Classical and slow-binding inhibitors of human type II arginase

Arginases catalyze the hydrolysis of L-arginine to yield L-ornithine and urea. Recent studies indicate that arginases, both the type I and type II isozymes, participate in the regulation of nitric oxide production by modulating the availability of arginine for nitric oxide synthase. Due to the reciprocal regulation between arginase and nitric oxide synthase, arginase inhibitors have therapeutic potential in treating nitric oxide-dependent smooth muscle disorders, such as erectile dysfunction. We demonstrate the competitive inhibition of the mitochondrial human type II arginase by N(omega)-hydroxy-L-arginine, the intermediate in the reaction catalyzed by nitric oxide synthase, and its analogue N(omega)-hydroxy-nor-L-arginine, with K(i) values of 1.6 microM and 51 nM at pH 7.5, respectively. We also demonstrate the inhibition of human type II arginase by the boronic acid-based transition-state analogues 2(S)-amino-6-boronohexanoic acid (ABH) and S-(2-boronoethyl)-L-cysteine (BEC), which are known inhibitors of type I arginase. At pH 7.5, both ABH and BEC are classical, competitive inhibitors of human type II arginase with K(i) values of 0.25 and 0.31 microM, respectively. However, at pH 9.5, ABH and BEC are slow-binding inhibitors of the enzyme with K(i) values of 8.5 and 30 nM, respectively. The findings presented here indicate that the design of arginine analogues with uncharged, tetrahedral functional groups will lead to the development of more potent inhibitors of arginases at physiological pH.

Expression, Purification, and Characterization of Human Type II Arginase

Human type II arginase, which is extrahepatic and mitochondrial in location, catalyzes the hydrolysis of arginine to form ornithine and urea. While type I arginases function in the net production of urea for excretion of excess nitrogen, type II arginases are believed to function primarily in the net production of ornithine, a precursor of polyamines, glutamate, and proline. Type II arginases may also regulate nitric oxide biosynthesis by modulating arginine availability for nitric oxide synthase. Recombinant human type II arginase was expressed in Escherichia coli and purified to apparent homogeneity. The Km of arginine for type II arginase is approximately 4.8 mM at physiological pH. Type II arginase exists primarily as a trimer, although higher order oligomers were observed. Borate is a noncompetitive inhibitor of the enzyme, with a Kis of 0.32 mM and a Kii of 0.3 mM. Ornithine, a product of the reaction catalyzed by arginase and a potent inhibitor of type I arginase, is a poor inhibitor of the type II isozyme. The findings presented here indicate that isozyme-selectivity exists between type I and type II arginases for binding of substrate and products, as well as inhibitors. Therefore, inhibitors with greater isozyme-selectivity for type II arginase may be identified and utilized for the therapeutic treatment of smooth muscle disorders, such as erectile dysfunction.

Subunit-subunit interactions in trimeric arginase. Generation of active monomers by mutation of a single amino acid

The structure of the trimeric, manganese metalloenzyme, rat liver arginase, has been previously determined at 2.1-A resolution (Kanyo, Z. F., Scolnick, L. R., Ash, D. E., and Christianson, D. W., (1996) Nature 383, 554-557). A key feature of this structure is a novel S-shaped oligomerization motif at the carboxyl terminus of the protein that mediates approximately 54% of the intermonomer contacts. Arg-308, located within this oligomerization motif, nucleates a series of intramonomer and intermonomer salt links. In contrast to the trimeric wild-type enzyme, the R308A, R308E, and R308K variants of arginase exist as monomeric species, as determined by gel filtration and analytical ultracentrifugation, indicating that mutation of Arg-308 shifts the equilibrium for trimer dissociation by at least a factor of 10(5). These monomeric arginase variants are catalytically active, with k(cat)/K(m) values that are 13-17% of the value for wild-type enzyme. The arginase variants are characterized by decreased temperature stability relative to the wild-type enzyme. Differential scanning calorimetry shows that the midpoint temperature for unfolding of the Arg-308 variants is in the range of 63.6-65.5 degrees C, while the corresponding value for the wild-type enzyme is 70 degrees C. The three-dimensional structure of the R308K variant has been determined at 3-A resolution. At the high protein concentrations utilized in the crystallizations, this variant exists as a trimer, but weakened salt link interactions are observed for Lys-308.

Mechanistic and metabolic inferences from the binding of substrate analogues and products to arginase

Arginase is a binuclear Mn(2+) metalloenzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. X-ray crystal structures of arginase complexed to substrate analogues N(omega)-hydroxy-L-arginine and N(omega)-hydroxy-nor-L-arginine, as well as the products L-ornithine and urea, complete a set of structural "snapshots" along the reaction coordinate of arginase catalysis when interpreted along with the X-ray crystal structure of the arginase-transition-state analogue complex described in Kim et al. [Kim, N. N., Cox, J. D., Baggio, R. F., Emig, F. A., Mistry, S., Harper, S. L., Speicher, D. W., Morris, Jr., S. M., Ash, D. E., Traish, A. M., and Christianson, D. W. (2001) Biochemistry 40, 2678-2688]. Taken together, these structures render important insight on the structural determinants of tight binding inhibitors. Furthermore, we demonstrate for the first time the structural mechanistic link between arginase and NO synthase through their respective complexes with N(omega)-hydroxy-L-arginine. That N(omega)-hydroxy-L-arginine is a catalytic intermediate for NO synthase and an inhibitor of arginase reflects the reciprocal metabolic relationship between these two critical enzymes of L-arginine catabolism.

