 0.3) × 10³ M^-1 s^-1). These data expand the multiple roles of ^·NO in modulating heme-protein actions, envisaging a delicate balancing between peroxynitrite and ^·NO, which is modulated through the relative amount of Mb(III) and Mb(III)-NO.

Hydroxylamine-induced oxidation of ferrous CO-bound carboxymethylated-cytochrome c

The hexa-coordinated metal center of horse heart cyt[Formula: see text] (cyt[Formula: see text] is at the root of its low reactivity. In contrast, carboxymethylated cyt[Formula: see text] (CM-cyt[Formula: see text] displays myoglobin-like properties. Herein, kinetics of CO binding to ferrous CM-cyt[Formula: see text] (CM-cyt[Formula: see text](II)) and of the irreversible oxidation of ferrous carbonylated CM-cyt[Formula: see text] (CM-cyt[Formula: see text](II)-CO) by hydroxylamine (HA), at pH 5.8 and 20.0 [Formula: see text]C, are reported. HA irreversibly oxidizes CM-cyt[Formula: see text](II)-CO with the 1:2 stoichiometry leading to the formation of the ferric species (CM-cyt[Formula: see text](III)) without the observation of intermediates. Present data indicate that: (i) the rate of CO dissociation from CM-cyt[Formula: see text](II)-CO represents the rate-limiting step of HA-mediated oxidation of the carbonylated metal center, (ii) the fast oxidation of CM-cyt[Formula: see text](II)-CO from HA reflects the penta-coordination of the transient CM-cyt[Formula: see text](II) species, (iii) the HA-catalyzed conversion of CM-cyt[Formula: see text](II)-CO to CM-cyt[Formula: see text](III) could proceed via the geminate mechanism, (iv) values of the second-order rate constants for the carbonylation and the HA-mediated oxidation of ferrous heme-proteins are linearly correlated reflecting the penta- or hexa-coordination of the metal center, the free energy for the in-plane positioning of the heme-Fe atom in the unliganded species, and the arrangement of the distal portion of the heme pocket that affects ligand and/or electron transfer.

Reductive nitrosylation of ferric microperoxidase-11

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c, which is considered as a heme-protein model. Here, the reductive nitrosylation of ferric MP11 (MP11(III)) under anaerobic conditions has been investigated between pH 7.4 and 9.2, at T = 20.0 °C. At pH ≤ 7.7, NO binds reversibly to MP11(III) leading to the formation of the MP11(III)-NO complex. However, between pH 8.2 and 9.2, the addition of NO to MP11(III) leads to the formation of ferrous nitrosylated MP11(II) (MP11(II)-NO). In fact, the transient MP11{FeNO}⁶ species is converted to ferrous deoxygenated MP11 (MP11(II)) by OH^-- and H₂O-based catalysis, which represents the rate-limiting step of the whole reaction. Then, MP11(II) binds NO very rapidly leading to MP11(II)-NO formation. Over the whole pH range explored, the apparent values of k_on, k_off, and K (= k_off/k_on) for MP11(III)(-NO) (de)nitrosylation are essentially pH independent, ranging between 5.8 × 10⁵ M^-1 s^-1 and 1.6 × 10⁶ M^-1 s^-1, between 1.9 s^-1 and 3.7 s^-1, and between 1.4 × 10^-6 M and 4.6 × 10^-6 M, respectively. Values of the apparent pseudo-first-order rate constant for the MP11{FeNO}⁶ conversion to MP11(II) (i.e., h) increase linearly with pH; the apparent values [Formula: see text] and [Formula: see text] are 7.2 × 10² M^-1 s^-1 and 2.5 × 10^-4 s^-1, respectively. Present data confirm that MP11 is a useful molecular model to highlight the role of the protein matrix on the heme-based reactivity.

