ATP Complex Research Articles

Spectral analysis of Fe(III)-complex reduction by hemoglobin: Possible mechanisms of interaction

1. 1. Hemoglobin is capable of electron transfer to Fe(III)-complexes of ATP, EDTA, NTA, and citrate leading to formation of reduced Fe(II) and its concurrent release from these chelating compounds as evident in the formation of a Fe(II)-Tris 2,2' bipyridine complex. 2. 2. Multi-component analysis of kinetic spectra in the visible region (700–500 nm) has permitted a determination of the effect of various chelating molecules bound to Fe(III), pH, the effects of ionic strength, temperature, and the molecular nature of the Fe(III)-complex on reaction rates. 3. 3. We have examined and compared the reactivities of normal adult hemoglobin A (α 2 β 2) to reduce these Fe(III)-complexes and suggest possible mechanism(s) for the electron transfer process.

Correlation of ActoS1, myofibrillar, and muscle fiber ATPases.

Our objective was to determine a good in vitro model for muscle fiber ATPase, and we compared the kinetics of Ca(2+)-activated myofibrils and cross-linked actoS1 in a buffer of physiological ionic strength. The myofibrils were cross-linked chemically to mimic the isometric condition of fibers or were un-cross-linked (the isotonic condition), and temperature perturbation was used to probe their ATPase mechanisms. At 4 degrees C, we have already shown that the kinetics of cross-linked actoS1 and myofibrils (cross-linked or not) are similar: there were large P(i) bursts and kcat values of about 1 s-1, close to that obtained with fibers [Herrmann, C., Sleep, J., Chaussepied, P., Travers, F. & Barman, T. (1993) Biochemistry 32, 7255-7263]. So, at 4 degrees C cross-linked actoS1 and myofibrils are equally good as models for fiber ATPase. At 20 degrees C, this similarity vanishes: progress curves with the myofibrils (cross-linked or not) had large P(i) bursts, but with cross-linked actoS1, bursts could not be discerned. This shows that at 20 degrees C the predominant steady-state intermediates are ATP complexes with actoS1 but are products complexes with the myofibrils, as with fibers [Ferenczi, M.A. (1986) Biophys. J. 50, 471-477]. Further, the kcat values were different: 15.5 s-1 with cross-linked actoS1, 8.3 s-1 for myofibrils, and 3.5 s-1 for cross-linked myofibrils. With fibers, kcat = 3.3 s-1. These results show that cross-linked myofibrillar ATPase is a good model for muscle fibers contracting isometrically.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure effect of nucleotides in terbium(III)—nucleotide fluorescent reaction New evidence for the binding sites of terbium(III) on nucleotides

AbstractUpon addition of Tb3+ to 16 nucleotides and homopolynucleotides, all of them showed a characteristic green emission from Tb3+, but with much different intensity, upon excitation in the aromatic region of bases. The result suggested that nucleotides with at least one carbonyl group in nucleotide bases are better enhancers to the fluorescence of Tb3+. The complexes of ATP, GDP and GTP with Tb3+ are synthesized as two types of models. Guanine type nucleotides with one carbonyl group in the bases are the best enhancers, while adenine type nucleotides with no carbonyl group in the bases are poorest enhancers to the fluorescence of Tb3+. Comparing the IR spectra of ATP, GTP, GDP and their Tb3+ complexes suggested that C‐6 carbonyl group in GTP and GDP may be involved in complex formation, which may be responsible for the effective energy transfer. This is further supported by comparing the UV spectra of ATP, Poly(A), GTP, and Poly(G) with their Tb3+ complexes in water solution.

Crystal structure of cyclin-dependent kinase 2.

Cyclin-dependent kinase 2 (CDK2) is a member of a highly conserved family of protein kinases that regulate the eukaryotic cell cycle. The crystal structures of the human CDK2 apoenzyme and its Mg2+ ATP complex have been determined to 2.4 A resolution. The structure is bi-lobate, like that of the cyclic AMP-dependent protein kinase, but contains a unique helix-loop segment that interferes with ATP and protein substrate binding and probably plays a key part in the regulation of all cyclin-dependent kinases.