Probing erectile function: S-(2-boronoethyl)-L-cysteine binds to arginase as a transition state analogue and enhances smooth muscle relaxation in human penile corpus cavernosum

The boronic acid-based arginine analogue S-(2-boronoethyl)-L-cysteine (BEC) has been synthesized and assayed as a slow-binding competitive inhibitor of the binuclear manganese metalloenzyme arginase. Kinetic measurements indicate a K(I) value of 0.4-0.6 microM, which is in reasonable agreement with the dissociation constant of 2.22 microM measured by isothermal titration calorimetry. The X-ray crystal structure of the arginase-BEC complex has been determined at 2.3 A resolution from crystals perfectly twinned by hemihedry. The structure of the complex reveals that the boronic acid moiety undergoes nucleophilic attack by metal-bridging hydroxide ion to yield a tetrahedral boronate anion that bridges the binuclear manganese cluster, thereby mimicking the tetrahedral intermediate (and its flanking transition states) in the arginine hydrolysis reaction. Accordingly, the binding mode of BEC is consistent with the structure-based mechanism proposed for arginase as outlined in Cox et al. [Cox, J. D., Cama, E., Colleluori D. M., Pethe, S., Boucher, J. S., Mansuy, D., Ash, D. E., and Christianson, D. W. (2001) Biochemistry 40, 2689-2701.]. Since BEC does not inhibit nitric oxide synthase, BEC serves as a valuable reagent to probe the physiological relationship between arginase and nitric oxide (NO) synthase in regulating the NO-dependent smooth muscle relaxation in human penile corpus cavernosum tissue that is required for erection. Consequently, we demonstrate that arginase is present in human penile corpus cavernosum tissue, and that the arginase inhibitor BEC causes significant enhancement of NO-dependent smooth muscle relaxation in this tissue. Therefore, human penile arginase is a potential target for the treatment of sexual dysfunction in the male.

Biochemical and functional profile of a newly developed potent and isozyme-selective arginase inhibitor

An increase in arginase activity has been associated with the pathophysiology of a number of conditions, including an impairment in nonadrenergic and noncholinergic (NANC) nerve-mediated relaxation of the gastrointestinal smooth muscle. An arginase inhibitor may rectify this condition. We compared the effects of a newly designed arginase inhibitor, 2(S)-amino-6-boronohexanoic acid (ABH), with the currently available N(omega)-hydroxy-L-arginine (L-HO-Arg), on the NANC nerve-mediated internal anal sphincter (IAS) smooth-muscle relaxation and the arginase activity in the IAS and other tissues. Arginase caused an attenuation of the IAS smooth-muscle relaxations by NANC nerve stimulation that was restored by the arginase inhibitors. L-HO-Arg but not ABH caused dose-dependent and complete reversal of N(omega)-nitro-L-arginine-suppressed IAS relaxation that was similar to that seen with L-arginine. Both ABH and L-HO-Arg caused an augmentation of NANC nerve-mediated relaxation of the IAS. In the IAS, ABH was found to be approximately 250 times more potent than L-HO-Arg in inhibiting the arginase activity. L-HO-Arg was found to be 10 to 18 times more potent in inhibiting the arginase activity in the liver than in nonhepatic tissues. We conclude that arginase plays a significant role in the regulation of nitric oxide synthase-mediated NANC relaxation in the IAS. The advent of new and selective arginase inhibitors may play a significant role in the discrimination of arginase isozymes and have important pathophysiological and therapeutic implications in gastrointestinal motility disorders.

Molecular basis of hyperargininemia: structure-function consequences of mutations in human liver arginase

Hyperargininemia is a rare autosomal recessive disorder that results from a deficiency of hepatic type I arginase. At the genetic level, this deficiency in arginase activity is a consequence of random point mutations throughout the gene that lead to premature termination of the protein or to substitution mutations. Given the high degree of sequence homology between human liver and rat liver enzymes, we have mapped both patient and nonpatient mutations of the human enzyme onto the structure of the rat liver enzyme to rationalize the molecular basis for the low activities of these mutant arginases. Mutations identified in hyperargininemia patients affect the structure and function of the enzyme by compromising active-site residues, packing interactions in the protein scaffolding, and/or quaternary structure by destabilizing the assembly of the arginase trimer.

L-arginine binding to liver arginase requires proton transfer to gateway residue His141 and coordination of the guanidinium group to the dimanganese(II,II) center

Rat liver arginase contains a dimanganese(II,II) center per subunit that is required for catalytic hydrolysis of l-arginine to form urea and l-ornithine. A recent crystallographic study has shown that the Mn2 center consists of two coordinatively inequivalent manganese(II) ions, MnA and MnB, bridged by a water (hydroxide) molecule and two aspartate residues [Kanyo et al. (1996) Nature 383, 554-557]. A conserved residue, His141, is located near the proposed substrate binding region at 4.2 A from the bridging solvent molecule. The present EPR studies reveal that there is no essential alteration of the Mn2 site upon mutation of His141 to an Asn residue, which lacks a potential acid/base residue, while the catalytic activity of the mutant enzyme is 10 times lower vs wild-type enzyme. The binding affinity of l-lysine, l-arginine (substrate), and Nomega-OH-l-arginine (type 2 binders) increases inversely with the pKa of the side chain. Binding of l-lysine is more than 10 times weaker, and the substrate Michaelis constant (Km) is >6-fold greater (weaker binding) in the His141Asn mutant than in wild-type arginase. L-Lysine and Nomega-OH-L-arginine, type 2 binders, induce extensive loss of the EPR intensity, suggesting direct coordination to the Mn2 center. From these data and the pH dependence of type 2 binders, we conclude that His141 functions as the base for deprotonation of the side-chain amino group of L-lysine and the substrate guanidinium group, -NH-C(NH2)2+ and that the unprotonated side chain of these amino acids is responsible for binding to the active site. A different class of inhibitors (type 1), including L-isoleucine, L-ornithine, and L-citrulline, suppresses enzymatic activity, producing only minor change in the zero-field splitting of the Mn2 EPR signal and no change in the EPR intensity, suggestive of minimal conformational transformation. We propose that type 1 alpha-amino acid inhibitors do not bind directly to either Mn ion, but interact with the recognition site on arginase for the alpha-aminocarboxylate groups of the substrate. A new mechanism for the arginase-catalyzed hydrolysis of L-arginine is proposed which has general relevance to all binuclear hydrolases: (1) Deprotonation of substrate l-arginine(H+) by His141 permits entry of the neutral guanidinium group into the buried Mn2 region. Binding of the substrate imino group (>C=NH), most likely to MnB, is coupled to breaking of the MnB-(mu-H2O) bond, forming a terminal aquo ligand on MnA. (2) Proton transfer from the terminal MnA-aqua ligand to the substrate Ndelta-guanidino atom forms the nucleophilic hydroxide on MnA and the cationic NdeltaH2+-guanidino leaving group. Protonation of the substrate -NdeltaH2+-group is likely assisted by hydrogen bonding to the juxtaposed anionic carboxylate group of Glu277. (3) Attack of the MnA-bound hydroxide at the electrophilic guanidino C-atom forms a tetrahedral intermediate. (4) Formation of products is initiated by cleavage of the Cepsilon-NdeltaH2+ bond, yielding urea and L-ornithine(H+).