Reductive nitrosylation of ferric human hemoglobin bound to human haptoglobin 1-1 and 2-2

Haptoglobin (Hp) sequesters hemoglobin (Hb) preventing the Hb-based damage occurring upon its physiological release into plasma. Here, reductive nitrosylation of ferric human hemoglobin [Hb(III)] bound to human haptoglobin (Hp) 1-1 and 2-2 [Hp1-1:Hb(III) and Hp2-2:Hb(III), respectively] has been investigated between pH 7.5 and 9.5, at T=20.0 °C. Over the whole pH range explored, only one process is detected reflecting NO binding to Hp1-1:Hb(III) and Hp2-2:Hb(III). Values of the pseudo-first-order rate constant for Hp1-1:Hb(III) and Hp2-2:Hb(III) nitrosylation (k) do not depend linearly on the ligand concentration but tend to level off. The conversion of Hp1-1:Hb(III)-NO to Hp1-1:Hb(II)-NO and of Hp2-2:Hb(III)-NO to Hp2-2:Hb(II)-NO is limited by the OH^-- and H₂O-based catalysis. In fact, bimolecular NO binding to Hp1-1:Hb(III), Hp2-2:Hb(III), Hp1-1:Hb(II), and Hp2-2:Hb(II) proceeds very rapidly. The analysis of data allowed to determine the values of the dissociation equilibrium constant for Hp1-1:Hb(III) and Hp2-2:Hb(III) nitrosylation [K = (1.2 ± 0.1) × 10^-4 M], which is pH-independent, and of the first-order rate constant for Hp1-1:Hb(III) and Hp2-2:Hb(III) conversion to Hp1-1:Hb(II)-NO and Hp2-2:Hb(II)-NO, respectively (k'). From the dependence of k' on [OH^-], values of h_OH- [(4.9 ± 0.6) × 10³ M^-1 s^-1 and (6.79 ± 0.7) × 10³ M^-1 s^-1, respectively] and of [Formula: see text] [(2.6 ± 0.3) × 10^-3 s^-1] were determined. Values of kinetic and thermodynamic parameters for Hp1-1:Hb(III) and Hp2-2:Hb(III) reductive nitrosylation match well with those of the Hb R-state, which is typical of the αβ dimers of Hb bound to Hp.

Reductive nitrosylation of ferric carboxymethylated-cytochrome c

Horse heart carboxymethylated-cyt[Formula: see text] (CM-cyt[Formula: see text] displays myoglobin-like properties due to the cleavage of the heme-Fe-Met80 axial bond. Here, reductive nitrosylation of CM-cyt[Formula: see text](III) between pH 8.5 and 9.5, at [Formula: see text] 20.0 C, is reported. Under anaerobic conditions, the addition of NO to CM-cyt[Formula: see text](III) leads to the transient formation of CM-cyt[Formula: see text](III)-NO in equilibrium with CM-cyt[Formula: see text](II)-NO[Formula: see text]. In turn, CM-cyt[Formula: see text](II)-NO[Formula: see text] is converted to CM-cyt[Formula: see text](II) by OH[Formula: see text]-based catalysis. Then, CM-cyt[Formula: see text](II) binds NO very rapidly leading to CM-cyt[Formula: see text](II)-NO. Kinetics of NO binding to CM-cyt[Formula: see text](III) is independent of the ligand concentration, [Formula: see text] values ranging between 3.6 ± 0.4 s[Formula: see text] and 7.1 ± 0.7 s[Formula: see text]. This indicates that the formation of the CM-cytc(III)-NO complex is rate-limited by the cleavage of the weak heme-Fe(III) distal bond (likely Lys79). The conversion of CM-cyt[Formula: see text](III)-NO to CM-cyt[Formula: see text](II)-NO is rate-limited by the OH[Formula: see text]-mediated reduction of CM-cyt[Formula: see text](II)-NO[Formula: see text] ([Formula: see text] (1.2 ± 0.1) × 103 M[Formula: see text].s[Formula: see text]. Lastly, the very fast nitrosylation of CM-cyt[Formula: see text](II) takes place, values of [Formula: see text] ranging between[Formula: see text]5.3 × 106 M[Formula: see text].s[Formula: see text] and 1.4 × 107 M[Formula: see text].s[Formula: see text]. These results indicate that CM-cyt[Formula: see text] behaves as the cardiolipin-cyt[Formula: see text] complex highlighting the role of the sixth axial ligand of the heme-Fe atom in the modulation of the metal-based reactivity.

