Resonance Raman studies of xanthine oxidase: The reduced enzyme-product complex with violapterin

A study of the molecular, electronic, and vibrational characteristics of the molybdenum-containing enzyme complex xanthine oxidase with violapterin has been carried out using density functional theory calculations and resonance Raman spectroscopy. The electronic structure calculations were carried out on a model consisting of the enzyme molybdopterin cofactor [in the four-valent, reduced state; Mo(IV)O(SH)] covalently linked to violapterin (1H,3H,8H-pteridine-2,4,7-trione in the neutral form) via an oxygen bridge, Mo-O-C7. Full geometry optimizations were performed for all models using the SDD basis set and the three-parameter exchange functional of Becke combined with the Lee, Yang, and Parr correlational functional. Harmonic vibrational frequencies were determined for a variety of isotopes in an attempt to correlate experimentally observed shifts upon 18O-labeling of the Mo-OR bridge to bound product as well as shifts seen upon substitution of solvent-exchangeable protons in samples prepared in D2O. The theoretical vibrational frequencies compared favorably with experimentally observed vibrational modes in the resonance Raman spectra of the reduced xanthine oxidase-violapterin complex prepared in H2O and D2O and with 18O-labeled product. Correlating the isotopic shifts from the calculations with those from the resonance Raman experiments resulted in complete normal mode assignments for all modes observed in the 350-1750 cm(-1) range. The present work demonstrates that a model in which the violapterin is coordinated to the molybdenum of the active site in a simple end-on manner via the hydroxyl group introduced by an enzyme accurately predicts the observed resonance Raman spectrum of the complex. Given the numerous modes involving the bridging oxygen, a side-on binding mode can be eliminated.

The Role of Arginine 310 in Catalysis and Substrate Specificity in Xanthine Dehydrogenase from Rhodobacter capsulatus

The rapid reaction kinetics of wild-type xanthine dehydrogenase from Rhodobacter capsulatus and variants at Arg-310 in the active site have been characterized for a variety of purine substrates. With xanthine as substrate, k(red) (the limiting rate of enzyme reduction by substrate at high [S]) decreased approximately 20-fold in an R310K variant and 2 x 10(4)-fold in an R310M variant. Although Arg-310 lies on the opposite end of the substrate from the C-8 position that becomes hydroxylated, its interaction with substrate still contributed approximately 4.5 kcal/mol toward transition state stabilization. The other purines examined fell into two distinct groups: members of the first were effectively hydroxylated by the wild-type enzyme but were strongly affected by the exchange of Arg-310 to methionine (with a reduction in k(red) greater than 10(3)), whereas members of the second were much less effectively hydroxylated by wild-type enzyme but also much less significantly affected by the amino acid exchanges (with a reduction in k(red) less than 50-fold). The effect was such that the 4000-fold range in k(red) seen with wild-type enzyme was reduced to a mere 4-fold in the R310M variant. The data are consistent with a model in which "good" substrates are bound "correctly" in the active site in an orientation that allows Arg-310 to stabilize the transition state for the first step of the overall reaction via an electrostatic interaction at the C-6 position, thereby accelerating the reaction rate. On the other hand, "poor" substrates bound upside down relative to this "correct" orientation. In so doing, they are unable to avail themselves of the additional catalytic power provided by Arg-310 in wild-type enzyme but, for this reason, are significantly less affected by mutations at this position. The kinetic data thus provide a picture of the specific manner in which the physiological substrate xanthine is oriented in the active site relative to Arg-310 and how this residue is used catalytically to accelerate the reaction rate (rather than simply bind substrate) despite being remote from the position that is hydroxylated.