Multifrequency ESR and NMR analysis of the metal–ATP interaction

AbstractMultifrequency ESR analysis allows a new interpretation of the correlation times involved in nuclear relaxation. In this study the metal‐ATP interaction, so important in ecosystem energy mechanisms, was interpreted in the light of new multifrequency ESR and NMR experimental data. ESR spectra of the Mn(II)‐ATP complex were recorded at 2–4 GHz (S‐band), 9 GHz (X‐band) and 35 GHz (Q‐band). A loop‐gap resonator was used to enhance the ηQ0 product at low frequencies. The ESR line shapes at different temperatures and frequencies led to the interpretation of the predominant relaxation mechanism involved. The NMR relaxation rates, also recorded at different temperatures and frequencies, are discussed in terms of a motional model taking into account a distribution of correlation times. Structural and dynamic features were analysed in order to determine the effective correlation time contributing to nuclear paramagnetic relaxation. A comprehensive analysis of the multifrequency ESR spectra is crucial for the correct interpretation of the NMR spectra and of the metal–ATP interaction.

ラタンニドとアデノシン５′‐三リン酸との錯体の安定度定数

The stability constants of lanthanide (M)-ATP (M=Tb, Dy, Er, Tm, Yb) complexes have been determined by a competitive spectrophotometry. The indicator ligand, 5-Br-PAPS (L), as well as its metal complexes, ML, having intense color with sufficient separate absorption peaks permits the analysis of complexation between metal and ATP (A). The stability constants are found to decrease with increasing atomic number: log KmA=5.76 (Tb), 5.57 (Dy), 5.38 (Er), 5.28 (Tm), 5.17 (Yb).

Redox reactions of apo mammalian ferritin.

Apo horse spleen ferritin undergoes a 6.3 +/- 0.5 electron redox reaction at -310 mV at pH 6.0-8.5 and 25 degrees C to form reduced apoferritin (apoMFred). Reconstituted ferritin containing up to 50 ferric ions undergoes reduction at the same potential, taking up one electron per ferric ion and six additional electrons by the protein. We propose that apo mammalian ferritin (apoMF) contains six redox centers that can be fully oxidized forming oxidized apoferritin (apoMFox) or fully reduced forming apoMFred. ApoMFred can be prepared conveniently by dithionite or methyl viologen reduction. ApoMFred is slowly oxidized by molecular oxygen but more rapidly by Fe(CN)6(3-) to apoMFox. Fe(III)-cytochrome c readily oxidizes apoMFred to apoMFox with a stoichiometry of 6 Fe(III)-cytochrome c per apoMFred, demonstrating a rapid interprotein electron-transfer reaction. Both redox states of apoMF react with added Fe3+ and Fe2+. Addition of eight Fe2+ to apoMFox under anaerobic conditions produced apoMFred and Fe3+, as evidenced by the presence of a strong g = 4.3 EPR signal. Subsequent addition of bipyridyl produced at least six Fe(bipyd)3(2+) per MF, establishing the reversibility of this internal electron-transfer process between the redox centers of apoMF and bound iron. Incubation of apoMFred with the Fe(3+)-ATP complex under anaerobic conditions resulted in the formation and binding of two Fe2+ and four Fe3+ by the protein. The various redox states formed by the binding of Fe2+ and Fe3+ to apoMFox and apoMFred are proposed and discussed. The yellow color of apoMF appears to be an integral characteristic of the apoMF and is possibly associated with its redox activity.

An ATP-independent U2 small nuclear ribonucleoprotein particle/precursor mRNA complex requires both splice sites and the polypyrimidine tract.

A complex is formed upon incubation of a precursor mRNA (pre-mRNA) with HeLA cell nuclear extract in the absence of added ATP (-ATP complex). Pre-mRNAs with mutations in the 5' splice site, the 3' splice site, or the polypyrimidine tract did not form this complex. Once formed, the -ATP complex was stable to competition by excess pre-mRNA. The complex was shown to contain the U2 small nuclear ribonucleoprotein particle (snRNP) and was distinct from the previously described U2 snRNP/pre-mRNA complex, the prespliceosome. These complexes have different electrophoretic mobilities, ATP requirements, and sensitivities to mutations of the 5' splice site. Although U1 snRNP was not found in the -ATP complex, a requirement for the U1 snRNP was suggested by immunodepletion experiments. The possible implications for the study of spliceosome formation are discussed.

Functional sensitivity of the native skeletal Ca(2+)-release channel to divalent cations and the Mg-ATP complex.