Purification of a multipotent antideath activity from bovine liver and its identification as arginase: nitric oxide-independent inhibition of neuronal apoptosis

Catalase is an antioxidant enzyme that has been shown to inhibit apoptotic or necrotic neuronal death induced by hydrogen peroxide. We report the purification of a contaminating antiapoptotic activity from a commercial bovine liver catalase preparation by following its ability to inhibit apoptosis when applied extracellularly in multiple death paradigms. The antiapoptotic activity was identified by protein microsequencing as arginase, a urea cycle and nitric oxide synthase-regulating enzyme, and confirmed by demonstrating the presence of antiapoptotic activity in a >97% pure preparation of recombinant arginase. The pluripotency of recombinant arginase was demonstrated by its ability to inhibit apoptosis in multiple paradigms including rat cortical neurons induced to die by glutathione depletion and oxidative stress, by 100 nM staurosporine treatment, or by Sindbis virus infection. The protective effects of arginase in these apoptotic paradigms, in contrast to previous studies on excitotoxic neuronal necrosis, are independent of nitric oxide synthase inhibition. Rather, arginase-induced depletion of arginine leads to inhibition of protein synthesis, resulting in cell survival. Because inhibitors of nitric oxide synthesis and of protein synthesis have been shown to decrease necrotic and apoptotic death, respectively, in animal models of stroke and spinal cord injury, arginine-depleting enzymes, capable of simultaneously inhibiting protein synthesis and nitric oxide generation, may be propitious therapeutic agents for acute neurological diseases. Furthermore, our results suggest caution in attributing the cytoprotective effects of some catalase preparations to catalase.

Structural and functional investigations on the role of zinc in bifunctional rat peptidylglycine alpha-amidating enzyme

Bifunctional peptidylglycine alpha-amidating enzyme (alpha-AE) catalyzes the two-step conversion of C-terminal glycine-extended peptides to C-terminal alpha-amidated peptides and glyoxylate. The first step is the ascorbate-, O2-, and copper-dependent hydroxylation of the alpha-carbon of the glycyl residue, producing an alpha-hydroxyglycine-extended peptide. The second step is the ascorbate-, O2-, and copper-independent dealkylation of the carbinolamide intermediate. We show that alpha-AE requires 1.1 +/- 0. 2 mol of zinc/mol of enzyme for maximal (S)-N-dansyl-Tyr-Val-alpha-hydroxyglycine dealkylation activity. Treatment of the enzyme with EDTA abolishes both the peptide hydroxylation and the carbinolamide dealkylation activities. Addition of Zn(II), Co(II), Cd(II), and Mn(II) partially restores carbinolamide dealkylation activity to the EDTA-treated enzyme. Addition of Co(II) produces the greatest restoration of dealkylation activity, 32% relative to a control not treated with EDTA, while Mn(II) addition results in the smallest restoration of dealkylation activity, only 3% relative to an untreated control. The structure and coordination of the zinc center has been investigated by X-ray absorption spectroscopy. EXAFS data are best interpreted by an average coordination of 2-3 histidine ligands and 1-2 non-histidine O/N ligands. Since catalytic zinc centers in other zinc metalloenzymes generally exhibit only O/N ligands to the zinc atom, a zinc-bound water or hydroxide may serve as a general base for the abstraction of the hydroxyl proton from the carbinolamide intermediate. Alternatively, the zinc may function in a structural role.

VO2+(IV) complexes with pyruvate carboxylase: activation of oxaloacetate decarboxylation and EPR properties of enzyme-VO2+ complexes

Chicken liver pyruvate carboxylase catalyzes a nonclassical ping-pong mechanism in which the carboxylation of biotin at subsite 1 of the active site is coupled to the biotin-dependent carboxylation of pyruvate at subsite 2. The functions of two divalent cation cofactors and at least one monovalent cation cofactor in catalysis are not well understood. The oxyvanadyl cation, VO2+ does not support phosphoryl transfer at the first subsite, and uncouples the decarboxylation of oxaloacetate at subsite 2 from the formation of ATP at subsite 1. Stimulation of this oxaloacetate decarboxylase activity in the presence of substrates and cofactors of the first subsite, including VO2+, VOADP-, Pi, and acetyl CoA, suggests that these cofactors and substrates induce the movement of carboxybiotin from the second subsite to the first subsite, where it is decarboxylated. VO2+ EPR has provided evidence for enzymic and nucleotide divalent cation binding sites within the first subsite. The EPR properties of enzyme bound VO2+ were altered by bicarbonate, suggesting that this substrate ligands directly to VO2+ at the enzymic metal site. Fluorescence quenching experiments suggest that a monovalent cation may interact with bicarbonate at the first subsite as well. The results of this study provide evidence that (i) the extrinsic metal ion cofactors interact with the substrates at the first subsite, and that (ii) divalent cations play a role in coupling catalysis at the two nonoverlapping subsites by inducing the decarboxylation of carboxybiotin at the first subsite.

Altering the binuclear manganese cluster of arginase diminishes thermostability and catalytic function

Arginase is a thermostable (Tm = 75 degrees C) binuclear manganese metalloenzyme which hydrolyzes l-arginine to form l-ornithine and urea. The three-dimensional structures of native metal-depleted arginase, metal-loaded H101N arginase, and metal-depleted H101N arginase have been determined by X-ray crystallographic methods to probe the roles of the manganese ion in site A (Mn2+A) and its ligand H101 in catalysis and thermostability. We correlate these structures with thermal stability and catalytic activity measurements reported here and elsewhere [Cavalli, R. C., Burke, C. J., Kawamoto, S., Soprano, D. R., and Ash, D. E. (1994) Biochemistry 33, 10652-10657]. We conclude that the substitution of a wild-type histidine ligand to Mn2+A compromises metal binding, which in turn compromises protein thermostability and catalytic function. Therefore, a fully occupied binuclear manganese metal cluster is required for optimal catalysis and thermostability.