Characterization of superoxide production from aldehyde oxidase: an important source of oxidants in biological tissues

Aldehyde oxidase, a molybdoflavoenzyme that plays an important role in aldehyde biotransformation, requires oxygen as substrate and produces reduced oxygen species. However, little information is available regarding its importance in cellular redox stress. Therefore, studies were undertaken to characterize its superoxide and hydrogen peroxide production. Aldehyde oxidase was purified to >98% purity and exhibited a single band at approximately 290 kDa on native polyacrylamide gradient gel electrophoresis. Superoxide generation was measured and quantitated by cytochrome c reduction and EPR spin trapping with p-dimethyl aminocinnamaldehyde as reducing substrate. Prominent superoxide generation was observed with an initial rate of 295 nmol min(-1) mg(-1). Electrochemical measurements of oxygen consumption and hydrogen peroxide formation yielded values of 650 and 355 nmol min(-1) mg(-1). In view of the ubiquitous distribution of aldehydes in tissues, aldehyde oxidase can be an important basal source of superoxide that would be enhanced in disease settings where cellular aldehyde levels are increased.

Studies on the current 137Cs body burden of children in Belarus--can the dose be further reduced?

After the Chernobyl reactor accident wide areas of Belarus were contaminated with radioactive fallout. The verification and documentation of the long-term development of radiation doses is still going on. A population group of special concern are the children living in contaminated regions. The annual dose limit of 1 mSv is still exceeded in some cases, essentially due to high body burdens of 137Cs as indicated by screening measurements with portable incorporation monitors. In this situation the evaluation of possible dose reduction measures in addition to the control of food contamination is being investigated. Special attention is given to the therapeutic application of a pectin preparation (Vitapect), for which a dose-lowering effect is presumed by Belarusian scientists. In a placebo-controlled double-blind study, several groups of contaminated children received a pectin compound named Vitapect for a two-week period during their stay in a sanatorium. For comparison the same number of control groups were given a placebo preparation. The 137Cs body burden of the children was measured at the beginning and the end. The mean relative reduction of the specific activity within the Vitapect groups was found to be approximately 33%, whereas the specific activity of the children who received a placebo decreased only by approximately 14%, due to clean food supply. It is known that pectins chemically bind cations like caesium in the gastrointestinal tract and thereby increase faecal excretion. Theoretical calculations based on this assumption and considering metabolism processes are qualitatively consistent with the experimentally found retention of radiocaesium in the human body after pectin treatment.

Reduced nonprotein thiols inhibit activation and function of MMP-9: implications for chemoprevention

Clinical studies demonstrate a positive correlation between the extent of matrix metalloproteinase (MMP) activation and malignant progression of precancerous lesions. Therefore, identification of effective, well-tolerated MMP inhibitors represents a rational chemopreventive strategy. A variety of agents, including proteinases and thiol-oxidizing compounds, activate MMPs by initiating release of the propeptide's cysteine sulfur "blockage" of the MMP active site. Despite the importance of the propeptide's cysteine thiol in preserving MMP latency, limited studies have evaluated the effects of reduced thiols on MMP function. This study investigated the effects of two naturally occurring nonprotein thiols, i.e., glutathione (GSH) and N-acetylcysteine (NAC), on activation, function, and cellular-extracellular matrix interactions of the basement-membrane-degrading gelatinase, MMP-9. Our results reveal that NAC and GSH employ protein S-thiolation to inhibit organomercurial activation of pro-MMP-9. Gelatinase activity assays showed that GSH and NAC significantly inhibited MMP-9 but not MMP-2 function, implying isoform structural specificity. Immunoblot analyses, which suggested GSH interacts with MMP-9's active-site Zn, were corroborated by computational molecular modeling. Cell invasion assays revealed that NAC enhanced endostatin's ability to inhibit human cancer cell invasion. Collectively, these data demonstrate that nonprotein thiols suppress MMP-9 activation and function and introduce the prospect for their use in chemopreventive applications.