Sarcoplasmic reticulum (SR) vesicles, prepared from rabbit skeletal muscle, were characterized by functional and binding assays and incorporated into planar lipid bilayers. Single-channel activity was recorded in an asymmetric calcium buffer system and studied under voltage clamp conditions. Under these experimental conditions, a large conductance (100 pS in 50 mM Ca2+ trans) divalent cation selective channel displaying high ruthenium red and low Ca2+ sensitivity was identified. This pathway has been previously described as the Ca(2+)-release channel of the SR of skeletal muscle. We now report that in the presence of a Mg-ATP complex, the Ca2+ sensitivity of the open probability of this channel is increased. Furthermore, we show that micromolar cis Sr2+ concentrations also activated the Ca(2+)-release channel. The open probability of the Sr(2+)-activated channel was increased in the presence of a 2 mM Mg-ATP complex and adenine nucleotides on the cytoplasmic face of the Ca(2+)-release channel. These results were confirmed by isotopic flux measurements using passively 45Ca(2+)-loaded vesicles. In the latter case, the presence of extravesicular AMP-PCP (the nonhydrolysable ATP analog) enhanced the percentage of 45Ca2+ release induced either by Ca2+ or Sr2+ activation. In conclusion our findings emphasize the fact that the divalent cation activation of the Ca(2+)-release channel may be induced by Ca2+ and Sr2+, but not by Ba2+, in the presence of adenine nucleotides. Furthermore, they support the view that in situ Ca2+ and Mg-ATP complexes are involved in modulating the gating mechanism of this specific pathway.

Time-resolved fluorescence studies of tryptophan mutants of Escherichia coli glutamine synthetase: conformational analysis of intermediates and transition-state complexes.

Single tryptophan-containing mutants of low adenylylation state Escherichia coli glutamine synthetase have been studied by frequency-domain fluorescence spectroscopy in the presence of various substrates and inhibitors. At pH 6.5, the Mn-bound wild-type enzyme (wild type has two tryptophans/subunit) and the mutant enzymes exhibit heterogeneous fluorescence decay kinetics; the individual tryptophans are adequately described by a triple exponential decay scheme. The recovered lifetime values are 5.9 ns, 2.6 ns, and 0.4 ns for Trp-57 and 5.8 ns, 2.3 ns, and 0.4 ns for Trp-158. These values are nearly identical to the previously reported results at pH 7.5 (Atkins, W.M., Stayton, P.S., & Villafranca, J.J., 1991, Biochemistry 30, 3406-3416). In addition, Trp-57 and Trp-158 both exhibit an ATP-induced increase in the relative fraction of the long lifetime component, whereas only Trp-57 is affected by this ligand at pH 7.5. The transition-state analogue L-methionine-(R,S)-sulfoximine (MSOX) causes a dramatic increase in the fractional intensity of the long lifetime component of Trp-158. This ligand has no effect on the W158S mutant protein and causes a small increase in the fractional intensity of the long lifetime component of the W158F mutant protein. Addition of glutamate to the ATP complex, which affords the gamma-glutamylphosphate-ADP complex, results in the presence of new lifetime components at 7, 3.2, and 0.5 ns for Trp-158, but has no effect on Trp-57. Similar results were obtained when ATP was added to the MSOX complex; Trp-57 exhibits heterogeneous fluorescence decay with lifetimes of 7, 3.5, and 0.8 ns. Decay kinetics of Trp-158 are best fit to a nearly homogeneous decay with a lifetime of 5.5 ns in the MSOX-ATP inactivated complex. These results provide a model for the sequence of structural and dynamic changes that take place at the Trp-57 loop and the central loop (Trp-158) during several intermediate stages of catalysis.