Human serum albumin: from bench to bedside

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule, representing the main determinant of plasma oncotic pressure and the main modulator of fluid distribution between body compartments. HSA displays an extraordinary ligand binding capacity, providing a depot and carrier for many endogenous and exogenous compounds. Indeed, HSA represents the main carrier for fatty acids, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays (pseudo-)enzymatic properties. HSA is a valuable biomarker of many diseases, including cancer, rheumatoid arthritis, ischemia, post-menopausal obesity, severe acute graft-versus-host disease, and diseases that need monitoring of the glycemic control. Moreover, HSA is widely used clinically to treat several diseases, including hypovolemia, shock, burns, surgical blood loss, trauma, hemorrhage, cardiopulmonary bypass, acute respiratory distress syndrome, hemodialysis, acute liver failure, chronic liver disease, nutrition support, resuscitation, and hypoalbuminemia. Recently, biotechnological applications of HSA, including implantable biomaterials, surgical adhesives and sealants, biochromatography, ligand trapping, and fusion proteins, have been reported. Here, genetic, biochemical, biomedical, and biotechnological aspects of HSA are reviewed.

On the origin of selective nitrous oxide N-N bond cleavage by three-coordinate molybdenum(III) complexes

Reaction of Mo(N[R]Ar)(3) (R = (t)Bu or C(CD(3))(2)CH(3)) with N(2)O gives rise exclusively to a 1:1 mixture of nitride NMo(N[R]Ar)(3) and nitrosyl ONMo(N[R]Ar)(3), rather than the known oxo complex OMo(N[R]Ar)(3) and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)(3) with N(2)O and, independently, the heat of reaction of Mo(N[R]Ar)(3) with NO. Derived from the latter measurements is an estimate (155.3 +/- 3.3 kcal.mol(-1)) of the molybdenum-nitrogen bond dissociation enthalpy for the terminal nitrido complex, NMo(N[R]Ar)(3). Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar)(3) shows that the nitrous oxide N-N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)(3) and N(2)O, consistent with a mechanism featuring post-rate-determining dinuclear N-N bond scission, but also consistent with cleavage of the N-N bond at a single metal center in a mechanism requiring the intermediacy of nitric oxide. The new 2-adamantyl-substituted molybdenum complex Mo(N[2-Ad]Ar)(3) was synthesized and found also to split N(2)O, resulting in a 1:1 mixture of nitrosyl and nitride products; the reaction exhibited first-order kinetics and was found to be ca. 6 times slower than that for the tert-butyl-substituted derivative. Discussed in conjunction with studies of the 2-adamantyl derivative Mo(N[2-Ad]Ar)(3) is the role of ligand-imposed steric constraints on small-molecule, e.g. N(2) and N(2)O, activation reactivity. Bradley's chromium complex Cr(N(i)Pr(2))(3) was found to be competitive with Mo(N[R]Ar)(3) for NO binding, while on its own exhibiting no reaction with N(2)O. Competition experiments permitted determination of ratios of second-order rate constants for NO binding by the two molybdenum complexes and the chromium complex. Analysis of the product mixtures resulting from carrying out the N(2)O cleavage reactions with Cr(N(i)Pr(2))(3) present as an in situ NO scavenger rules out as dominant any mechanism involving the intermediacy of NO. Simplest and consistent with all the available data is a post-rate-determining bimetallic N-N scission process. Kinetic funneling of the reaction as indicated is taken to be governed by the properties of nitrous oxide as a ligand, coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.

Determination of H(2)S solubility via the reaction with ferric hemoglobin I from the bivalve mollusc Lucina pectinata

A new, simple and fast spectrophotometric method for the determination of the H(2)S concentration is reported. This method, based on the 1:1 reaction between H(2)S and the ferric derivative of hemoglobin I (HbI) from the bivalve mollusc Lucina pectinata, allows the quantitative determination of H(2)S dissolved in a given solution even at concentrations as low as 1 x 10(-6) M. Note that L. pectinata HbI is considered the physiological receptor of H(2)S.