Spectroscopic and kinetic studies of Arabidopsis thaliana sulfite oxidase: nature of the redox-active orbital and electronic structure contributions to catalysis

Plant sulfite oxidase from Arabidopsis thaliana has been characterized both spectroscopically and kinetically. The enzyme is unusual in lacking the heme domain that is present in the otherwise highly homologous enzyme from vertebrate sources. In steady-state assays, the enzyme exhibits a pH maximum of 8.5 and is also found to function as a selenite oxidase. Sulfite at the lowest experimentally feasible concentrations reduces the enzyme within the dead-time of a stopped-flow instrument at 5 degrees C, indicating that the A. thaliana enzyme has a limiting rate constant for reduction, k(red), at least 10 times greater than that of the chicken enzyme (190 s(-1)). The EPR parameters for the high- and low-pH forms of the A. thaliana enzyme have been determined, and the g-values are found to resemble those previously reported for the vertebrate enzymes. Finally, the A. thaliana enzyme has been probed by resonance Raman spectroscopy. A detailed analysis of the vibrational spectrum in the region where Mo=O stretching modes are anticipated to occur has been performed with the help of density functional theory calculations, evaluated in the context of the Raman data. Calculated frequencies obtained for two model systems have been compared to experimental resonance Raman spectra of oxidized A. thaliana sulfite oxidase catalytically cycled in both H2(16)O and H2(18)O. The vibrational frequency shifts observed upon (18)O-labeling of the enzyme are consistent with theoretical models in which either the equatorial oxygen or both equatorial and axial atoms of the dioxomolybdenum center are labeled. Importantly, the vibrational mode description is consistent with the active site possessing geometrically inequivalent oxo ligands and a Mo d(xy) redox-active molecular orbital oriented in the equatorial plane forming a pi-bonding interaction solely with the equatorial oxo, O(eq). Electron occupancy of this Mo=O(eq) pi* redox orbital upon interaction with substrates would effectively labilize the Mo=O(eq) bond, providing the dominant contribution to lowering the activation energy for oxygen atom transfer.

Structures of the Mo(V) forms of sulfite oxidase from Arabidopsis thaliana by pulsed EPR spectroscopy

The Mo(V) center of plant sulfite oxidase from Arabidopsis thaliana (At-SO) has been studied by continuous wave and pulsed EPR methods. Three different Mo(V) EPR signals have been observed, depending on pH and the technique used to generate the Mo(V) oxidation state. At pH 6, reduction by sulfite followed by partial reoxidation with ferricyanide generates an EPR spectrum with g-values similar to the low-pH (lpH) form of vertebrate SOs, but no nearby exchangeable protons can be detected. On the other hand, reduction of At-SO with Ti(III) citrate at pH 6 generates a Mo(V) signal with large hyperfine splittings from a single exchangeable proton, as is typically observed for lpH SO from vertebrates. Reduction of At-SO with sulfite at high pH generates the well-known high-pH (hpH) signal common to all sulfite oxidizing enzymes. It is proposed that, depending on the conformation of Arg374, the active site of At-SO may be in "closed" or "open" forms that differ in the degree of accessibility of the Mo center to substrate and water molecules. It is suggested that at low pH the sulfite-reduced At-SO has coordinated sulfate and is in the "closed form". Reoxidation to Mo(V) by ferricyanide leaves bound sulfate trapped at the active site, and consequently, there are no ligands with exchangeable protons. Reduction with Ti(III) citrate injects an electron directly into the active site to generate the [Mo(V)[triple bond]O(OH)]2+ unit that is well-known from model chemistry and which has a single exchangeable proton with a large isotropic hyperfine interaction. At high pH, the active site is in the "open form", and water can readily exchange into the site to generate the hpH SO.

Exploring the redox reactions between heme and tetrahydrobiopterin in the nitric oxide synthases

The NO synthases (NOSs) catalyze a two-step oxidation of L-arginine (Arg) to generate nitric oxide (NO) plus L-citrulline. Because NOSs are the only hemeproteins known to contain tetrahydrobiopterin (H4B) as a bound cofactor, the function and role of H4B in their heme-based oxygen activation and catalysis is of current interest. Distinct oxidative and reductive transitions of bound H4B cofactor occur during catalysis and are associated with distinct redox transitions of the NOS heme and flavin prosthetic groups. In this perspective, we discuss the redox transitions of H4B and heme with regard to their kinetics, regulation, role in the catalytic mechanism, and how and why they may be linked.