Potentiation of epidermal growth factor receptor protein-tyrosine kinase activity by sulfate

The protein-tyrosine kinase activity of the epidermal growth factor (EGF) receptor is critical for EGF-stimulated cell growth, although little is known about the molecular details of its enzymatic activity. Previous studies have found that EGF receptor kinase activity can be stimulated by factors such as ammonium sulfate ((NH 4) 2SO 4), but the manner in which (NH 4) 2SO 4 induces this effect is unclear. Therefore, we have explored the processes by which (NH 4) 2SO 4 potentiated tyrosine kinase activity to better understand not only the molecular events involved in (NH 4) 2SO 4 activation, but also the kinetic properties and mechanism of the EGF receptor. In this study, the addition of an optimum concentration of (NH 4) 2SO 4 (250 mM) resulted in a 5-fold stimulation of kinase activity toward the peptide substrate, angiotensin II. The sulfate group is primarily involved in this action, since other salts containing SO 4 2− increased kinase activity similarly, whereas salts containing Cl − and F − had less of an effect, and divalent salts such as HPO 4 2− were inhibitory at doses of 1 mM or more. In addition, EGF receptor kinase activation by (NH 4) 2SO 4 did not strictly correlate with changes in the ionic strength or conductivity of the solution. However, several lines of evidence suggest that SO 4 2− directly alters the kinetic properties of the EGF receptor kinase: (1) the maximum velocity ( V max) and K m (ATP) for EGF receptor phosphorylation of angiotensin II were substantially higher in the presence of (NH 4) 2SO 4. (2) EGF receptor kinase activity in the absence of (NH 4) 2SO 4 required either Mn 2+ or Mg 2+, yet in the presence of (NH 4) 2SO 4, only Mn 2+ supported the increase in kinase activity. (3) Ammonium sulfate addition altered the product inhibition pattern of ADP versus angiotensin II, suggesting that an enzyme-angiotensin II-ADP complex can form in the presence of (NH 4) 2SO 4 but not in its absence. (4) The near-maximal rate of self-phosphorylation was not affected by (NH 4) 2SO 4, but the apparent K m(ATP) was greatly increased. From these results, we propose a model for (NH 4) 2SO 4 stimulation of EGF receptor kinase activity in which SO 4 2− interacts directly with the receptor or receptor-Mn 2+-ATP complex and alters reactant binding and the catalytic efficiency of the tyrosine kinase.

Complexation studies on inositol-phosphates: IV. Ca(II) complexes of myo-inositol 1,4,5-trisphosphate

The stability constants of the complexes formed between Ca 2+ and the myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3) were determined by potentiometric titration in two different media and temperature conditions (medium 1: I = 0.1 M But 4NBr, 25°C; medium 2: I = 0.2 M KCl, 37°C). Mainly because of the presence of potassium the results obtained in these media show large differences in both the nature and the stability of the complexes. In medium 1, MH 2L and M 2L species are formed along with the ML and MHL species which also exist in medium 2. In addition, the stability of the latter species decreases by more than one log unit in going from medium 1 to medium 2. In an attempt to assess the biological significance of the metal binding to Ins(1,4,5)P 3, the results were compared to the Ca 2+-ATP complexes that form in the same media conditions. Taking into account the relative stability of the complexes of both systems, it is likely that the action or metabolism of Ins(1,4,5)P 3 may be influenced by coordination of either alkali or alkali-earth cations.

Chapter 30: Astrocytic response to injury

The chapter discusses the role of the astrocyte in the central nervous system (CNS) injury and disease. Astrocytes comprise as much as 25% of the cells and 35% of the total mass of the CNS. Astrocytes form barriers around blood vessels and connections between nerve cells. Numerous functions have been assigned to the astrocyte depending on its stage of maturation, location in the CNS, and response to CNS insult. Some signals that regulate gene expression in development and response to astrocyte injury are: growth factors, prion protein from Scrapies, neural and immunological adhesion molecules, such as NCAM, LFA-1, gangliosides, low density lipoproteins, cytokines from T-cells, macrophages and other glia, neurotransmitters and neuropeptides, such as catecholamines, monoamines, glutamate, ATP, substance P, and antigen-antibody complexes. Astrocytic responses to these signals include: (1) proliferation, movement and differentiation; (2) changes in shape, cell volume, cytoskeletal organization, endocytic activity, lysosomal fragility, and enzyme content; (3) buffering capacity for K + , glutamate and GABA; (4) expression of nerve growth factor, tumor necrosis factor, interferon α and β , interleukin 1 and 6, colony stimulating factor-1, fibroblast growth factor, neurotropic factors, neurite promoting agents, MHC class I and I1 histocompatibility antigens, amyloid protein, GD3 ganglioside, ICAM- 1, Na + channel protein, GFAP, crystallin, vimentin and heat shock proteins.