EXAFS comparison of the dimanganese core structures of manganese catalase, arginase, and manganese-substituted ribonucleotide reductase and hemerythrin

The solution structures of the binuclear Mn centers in arginase, Mn catalase, and the Mn-substituted forms of the Fe enzymes ribonucleotide reductase and hemerythrin have been determined using X-ray absorption spectroscopy (XAS). X-ray absorption near edge structure (XANES) spectra for these proteins were compared to those obtained for Mn(II) models. The Mn model spectra show an inverse correlation between the XANES peak maximum and the root-mean-square (RMS) deviation in metal-ligand bond lengths. For these complexes, the XANES maxima appear to be more effective than the 1s --> 3d areas as an indicator of metal-site symmetry. Arginase and Mn-substituted ribonucleotide reductase have symmetric nearest neighbor environments with low RMS deviation in bond length, while Mn catalase and Mn-substituted hemerythrin appear to have a larger RMS bond length deviation. The 1s --> 3d areas for arginase and Mn-substituted ribonucleotide reductase are consistent with six coordinate Mn, while the 1s --> 3d areas for Mn catalase and Mn-substituted hemerythrin are larger, suggesting that one or both of the Mn ions are five-coordinate in these proteins. Extended x-ray absorption fine structure (EXAFS) spectra were used to determine the Mn2 core structure for the four proteins. In order to quantitate the number of histidine residues bound to the Mn2 centers, EXAFS data for the crystallographically characterized model hexakis-imidazole Mn(II) dichloride tetrahydrate were used to calibrate the Mn-imidazole multiple scattering interactions. These calibrated parameters allowed the outer shell EXAFS to be fit to give a lower limit on the number of bound histidine residues. The EXAFS spectra for Mn-substituted ribonucleotide reductase and arginase are nearly identical, with symmetric Mn-nearest neighbor environments and outer shell scattering consistent with a lower limit of one histidine per Mn2 core. In contrast, the EXAFS data for Mn catalase and Mn-substituted hemerythrin show two distinct Mn-nearest neighbor shells, modeled as Mn-O at ca. 2.1 A and Mn-N at ca. 2.3 A, and outer shell carbon scattering consistent with a lower limit of ca. 2-3 His residues per Mn2 core. Only Mn catalase shows clear evidence for Mn...Mn scattering. The observed Mn...Mn distance is 3.53 A, which is significantly longer than the approximately 3.3 A distances that are typically observed for Mn(II)2 cores with two single atom bridges, but which is typical of the distances seen in Mn(II)2 cores having one single atom bridge (e.g., aqua or hydroxo) together with one or two carboxylate bridges. The absence of EXAFS-detectable Mn...Mn interactions for the other three proteins suggests either that there are no single atom bridges in these cases or that the Mn...Mn interactions are more disordered.

Structure of a unique binuclear manganese cluster in arginase

Each individual excretes roughly 10 kg of urea per year, as a result of the hydrolysis of arginine in the final cytosolic step of the urea cycle. This reaction allows the disposal of nitrogenous waste from protein catabolism, and is catalysed by the liver arginase enzyme. In other tissues that lack a complete urea cycle, arginase regulates cellular arginine and ornithine concentrations for biosynthetic reactions, including nitric oxide synthesis: in the macrophage, arginase activity is reciprocally coordinated with that of NO synthase to modulate NO-dependent cytotoxicity. The bioinorganic chemistry of arginase is particularly rich because this enzyme is one of very few that specifically requires a spin-coupled Mn2+-Mn2+ cluster for catalytic activity in vitro and in vivo. The 2.1 angstrom-resolution crystal structure of trimeric rat liver arginase reveals that this unique metal cluster resides at the bottom of an active-site cleft that is 15 angstroms deep. Analysis of the structure indicates that arginine hydrolysis is achieved by a metal-activated solvent molecule which symmetrically bridges the two Mn2+ ions.

Chemical modification and inactivation of rat liver arginase by N-bromosuccinimide: reaction with His141

Treatment of rat liver arginase with N-bromosuccinimide results in modification of six tryptophan residues per enzyme molecule and is accompanied by loss of catalytic activity (E. Ber and G. Muzynska (1979) Acta Biochim. Pol. 26, 103-114). In order to probe the chemistry of N-bromosuccinimide inactivation and the role of tryptophan residues in catalysis, the two tryptophan residues of rat liver arginase, Trp122 and Trp164, have been separately mutated to phenylalanine using site-directed mutagenesis of the protein expressed in Escherichia coli. Both single Trp -> Phe mutant enzymes have kinetic parameters nearly identical to those for the wild-type enzyme. Treatment of native, wild-type, and each of the Trp -> Phe mutant enzymes with N-bromosuccinimide results in loss of absorbance at 280 nm and is accompanied by a loss of catalytic activity. However, treatment of the wild-type enzyme with N-bromosuccinimide in the presence of the arginase inhibitors NG-hydroxy-L-arginine or the combination of L-ornithine and borate protects against inactivation, even though tryptophan residues are modified. Treatment of the H101N and H126N mutant arginases with N-bromosuccinimide also results in loss of catalytic activity and modification of tryptophan residues. In contrast, the H141N mutant arginase is not inactivated by N-bromosuccinimide, indicating that His141 is the critical target for the N-bromosuccinimide inactivation of the enzyme.

The irreversible inactivation of two copper-dependent monooxygenases by sulfite: peptidylglycine alpha-amidating enzyme and dopamine beta-monooxygenase

Peptidylglycine alpha-amidating enzyme (alpha-AE) and dopamine beta-monooxygenase (D beta M), two copper-dependent monooxygenases that have catalytic and structural similarities, are irreversibly inactivated by sodium sulfite in a time- and concentration-dependent manner. Studies with alpha-AE show that the sulfite-mediated inactivation is dependent on the presence of redox active transition metals free in solution, with Cu(II) being the most effective in supporting the inactivation reaction. Sulfite inactivation of alpha-AE is specific for the monooxygenase reaction of this bifunctional enzyme and amidated peptides provide protection against the inactivation. Consequently, the sulfite-mediated inactivation of alpha-AE and D beta M most likely results from the transition metal-catalyzed oxidation of sulfite to the sulfite radical, SO3-.