Electron Transfer within Complex II

Electron transfer within Escherichia coli succinate:ubiquinone oxidoreductase has been examined by the pulse radiolysis technique using spectrophotometric detection. Electrons have been introduced into the protein by the bimolecular reaction with quantified concentrations of the low potential N-methylnicotinamide radical at a rate constant of 7 x 10(8) M(-1) s(-1). Two redox-active centers in the protein are initially reduced, assigned as the high potential [3Fe-4S] center and the bound ubiquinone, followed by intramolecular equilibration with the b heme in both cases. Electron equilibration at 25 degrees C from the ubisemiquinone proceeds with an observed rate constant of 7,200 s(-1) and from the more distant [3Fe-4S] reduced center at a rate constant of 1,200 s(-1). Temperature dependence studies have revealed that both reactions have large free energies of activation, with deltaG(double dagger) values of +0.53 and +0.58 eV, respectively. Cumulative spectral changes, as well as accompanying decreases in the rates of intramolecular electron transfer, observed upon adding electrons to progressively reduced protein, indicate that 4 electrons must be introduced into the protein before the heme center is fully reduced. Overall, evidence is presented that the heme, far from being a bystander in the efficient transfer of reducing equivalents from succinate to the ubiquinone via the flavin-Fe/S centers, plays a pivotal role in providing a lower energy pathway for the transfer of an electron from the high potential [3Fe-4S] center to ubiquinone.

The three nitric-oxide synthases differ in their kinetics of tetrahydrobiopterin radical formation, heme-dioxy reduction, and arginine hydroxylation

The nitric-oxide synthases (NOSs) make nitric oxide and citrulline from l-arginine. How the bound cofactor (6R)-tetrahydrobiopterin (H4B) participates in Arg hydroxylation is a topic of interest. We demonstrated previously that H4B radical formation in the inducible NOS oxygenase domain (iNOSoxy) is kinetically coupled to the disappearance of a heme-dioxy intermediate and to Arg hydroxylation. Here we report single turnover studies that determine and compare the kinetics of these transitions in Arg hydroxylation reactions catalyzed by the oxygenase domains of endothelial and neuronal NOSs (eNOSoxy and nNOSoxy). There was a buildup of a heme-dioxy intermediate in eNOSoxy and nNOSoxy followed by a monophasic transition to ferric enzyme during the reaction. The rate of heme-dioxy decay matched the rates of H4B radical formation and Arg hydroxylation in both enzymes. The rates of H4B radical formation differed such that nNOSoxy (18 s(-1)) > iNOSoxy (11 s(-1)) > eNOSoxy (6 s(-1)), whereas the lifetimes of the resulting H4B radical followed an opposite rank order. 5MeH4B supported a three-fold faster radical formation and greater radical stability relative to H4B in both eNOSoxy and nNOSoxy. Our results indicate the following: (i) the three NOSs share a common mechanism, whereby H4B transfers an electron to the heme-dioxy intermediate. This step enables Arg hydroxylation and is rate-limiting for all subsequent steps in the hydroxylation reaction. (ii) A direct correlation exists between pterin radical stability and the speed of its formation in the three NOSs. (iii) Uncoupled NO synthesis often seen for eNOS at low H4B concentrations may be caused by the slow formation and poor stability of its H4B radical.

The interaction of trimethylamine dehydrogenase and electron-transferring flavoprotein

The interaction between the physiological electron transfer partners trimethylamine dehydrogenase (TMADH) and electron-transferring flavoprotein (ETF) from Methylophilus methylotrophus has been examined with particular regard to the proposal that the former protein "imprints" a conformational change on the latter. The results indicate that the absorbance change previously attributed to changes in the environment of the FAD of ETF upon binding to TMADH is instead caused by electron transfer from partially reduced, as-isolated TMADH to ETF. Prior treatment of the as-isolated enzyme with the oxidant ferricenium essentially abolishes the observed spectral change. Further, when the semiquinone form of ETF is used instead of the oxidized form, the mirror image of the spectral change seen with as-isolated TMADH and oxidized ETF is observed. This is attributable to a small amount of electron transfer in the reverse of the physiological direction. Kinetic determination of the dissociation constant and limiting rate constant for electron transfer within the complex of (reduced) TMADH with (oxidized) ETF is reconfirmed and discussed in the context of a recently proposed model for the interaction between the two proteins that involves "structural imprinting" of ETF.