Competition between Li + and Mg 2+ for ATP and ADP in aqueous solution: A multinuclear NMR study

We used 7Li NMR spin-lattice relaxation times and 31P NMR chemical shifts to study the binding of Li + and Mg 2+ to the phosphate moieties of ATP and ADP. To examine the binding of Li + and Mg 2+ to the base and ribose moieties, we used 1H and 13C NMR chemical shifts. The 7Li NMR relaxation times of Li +/Mg 3+ mixtures of ATP or ADP increased with increasing concentrations of Mg 2+, suggesting competition between the two ions for adenine nucleotides. No significant binding of Li + and Mg 2+ to the base and ribose moieties occurred. At the pH and ionic strength used, 2:1 and 1:1 species of the Li +-ATP and Li +-ADP complexes were present, with the 2:1 species predominating. In contrast, 1:1 species predominated for the Mg 2+-ADP and Mg 2+-ATP complexes. We calculated the Li +-nucleotide binding constants in the presence and absence of Mg 2+ and found them to be somewhat greater in the presence of Mg 2+. Although competition between Li + and Mg 2+ for ATP and ADP phosphate binding sites in solution is consistent with the 31P chemical shift data, the possibility that the Li + and Mg 2+ form mixed complexes with the phosphate groups of ATP or ADP cannot be ruled out.

Site-directed mutagenesis of the RecA protein of Escherichia coli. Tyrosine 264 is required for efficient ATP hydrolysis and strand exchange but not for LexA repressor inactivation.

The role of Tyr264 in nucleotide binding and hydrolysis catalyzed by the RecA protein of Escherichia coli was investigated by constructing Gly, Ser, and Phe substitution mutations using oligonucleotide-directed mutagenesis. The corresponding mutant recA genes neither restored resistance to killing by ultraviolet irradiation nor increased homologous recombination in a recA strain. The purified RecA(Gly264) protein was unable to bind nucleotide, hydrolyze ATP, or form stable ternary complexes with adenosine 5'-O-thiotriphosphate and DNA although the mutant protein bound DNA normally in the absence of nucleotide. The RecA (Phe264) and RecA(Ser264) proteins hydrolyzed ATP poorly and the rates were reduced approximately 8- and 18-fold, respectively. Although capable of low levels of ATP hydrolysis, neither the RecA(Phe264) nor the RecA(Ser264) protein promoted DNA pairing or strand exchange reactions in vitro. Furthermore, these mutant RecA proteins were impaired in their ability to form salt-resistant ternary complexes with adenosine 5'-O-thiotriphosphate) and DNA as judged by filter binding. Nevertheless, nucleoprotein complexes formed with either RecA(Phe264) or RecA(Ser264) protein directed efficient cleavage of LexA repressor in vitro. These results demonstrate that Tyr264 is required for efficient ATP hydrolysis and for homologous pairing of DNA but does not participate in activating RecA protein for LexA repressor autodigestion.

Chelation of divalent cations by ATP, studied by titration calorimetry

Thermodynamic parameters and stoichiometry for the formation of complexes of ATP with Mg 2+, Ca 2+, and Sr 2+ were determined by titration calorimetry. In each case, 1:1 stoichiometry was observed and complex formation was entropy driven. Binding constants for formation of complexes decreased in the order of Mg 2+ > Ca 2+ > Sr 2+, as expected from charge density considerations. Monovalent cations hindered complex formation with Mg 2+, apparently by competing with the divalent cation for complexation with ATP. Analysis of this competitive effect provided estimates of the binding constants for complexes of ATP with monovalent cations, which decreased in the order expected from charge density considerations (Li + > Na + > K +).

Macrocyclic polyamines [16]-N3 and [21]-N4: synthesis and study of their ATP complexation by 31P nuclear magnetic resonance spectroscopy

The macrocyclic polyamines [16]-N3, compound 1, and [21]-N4, compound 2, possessing a hydroxymethyl side-chain have been synthesized. The macrocyclisation was achieved by condensation of lysine ditosate 3b with appropriate components 5 and 6 in the presence of Cs2CO3 to obtain 7 and 8 in 33 and 37% yield, respectively. The complexation of polyamines 1 and 2 with ATP was studied by 31P NMR spectroscopy, which indicated a specific recognition of γ-phosphorus of ATP (NMe4 salt) by the polyamines. The binding constant estimates indicated that ATP binds compound 2ca. 35-times stronger than it does compound 1. The binding curves indicated a definite 1 : 1 stoicheiometry for 2 :ATP complex, and not so well defined stoicheiometry for 1 :ATP binding. The mononucleotides, AMP and cAMP, and dinucleotide TpT did not show significant complexation with either compound 1 or 2. The binding of ATP by macrocyclic polyamines 1 and 2 and the presence of a hydroxymethyl side-chain to link with a nucleophile may aid rational design of chemical nucleases.