The inactivation of bifunctional peptidylglycine alpha-amidating enzyme by benzylhydrazine: evidence that the two enzyme-bound copper atoms are nonequivalent

Peptidylglycine alpha-amidating enzyme catalyzes the two-step conversion of C-terminal glycine-extended peptides to C-terminal alpha-amidated peptides and glyoxylate in a reaction that requires O2, ascorbate and 2 mol of copper per mole of enzyme [Kulathila et al. (1994) Arch. Biochem. Biophys. 311, 191-195]. Peptides with a C-terminal alpha-hydroxyglycine residue are intermediates in the amidation reaction. Benzylhydrazine inactivates the enzymatic conversion of dansyl-Tyr-Val-Gly to dansyl-Tyr-Val-NH2 in a time- and concentration-dependent manner. In contrast, the enzymatic conversion of dansyl-Tyr-Val-alpha-hydroxyglycine to dansyl-Tyr-Val-NH2 is unaffected by benzylhydrazine. The plot of 1/(inactivation rate) vs 1/[benzylhydrazine] is parabolic, indicating that the inactivation results from the interaction of 2 mol of benzylhydrazine per mole of enzyme. EPR spectra obtained from benzylhydrazine inactivation reactions carried out in the presence of a radical trap, alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone, show the formation of a carbon-centered benzyl radical. The benzyl radical most likely results from redox chemistry between benzylhydrazine and the enzyme-bound Cu(II) ions because EPR studies show that enzyme-bound Cu(II) is reduced to Cu(I) in the presence of benzylhydrazine. The kinetic constants for benzylhydrazine as a reductant in the amidation reaction were determined at benzylhydrazine concentrations too low to cause significant enzyme inactivation. Mimosine exhibits mixed inhibition vs benzylhydrazine; however, previous results have shown that benzylhydrazine is competitive vs ascorbate [Miller et al. (1992) Arch. Biochem. Biophys. 298, 380-388]. This change in kinetic mechanism coupled with the nonlinear inactivation kinetics have lead to a proposal that the two enzyme-bound Cu(II) atoms are nonequivalent with respect to their reduction by benzylhydrazine.

Determination of the metal ion separation and energies of the three lowest electronic states of dimanganese (II,II) complexes and enzymes: catalase and liver arginase

The dimanganese (II,II) catalase from Thermus thermophilus, MnCat(II,II), arginase from rat liver, Arg(II,II), and several dimanganese(II,II) compounds, LMn2XY2, which are functional catalase mimics, all possess a pair of coupled Mn(II) ions in their catalytic sites. For each of these, we have measured by EPR spectroscopy the relative energies separating the three lowest electronic states (singlet, triplet, and quintet), described a general method for extracting the individual spectra for these states by multicomponent analysis, and determined the Mn-Mn separation. The triplet-singlet and quintet-singlet energy gaps were modeled well by fitting the temperature dependence of the EPR intensities to a Boltzmann expression for a pair of Mn(II) ions coupled by isotropic Heisenberg spin exchange (-2JS1S2). This dependence indicates diamagnetic ground states with delta E10 (cm-1) = magnitude of 2J = 4 and 11.2 cm-1 for Arg-(II,II)(+borate) and MnCat(II,II)(phosphate), respectively. This large difference in magnitude of 2J reflects either a difference in the bridging ligands or, possibly, a weaker ligand field (larger ionization potential) for the Mn(II) ions in arginase. In n-butanol/CH2Cl2 the triplet-singlet energy gaps for [LMn2(CH3CO2)](C1O4)2 (1), [LMn2(CH3CO2)3] (2), and [LMn2Cl3] (3), where HL = N,N,N',N'-tetrakis(2-methylenebenzimidazole)-1,3-diaminopropan+ ++-2-ol, are 23-24 cm-1. Comparison of the Heisenberg exchange interaction constants for more than 30 dimanganese(II,II) complexes suggests a possible bridging structure of (mu-OH)(mu-carboxylate)1-2 for MnCat(II,II), while the 3-fold weaker coupling in Arg(II,II) suggests mu-aqua in place of mu-hydroxide. EPR spectra of both the triplet and quintet electronic states were extracted and found to exhibit zero-field splittings (ZFS) and resolved 55Mn hyperfine splittings indicating spin-coupled Mn2-(II,II) species. The major ZFS interaction could be attributed to the magnetic dipole-dipole interaction between the Mn(II) ions. A linear correlation is observed between the crystallographically determined Mn-Mn distance and the ZFS of the quintet state (D2) for five dimanganese pairs for which both data sets are available. Using this correlation, the Mn-Mn distance in Arg(II,II) is predicted to be 3.36-3.57 A for the native enzyme (multiple forms) and 3.59 A for MnCat(II,II)(phosphate). Addition of the inhibitor borate to Arg(II,II) simplifies the ZFS, indicative of conversion to a single species with mean Mn-Mn separation of 3.50 A. The second metal ion in dinuclear complexes possessing a shared bridging ligand has been shown to attenuate the strength of the mu-ligand field potential, as monitored by the strength of the single ion ZFS.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutagenesis of rat liver arginase expressed in Escherichia coli: role of conserved histidines

Rat liver arginase has been overexpressed in Escherichia coli using a T7-based expression system. The kinetic properties of the recombinant wild-type protein are essentially identical to those of the native rat liver enzyme. The recombinant wild-type protein contains six Mn(II) ions per trimer, in good agreement with results obtained with the fully active native enzyme. However, in contrast to the native enzyme which loses three Mn(II) per trimer upon extended dialysis, the recombinant protein binds Mn(II) tenaciously, and retains six Mn(II) per trimer even after extensive dialysis. Three histidine residues, corresponding to His101, His126, and His141 in the rat liver enzyme, are highly conserved in arginases from evolutionarily divergent species. The replacement of His101 and His126 with Asn by site-directed mutagenesis produced only modest effects on enzymatic activity when measured in the presence of Mn(II) ions. However, EDTA treatment of these mutant enzymes reduced activity to < 0.2% of that for the wild-type enzyme. The activity of wild-type enzyme and the His141 Asn mutant was unaffected by treatment with EDTA. Thus, His101 and His126 are proposed to be ligands to the binuclear Mn(II) center of the enzyme. The His141 Asn mutation produced an enzyme which, in contrast to the native, wild-type, His101 Asn, and His126 Asn arginases, was not inactivated by diethyl pyrocarbonate. These results suggest a catalytic role for His141.