Nature of the Catalytically Labile Oxygen at the Active Site of Xanthine Oxidase

In this paper we report the results of molybdenum K-edge X-ray absorption studies performed on the oxidized active site of xanthine oxidase at pH 6 and 10. These results indicate that the active site possesses one terminal oxygen ligand (Mo=O), two thiolate ligands (Mo-S), one terminal sulfido ligand (Mo=S), and one Mo-OH moiety. EXAFS analysis demonstrates that the Mo-OH bond shortens from 1.97 A at pH 6 to 1.75 A at pH 10, which is consistent with the generation of a Mo-O- moiety. This study provides convincing structural evidence that the catalytic oxygen donor at the oxidized active site of xanthine oxidase is Mo-OH rather than the Mo-OH2 ligation previously suggested by X-ray crystallography. These results support a mechanism initiated by base-assisted nucleophilic attack of the substrate by Mo-OH.

Mechanistic Studies of Rhodobacter sphaeroides Me2SO Reductase

Studies of the molybdenum-containing dimethyl sulfoxide reductase from Rhodobacter sphaeroides have yielded new insight into its catalytic mechanism. A series of reductive titrations, performed over the pH range 6-10, reveal that the absorption spectrum of reduced enzyme is highly sensitive to pH. The reaction of reduced enzyme with dimethyl sulfoxide is found to be clearly biphasic throughout the pH range 6-8 with a fast, initial substrate-binding phase and substrate-concentration independent catalytic phase. The intermediate formed at the completion of the fast phase has the characteristic absorption spectrum of the established dimethyl sulfoxide-bound species. Quantitative reductive and oxidative titrations of the enzyme demonstrate that the molybdenum center takes up only two reducing equivalents, implying that the two pyranopterin equivalents of the molybdenum center are not formally redox active. Finally, the visible spectrum associated with the catalytically relevant "high-g split" Mo(V) species has been determined. Spectral deconvolution and EPR quantitation of enzyme-monitored turnover experiments with trimethylamine N-oxide as substrate reveal that no substrate-bound intermediate accumulates and that Mo(V) content remains near unity for the duration of the reaction. Similar experiments with dimethyl sulfoxide show that significant quantities of both the Mo(V) species and the dimethyl sulfoxide-bound complex accumulate during the course of reaction. Accumulation of the substrate-bound complex in the steady-state with dimethyl sulfoxide arises from partial reversal of the physiological reaction in which the accumulating product, dimethyl sulfide, reacts with oxidized enzyme to yield the substrate-bound intermediate, a process that significantly slows turnover.

Molybdenum-containing hydroxylases

Unlike monooxygenases, molybdenum-containing hydroxylases catalyze the hydroxylation of carbon centers using oxygen derived ultimately from water, rather than O(2), as the source of the oxygen atom incorporated into the product, and do not require an external source of reducing equivalents. The mechanism by which this interesting chemistry takes place has been the subject of investigation for some time, and in the last several years the chemical course of the reaction has become increasingly well understood. The present minireview summarizes recent mechanistic and structure/function studies of members of this large and growing family of enzymes.