Phosphate and thiophosphate group donating adenine and guanine nucleotides inhibit glibenclamide binding to membranes from pancreatic islets

In microsomes obtained from mouse pancreatic islets, the Mg complex of adenosine 5'-triphosphate (MgATP) increased the dissociation constant (KD) for binding of [3H]glibenclamide by sixfold. In the presence of Mg2+, not only ATP but also adenosine 5'-0-(3-thiotriphosphate) (ATP gamma S), adenosine 5'-diphosphate (ADP), guanosine 5'-triphosphate (GTP), guanosine 5'-diphosphate (GDP), guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S) and guanosine 5'-0-(2-thiodiphosphate) (GDP beta S) inhibited binding of [3H]glibenclamide. These effects were not observed in the absence of Mg2+. Half maximally effective concentrations of the Mg complexes of ATP, ADP, ATP gamma S and GDP were 11.6, 19.0, 62.3 and 90.1 mumol/l, respectively. The non-hydrolyzable analogues adenosine 5'-(beta,gamma-imidotriphosphate) (AMP-PNP) and guanosine 5'-(beta,gamma-imidotriphosphate) (GMP-PNP) did not alter [3H]glibenclamide binding in the presence of Mg2+, MgADP acted much more slowly than MgATP and both MgADP and MgGDP did not inhibit [3H]glibenclamide binding when the concentrations of MgATP and MgGTP were kept low by the hexokinase reaction. Development of MgATP-induced inhibition of [3H]glibenclamide binding and dissociation of [3H]glibenclamide binding occurred at similar rates. However, the reversal of MgATP-induced inhibition of [3H]glibenclamide binding was slower than the association of [3H]glibenclamide with its binding site. Exogenous alkaline phosphatase accelerated the reversal of MgATP-induced inhibition of [3H]glibenclamide binding. MgATP enhanced displacement of [3H]glibenclamide binding by diazoxide. The data suggest that sulfonylureas and diazoxide exert their effects by interaction with the same binding site at the sulfonylurea receptor and that protein phosphorylation modulates the affinity of the receptor.

A1-ATP as an intracellular carrier of A1(III) ion

1. 1. Using 27Al and 31P NMR spectroscopy in conjunction with an Al lactate aqueous reagent at pH 7.2, Al complexes of ATP and of phospholipids were characterized in synthetic-aqueous and organic-phospholipid chemical systems and in the intact human red blood cell. 2. 2. The observed 31P NMR chemical shifts of the Al-ATP complex in aqueous laboratory preparations or the intact human red blood cell were, respectively, α phosphate, −11.53 δ; β phosphate, − 22.65 δ; and γ phosphate, −10.95 δ. 3. 3. The observed complexed 27Al chemical shift was −2.22 δ. 4. 4. The relative affinities for Al of the phospholipids determined from 31P NMR spectroscopic titrations were PA > Cl > PS > PG ▪ PI > PE plas ▪ PE > SPH > PC.

Reactions of Fe(II)-ATP and Fe(II)-citrate complexes with t-butyl hydroperoxide and cumyl hydroperoxide

Rate constants for the reactions of cumyl hydroperoxide and t-butyl hydroperoxide with ferrous complexes of ATP and citrate were measured at pH 7.4. These ligands are potential chelators of iron(II) in the low-molecular weight iron pool that may catalyze oxidative degradation of biomolecules. The second-order rate constants for the reduction of cumyl hydroperoxide and t-butyl hydroperoxide by ferrous ATP are 3.1 × 10 3 and 1.3 × 10 3 M −1·s −1, respectively, at 25°C and 0.11 M ionic strength. Rates of reduction by ferrous citrate are similar. Activation enthalpies for these reactions average 10 kcal/mol.