Inhibition of rat liver arginase by an intermediate in NO biosynthesis, NG-hydroxy-L-arginine: implications for the regulation of nitric oxide biosynthesis by arginase

NG-hydroxy-L-arginine, an intermediate in the biosynthesis of nitric oxide (NO), has been found to be a uniquely potent competitive inhibitor of rat liver arginase. Among previously reported inhibitors of arginase and the eight arginine analogs tested herein, only NG-hydroxy-L-arginine was found to be strongly inhibitory. Significantly, the Ki (42 microM) for inhibition of rat liver arginase by NG-hydroxy-L-arginine was found to be 20-40-fold lower than the KM (1-1.7 mM) for its natural substrate, L-arginine. Since NG-hydroxy-L-arginine is the only known intermediate in the biosynthesis of NO from L-arginine, this finding may have significant implications for the regulation of NO levels in tissues or cells, such as liver or macrophages, which synthesize both NO and contain arginase.

Rat liver arginase: kinetic mechanism, alternate substrates, and inhibitors

The activity of guanidino compounds as alternate substrates for rat liver arginase is critically dependent on the length of the amino acid side chain and the substituents about C-alpha. In addition to L-arginine, the enzyme catalyzes the hydrolysis of L-argininamide, L-canavanine, L-homoarginine, L-argininic acid, and agmatine. The kcat values for these substrates are 15- to 5000-fold slower than the kcat for L-arginine. Guanidobutyrate, D-arginine, and NG-methyl-L-arginine are not substrates. Competitive inhibition by the products L-ornithine and urea indicates a rapid-equilibrium random mechanism for the enzyme. Despite the requirement for added divalent cations in the activation of the enzyme, metal chelators such as EDTA and citrate do not inhibit the enzyme. These results suggest that the metal site is not readily accessible to solvent. Multiple inhibition experiments with the noncompetitive inhibitor borate demonstrate that borate and urea bind in a mutually exclusive manner, while L-ornithine and borate can bind simultaneously to the enzyme. Borate inhibition is proposed to arise from chelation of Mn(II) in the binuclear Mn(II) center, thus displacing a metal-bound water molecule that is responsible for nucleophilic attack on the guanidium carbon.

Threonine synthase of Escherichia coli: inhibition by classical and slow-binding analogues of homoserine phosphate

L-threo-3-Hydroxyhomoserine phosphate, derived from the antimetabolites L-threo-3-hydroxyaspartate and L-threo-3-hydroxyhomoserine [Shames, S. L., Ash, D. E., Wedler, F. C., and Villafranca, J. J. (1984) J. Biol. Chem. 258, 15331-15339], is a classical competitive inhibitor of threonine synthase (Ki = 6 microM) with structural elements of both substrate and product. L-2-Amino-5-phosphonovaleric acid also inhibits the enzyme competitively with a Ki (31 microM), comparable to Km for L-homoserine phosphate. In contrast, a structural analogue of Hse-P, L-2-amino-3-[(phosphonomethyl)thio]propionic acid exhibits a Ki = 0.11 microM (ca. 100-fold less than Km for L-Hse-P), along with "slow, tight" inhibition kinetics. Nuclear magnetic resonance was used with these inhibitors to probe for pyridoxal phosphate-catalyzed hydrogen-deuterium exchange reactions characteristic of substrates. With L-threo-3-hydroxy-homoserine phosphate, H-D exchange occurs only at the C-alpha position, but for homoserine in the presence of phosphate and for L-2-amino-5-phosphonovaleric acid and L-amino-3[(phosphonomethyl)thio]propionic acid (APMTP), H-D exchange occurs at C-alpha and stereospecifically at C-beta. For L-homoserine plus phosphate and L-2-amino-5-phosphonovaleric acid, the rate of H-D exchange at C-alpha is 8-45 times faster than at C-beta. For L-2-amino-3-[(phosphonomethyl)thio]propionic acid, the C-alpha to C-beta exchange rate ratio is near unity, due to a 700-fold decrease in the C-alpha rate for the analogue. Taken with information from molecular modeling, these data can be interpreted in terms of the current working hypothesis for the catalytic mechanism. Specifically, the slow, tight inhibition by APMTP results from its being carried further into the catalytic cycle than other analogues prior to forming an intermediate that is blocked from further catalysis.

Chemical modifications of chicken liver pyruvate carboxylase: evidence for essential cysteine-lysine pairs and a reactive sulfhydryl group

Inactivations of chicken liver pyruvate carboxylase with N-(7-dimethylamino-4-methyl-3-coumarinyl)maleimide (DACM) and o-phthalaldehyde (o-PA) have identified cysteine and lysine residues that are essential for catalytic activity. Protection experiments suggest that the modified residues are located in or near the first and second subsites. At a one- to two-fold molar excess over active site concentration, DACM inactivated approximately 80-90% of the pyruvate carboxylase and ADP/Pi linked oxaloacetate decarboxylase activities by forming a sulfhydryl-DACM adduct with a fluorescence excitation maximum at 385 nm and an emission maximum at 476 nm. o-PA reacted with the enzyme by cross-linking lysine and cysteine residues to form an inactive isoindole-enzyme derivative with a fluorescence excitation maximum at 337 nm and an emission maximum at 415 nm. Incorporation of one equivalent of either DACM or isoindole derivative resulted in an 80-90% decrease in all activities involving chemistry at the first subsite, suggesting that the modification of a sulfhydryl group or a cysteine-lysine ion pair in or near the first subsite inactivates the enzyme. A cysteine-lysine ion pair in the first subsite could function to remove the N-1 proton of biotin to yield enol-biotin, which could be readily carboxylated by the carboxyphosphate intermediate. In the reverse direction, a cysteine-lysine ion pair in or near the second subsite has been proposed to enolize biotin prior to carboxylation by oxaloacetate (P. V. Attwood and W. W. Cleland, 1986, Biochemistry 25, 8197-8205). Enzyme modified with 2 equivalents of isoindole retained only 7% of the oxamate-induced, ADP/Pi-independent oxaloacetate decarboxylase activity, suggesting that there is at least one essential cysteine-lysine ion pair at or near the second subsite.