The role of active site glutamate residues in catalysis of Rhodobacter capsulatus xanthine dehydrogenase

Xanthine dehydrogenase (XDH) from the bacterium Rhodobacter capsulatus catalyzes the hydroxylation of xanthine to uric acid with NAD+ as the electron acceptor. R. capsulatus XDH forms an (alphabeta)2 heterotetramer and is highly homologous to homodimeric eukaryotic xanthine oxidoreductases. Here we first describe reductive titration and steady state kinetics on recombinant wild-type R. capsulatus XDH purified from Escherichia coli, and we then proceed to evaluate the catalytic importance of the active site residues Glu-232 and Glu-730. The steady state and rapid reaction kinetics of an E232A variant exhibited a significant decrease in both kcat and kred as well as increased Km and Kd values as compared with the wild-type protein. No activity was determined for the E730A, E730Q, E730R, and E730D variants in either the steady state or rapid reaction experiments, indicating at least a 10(7) decrease in catalytic effectiveness for this variant. This result is fully consistent with the proposed role of this residue as an active site base that initiates catalysis.

A mechanism for substrate-Induced formation of 6-hydroxyflavin mononucleotide catalyzed by C30A trimethylamine dehydrogenase

Experiments are described to determine the origin of the 6-hydroxyl group of 6-hydroxyFMN produced by the substrate-induced transformation of FMN in the C30A mutant of trimethylamine dehydrogenase. The conversion of FMN to 6-hydroxyFMN is carried out in the presence of H(2)(18)O and 18O(2), and the results clearly show that the 6-hydroxyl group is derived from molecular oxygen and not from water.

The crystal structure of xanthine oxidoreductase during catalysis: implications for reaction mechanism and enzyme inhibition

Molybdenum is widely distributed in biology and is usually found as a mononuclear metal center in the active sites of many enzymes catalyzing oxygen atom transfer. The molybdenum hydroxylases are distinct from other biological systems catalyzing hydroxylation reactions in that the oxygen atom incorporated into the product is derived from water rather than molecular oxygen. Here, we present the crystal structure of the key intermediate in the hydroxylation reaction of xanthine oxidoreductase with a slow substrate, in which the carbon-oxygen bond of the product is formed, yet the product remains complexed to the molybdenum. This intermediate displays a stable broad charge-transfer band at approximately 640 nm. The crystal structure of the complex indicates that the catalytically labile Mo-OH oxygen has formed a bond with a carbon atom of the substrate. In addition, the MoS group of the oxidized enzyme has become protonated to afford Mo-SH on reduction of the molybdenum center. In contrast to previous assignments, we find this last ligand at an equatorial position in the square-pyramidal metal coordination sphere, not the apical position. A water molecule usually seen in the active site of the enzyme is absent in the present structure, which probably accounts for the stability of this intermediate toward ligand displacement by hydroxide.

Studies on the mechanism of action of xanthine oxidase

Recent studies of the reaction mechanism of the molybdenum-containing enzyme xanthine oxidase are presented. The pH-dependence of both the steady-state and rapid reaction kinetics of the enzyme exhibits is bell-shaped, with pK(a)s for the acid and alkaline limbs of 6.6 and 7.4, respectively. These are assigned to ionizations of an active site base and substrate, respectively, with the implication that enzyme acts on the neutral rather than monoanionic form of the purine substrate. A computational study provides evidence that in the course of the reaction tautomerization of substrate occurs, with a proton moving from N-3 to N-9 in the course of the reaction - enzyme facilitation of this tautomerization may contribute as much as 24 kcal/mol in transition state stabilization for the reaction. Electron spin echo (ESEEM) and electron-nuclear double resonance (ENDOR) studies of the so-called "very rapid" Mo(V) intermediate of the reaction, the latter work using a newly synthesized form of the substrate 2-hydroxy-6-methylpurine that has been selectively isotopically labeled at C-8, indicates that product is bound to the molybdenum of the active site in a simple, end-on fashion, consistent with a reaction mechanism involving nucleophilic attack of a (deprotonated) Mo-OH on the C-8 position of substrate. A kinetic study using a series of purines has failed to identify a correlation between the one-electron reduction potential for substrate and catalytic effectiveness, indicating that a reaction mechanism initiated by one-electron, outer-sphere electron transfer is unlikely. Finally, a consideration of the active site structure in the context of the above work suggests specific amino acid residues to target for site-directed mutagenesis studies. Preliminary experiments with two such mutants are entirely consistent with the proposed catalytic roles of two active site glutamate residues.

Electrochemical Studies of Arsenite Oxidase: An Unusual Example of a Highly Cooperative Two-Electron Molybdenum Center

Arsenite oxidase from Alcaligenes faecalis, an unusual molybdoenzyme that does not exhibit a Mo(V) EPR signal during oxidative-reductive titrations, has been investigated by protein film voltammetry. A film of the enzyme on a pyrolytic graphite edge electrode produces a sharp two-electron signal associated with reversible reduction of the oxidized Mo(VI) molybdenum center to Mo(IV). That reduction or oxidation of the active site occurs without accumulation of Mo(V) is consistent with the failure to observe a Mo(V) EPR signal for the enzyme under a variety of conditions and is indicative of an obligate two-electron center. The reduction potential for the molybdenum center, 292 mV (vs SHE) at pH 5.9 and 0 degrees C, exhibits a linear pH dependence for pH 5-10, consistent with a two-electron reduction strongly coupled to the uptake of two protons without a pK in this range. This suggests that the oxidized enzyme is best characterized as having an L(2)MoO(2) rather than L(2)MoO(OH) center in the oxidized state and that arsenite oxidase uses a "spectator oxo" effect to facilitate the oxo transfer reaction. The onset of the catalytic wave observed in the presence of substrate correlates well with the Mo(VI/IV) potential, consistent with catalytic electron transport that is limited only by turnover at the active site. The one-electron peaks for the iron-sulfur centers are difficult to observe by protein film voltammetry, but spectrophotometric titrations have been carried out to measure their reduction potentials: at pH 6.0 and 20 degrees C, that of the [3Fe-4S] center is approximately 260 mV and that of the Rieske center is approximately 130 mV.

A tetrahydrobiopterin radical forms and then becomes reduced during Nomega-hydroxyarginine oxidation by nitric-oxide synthase

Nitric-oxide synthases are flavoheme enzymes that catalyze two sequential monooxygenase reactions to generate nitric oxide (NO) from l-arginine. We investigated a possible redox role for the enzyme-bound cofactor 6R-tetrahydrobiopterin (H4B) in the second reaction of NO synthesis, which is conversion of N-hydroxy-l-arginine (NOHA) to NO plus citrulline. We used stopped-flow spectroscopy and rapid-freeze EPR spectroscopy to follow heme and biopterin transformations during single-turnover NOHA oxidation reactions catalyzed by the oxygenase domain of inducible nitric-oxide synthase (iNOSoxy). Significant biopterin radical (>0.5 per heme) formed during reactions catalyzed by iNOSoxy that contained either H4B or 5-methyl-H4B. Biopterin radical formation was kinetically linked to conversion of a heme-dioxy intermediate to a heme-NO product complex. The biopterin radical then decayed within a 200-300-ms time period just prior to dissociation of NO from a ferric heme-NO product complex. Measures of final biopterin redox status showed that biopterin radical decay occurred via an enzymatic one-electron reduction process that regenerated H4B (or 5MeH4B). These results provide evidence of a dual redox function for biopterin during the NOHA oxidation reaction. The data suggest that H4B first provides an electron to a heme-dioxy intermediate, and then the H4B radical receives an electron from a downstream reaction intermediate to regenerate H4B. The first one-electron transition enables formation of the heme-based oxidant that reacts with NOHA, while the second one-electron transition is linked to formation of a ferric heme-NO product complex that can release NO from the enzyme. These redox roles are novel and expand our understanding of biopterin function in biology.