18O isotopic 13C NMR shift as proof that bifunctional peptidylglycine alpha-amidating enzyme is a monooxygenase

The biosynthesis of C-terminal alpha-amidated peptides from their corresponding C-terminal glycine-extended precursors is catalyzed by peptidylglycine alpha-amidating enzyme (alpha-AE) in a reaction that requires copper, ascorbate, and molecular oxygen. Using bifunctional type A rat alpha-AE, we have shown that O2 is the source of the alpha-carbonyl oxygen of pyruvate produced during the amidation of dansyl-Tyr-Val-[alpha-13C]-D-Ala, as demonstrated by the 18O isotopic shift in the 13C NMR spectrum of [alpha-13C]lactate generated from [alpha-13C]pyruvate in the presence of lactate dehydrogenase and NADH. In addition, one-to-one stoichiometries have been determined for glyoxylate formed/dansyl-Tyr-Val-Gly consumed, pyruvate formed/dansyl-Tyr-Val-D-Ala consumed, dansyl-Tyr-Val-NH2 formed/ascorbate oxidized, and dansyl-Tyr-Val-NH2 formed/O2 consumed. Quantitative coupling of NADH oxidation to dansyl-Tyr-Val-NH2 production using Neurospora crassa semidehydroascorbate reductase showed that two one-electron reductions by ascorbate occurred per alpha-AE turnover. The stoichiometry of approximately 1.0 dansyl-Tyr-Val-NH2 produced/ascorbate oxidized observed in the absence of a semidehydroascorbate trap resulted from the disproportionation of two semidehydroascorbate molecules to ascorbate and dehydroascorbate.

Crystallization and oligomeric structure of rat liver arginase

Rat liver arginase, a manganese-metalloenzyme, has been crystallized from polyethylene glycol 8000 in N,N-bis(2-hydroxyethyl)glycine (Bicine) buffer at pH 8.5. Crystals form as either cubes or pyramids and belong to space group P3(1) (or P3(2)) with hexagonal unit cell dimensions a = b = 88.9 A, c = 114.8 A, or a = b = 88.5 A, c = 104.5 A; the variation along the c axis does not correlate with the external crystal morphology of cube or pyramid-shaped. X-ray diffraction data are measured to a limiting resolution of 2.4 A. Given the volume constraints of the unit cell it is likely that rat liver arginase is a trimer, with three 35,000 Da monomers in the asymmetric unit. This resolves a persistent ambiguity regarding the oligomeric structure of this enzyme.

Inactivation of chicken mitochondrial phosphoenolpyruvate carboxykinase by o-phthalaldehyde

Chicken liver mitochondrial phosphoenolpyruvate carboxykinase is inactivated by o-phthalaldehyde. The inactivation followed pseudo first-order kinetics, and the second-order rate constant for the inactivation process was 29 M-1 s-1 at pH 7.5 and 25 degrees C. The modified enzyme showed maximal fluorescence at 427 nm upon excitation at 337 nm, consistent with the formation of isoindole derivatives by the cross-linking of proximal cysteine and lysine residues. Activities in the physiologic reaction and in the oxaloacetate decarboxylase reaction were lost in parallel upon modification with o-phthalaldehyde. Plots of (percent of residual activity) versus (mol of isoindole incorporated/mol of enzyme) were biphasic, with the initial loss of enzymatic activity corresponding to the incorporation of one isoindole derivative/enzyme molecule. Complete inactivation of the enzyme was accompanied by the incorporation of 3 mol of isoindole/mol of enzyme. beta-Sulfopyruvate, an isoelectronic analogue of oxaloacetate, completely protected the enzyme from reacting with o-phthalaldehyde. Other substrates provided protection from inactivation, in decreasing order of protection: oxaloacetate greater than phosphoenolpyruvate greater than MgGDP, MgGTP greater than oxalate. Cysteine 31 and lysine 39 have been identified as the rapidly reacting pair in isoindole formation and enzyme inactivation. Lysine 56 and cysteine 60 are also involved in isoindole formation in the completely inactivated enzyme. These reactive cysteine residues do not correspond to the reactive cysteine residue identified in previous iodoacetate labeling studies with the chicken mitochondrial enzyme (Makinen, A. L., and Nowak, T. (1989) J. Biol. Chem. 264, 12148-12157). Protection experiments suggest that the sites of o-phthalaldehyde modification become inaccessible when the oxaloacetate/phosphoenolpyruvate binding site is saturated, and sequence analyses indicate that cysteine 31 is located in the putative phosphoenolpyruvate binding site.

Mammalian and avian liver phosphoenolpyruvate carboxykinase. Alternate substrates and inhibition by analogues of oxaloacetate

Phosphoenolpyruvate carboxykinase from chicken liver mitochondria and rat liver cytosol catalyzes the phosphorylation of alpha-substituted carboxylic acids such as glycolate, thioglycolate, and DL-beta-chlorolactate in reactions with absolute requirements for divalent cation activators. 31P NMR analysis of the reaction products indicates that phosphorylation occurs at the alpha-position to generate the corresponding O- or S-bridged phosphate monoesters. In addition, the enzymes catalyze the bicarbonate-dependent phosphorylation of hydroxylamine. The chicken liver enzyme also catalyze the bicarbonate-dependent phosphorylation of hydroxylamine. The chicken liver enzyme also catalyzes the bicarbonate-dependent phosphorylation of fluoride ion. The kappa cat values for these substrates are 20-1000-fold slower than the kappa cat for oxaloacetate. Pyruvate and beta-hydroxypyruvate are not phosphorylated, since the enzyme does not catalyze the enolization of these compounds. Oxalate, a structural analogue of the enolate of pyruvate, is a competitive inhibitor of phosphoenolpyruvate carboxykinase (Ki of 5 microM) in the direction of phosphoenolpyruvate formation. Oxalate is also an inhibitor of the chicken liver enzyme in the direction of oxaloacetate formation and in the decarboxylation of oxaloacetate. The chicken liver enzyme is inhibited by beta-sulfopyruvate, an isoelectronic analogue of oxaloacetate. The extensive homologies between the reactions catalyzed by phosphoenolpyruvate carboxykinase and pyruvate kinase suggest that the divalent cation activators in these reactions may have similar functions. The substrate specificity indicates that phosphoenolpyruvate carboxykinase decarboxylates oxaloacetate to form the enolate of pyruvate which is then phosphorylated by MgGTP on the enzyme.