Insulin-like growth factor I (IGF-I) stimulates proliferation but also increases caspase-3 activity, Annexin-V binding, and DNA-fragmentation in human MG63 osteosarcoma cells: co-activation of pro- and anti-apoptotic pathways by IGF-I

Insulin-like growth factor-I (IGF-I) was found to promote proliferation, cell survival, and inhibition of apoptosis. But in some instances, IGF-I was found to mildly induce apoptosis, i. e. Fas-mediated apoptosis in human MG63 osteosarcoma cells. In the present study, we intended to further investigate IGF-I dependent pathways leading either to proliferation and cell survival or to cell death. MG63 osteosarcoma cells were treated with serum free medium alone or in combination with IGF-I, a neutralizing antibody against the human IGF-I receptor (alphaIR-3) or non-immune control IgG (1) for two to six days. We investigated cell survival (cell count), proliferation (CD71-FACS), apoptosis (Annexin-V-FACS, Caspase-3 activity, PCD) and anti-apoptosis (112-Ser Bad phosphorylation), and regulation of IGF-I receptor surface expression (IGF-I receptor-FACS). We found that IGF-I treatment (48 h) stimulated cell growth and proliferation, but also mildly induced apoptosis. IGF-I activated specific apoptotic pathways (Caspase-3 activation, Annexin-V binding and DNA degradation), as well as anti-apoptotic signals (Bad phosphorylation at serine 112). alphaIR-3 blocked cell proliferation, strongly induced apoptosis, and inhibited Bad-phosphorylation. Thus, IGF-I treatment overall resulted in increased tumour cell mass, despite a detectable stimulation of apoptosis; in other words proliferation exceeded cell death. If IGF-I was first added on day 0, 2, or 4 of serum free culture, we found decreasing IGF-I specific effects on proliferation and apoptosis. In parallel, we found a down-regulation of IGF-I receptors (FACS) by serum withdrawal, which was partly reversed if IGF-I was added. Therefore receptor number might have an impact on IGF-I function in MG63 cells. In conclusion, co-activation of apoptosis and proliferation by IGF-I might result in higher cell turnover in MG63 osteosarcoma cells. Furthermore, in sarcomas or carcinomas showing clinical association to IGF-I levels and malignancy, IGF-I dependent apoptosis and proliferation could be a significant mechanism of malignant tumour growth.

Structure of tetrahydrobiopterin tunes its electron transfer to the heme-dioxy intermediate in nitric oxide synthase

How 6R-tetrahydrobiopterin (H(4)B) participates in Arg hydroxylation as catalyzed by the nitric oxide synthases (NOSs) is a topic of current interest. Previous work with the oxygenase domain of inducible NOS (iNOSoxy) demonstrated that H(4)B radical formation is kinetically coupled to disappearance of an initial heme-dioxy intermediate and to Arg hydroxylation in a single turnover reaction run at 10 degrees C [Wei, C.-C., Wang, Z.-Q., Wang, Q., Meade, A. L., Hemann, C., Hille, R., and Stuehr, D. J. (2001) J. Biol. Chem. 276, 315-319]. Here we used 5-methyl-H(4)B to investigate how pterin structure influences radical formation and associated catalytic steps. In the presence of Arg, the heme-dioxy intermediate in 5-methyl-H(4)B-bound iNOSoxy reacted at a rate of 35 s(-)(1), which is 3-fold faster than with H(4)B. This was coupled to a faster rate of 5-methyl-H(4)B radical formation (40 vs 12.5 s(-)(1)) and to a faster and more productive Arg hydroxylation. The EPR spectrum of the enzyme-bound 5-methyl-H(4)B radical had different hyperfine structure than the bound H(4)B radical and exhibited a 3-fold longer half-life after its formation. A crystal structure of 5-methyl-H(4)B-bound iNOSoxy revealed that there are minimal changes in conformation of the bound pterin or in its interactions with the protein as compared to H(4)B. Together, we conclude the following: (1) The rate of heme-dioxy reduction is linked to pterin radical formation and is sensitive to pterin structure. (2) Faster heme-dioxy reduction increases the efficiency of Arg hydroxylation but still remains rate limiting for the reaction. (3) The 5-methyl group influences heme-dioxy reduction by altering the electronic properties of the pterin rather than changing protein structure or interactions. (4) Faster electron transfer from 5-methyl-H(4)B may be due to increased radical stability afforded by the N-5 methyl group.