Coordination of manganous ion at the active site of pyruvate, phosphate dikinase: the complex of oxalate with the phosphorylated enzyme

Electron paramagnetic resonance spectroscopy has been used to investigate the structure of the complex of manganous ion with the phosphorylated form of pyruvate,phosphate dikinase (Ep) and the inhibitor oxalate. Oxalate, an analogue of the enolate of pyruvate, is competitive with respect to pyruvate in binding to the phosphorylated form of the enzyme [Michaels, G., Milner, Y., & Reed, G.H. (1975) Biochemistry 14, 3213-3219]. Superhyperfine coupling between the unpaired electrons of Mn(II) and ligands specifically labeled with 17O has been used to identify oxygen ligands to Mn(II) in the complex with oxalate and the phosphorylated form of the enzyme. Oxalate binds at the active site as a bidentate chelate with Mn(II). An oxygen from the 3'-N-phosphohistidyl residue of the protein is in the coordination sphere of Mn(II), and at least two water molecules are also bound to Mn(II) in the complex. Oxalate also binds directly to Mn(II) in a complex with nonphosphorylated enzyme. The structure for the Ep-Mn(II)-oxalate complex implies that simultaneous coordination of a phospho group and of the attacking nucleophile to the divalent cation is likely an important factor in catalysis of this phospho-transfer reaction.

Biosynthesis of delta-aminolevulinate in greening barley leaves. IX. Structure of the substrate, mode of gabaculine inhibition, and the catalytic mechanism of glutamate 1-semialdehyde aminotransferase

Glutamic acid 1-semialdehyde hydrochloride was synthesized and purified. Its prior structural characterization was extended and confirmed by 1H NMR spectroscopy and chemical analyses. In aqueous solution at pH 1 to 2 glutamic acid 1-semialdehyde exists in a stable hydrated form, but at pH 8.0 it has a half-life of 3 to 4 min. Spontaneous degradation of the material at pH 8.0 generated some undefined condensation products, but coincidentally a significant amount isomerized to 5-aminolevulinate. At pH 6.8 to 7.0, glutamate 1-semialdehyde is sufficiently stable to permit routine and reproducible assay for glutamate 1-semialdehyde aminotransferase activity. Only about 20% of the enzyme extracted from chloroplasts was sensitive to inactivation by gabaculine with no pretreatment. However, when the enzyme was exposed to 5-aminolevulinate, levulinate or 4,5-dioxovalerate in the absence of glutamate 1-semialdehyde, it was completely inactivated by gabaculine; 4,6-dioxoheptanoate had no effect on the enzyme. These results lead to the hypothesis that the aminotransferase exists in the chloroplast in a complex with pyridoxamine phosphate, which must be converted to the pyridoxal form before it can form a stable adduct with gabaculine. We propose that the enzyme catalyzes the conversion of glutamate 1-semialdehyde to 5-aminolevulinate via 4,5-diaminovalerate.

Characterization of highly purified dopamine beta-hydroxylase

A modified purification procedure has been developed for dopamine beta-hydroxylase isolated from bovine adrenal medulla. Catalase is included in the homogenization step starting with a suspension of either chromaffin granules or adrenal medulla tissue. With this precaution, the enzyme remains stable in the supernatant solution in preparation for the subsequent purification step involving concanavalin A-Sepharose chromatography. The homogeneous enzyme has a specific activity in the range of 60-70 mumol O2 consumed/min/mg. Using radiolabeled metal ion chelators, it was determined that several of the chelators remained tightly bound to the enzyme after removal of the copper leading to difficulties in establishing stoichiometry of enzyme-bound metal ions.

Interaction of aspartate and aspartate-derived antimetabolites with the enzymes of the threonine biosynthetic pathway of Escherichia coli

The five enzymes responsible for the conversion of L-aspartate to L-threonine in Escherichia coli were purified to homogeneity and subsequently reconstituted in vitro in ratios approximating those found in vivo. 31P NMR was used to conveniently monitor the rates of consumption of the substrates ATP and NADPH, the accumulation of the intermediates beta-aspartyl phosphate and homoserine phosphate, and the formation of the products ADP, NADP+, and Pi in a single experiment. By this method, the flux of aspartic acid through the enzymes of the pathway was monitored in the absence and in the presence of several alternative substrates and inhibitors. Several known antimetabolites were found to be alternative substrates that ultimately became inhibitors of pathway flux. L-threo-3-Hydroxyaspartic acid was converted to 3-hydroxyhomoserine phosphate by the first four enzymes of the pathway. The antimetabolite L-threo-3-hydroxyhomoserine was found to bind to and inhibit aspartokinase-homoserine dehydrogenase I in a cooperative fashion (I 0.5 = 3 mM, nH = 2.5), similar to the action of the allosteric end product inhibitor L-threonine (I 0.5 = 0.36 mM, nH = 2.4). In the presence of the remaining enzymes of the pathway, however, L-threo-3-hydroxyhomoserine was phosphorylated to the apparent ultimate antimetabolite L-threo-3-hydroxyhomoserine phosphate that was a potent inhibitor of threonine synthase and consequently of L-threonine biosynthesis. When aspartic acid alone was examined as a substrate of the enzymes of the pathway, no accumulation of the beta-aspartyl phosphate and homoserine phosphate intermediates was observed. However, in the presence of either 5 mM L-threo-3-hydroxyhomoserine or 5 mM L-threo-3-hydroxyhomoserine phosphate, homoserine phosphate was found to accumulate. In contrast to the homoserine phosphate and 3-hydroxyhomoserine phosphate intermediates, both of which were very stable, the acylphosphate intermediates beta-aspartyl phosphate and beta-3-hydroxyaspartyl phosphate were highly susceptible to hydrolysis, with first-order rate constants of 4.6 X 10(-3) min-1 and 4.5 X 10(-2) min-1 (pH 7.8, 25 degrees C), respectively.

Kinetic and spectroscopic studies of the interaction of copper with dopamine beta-hydroxylase

The question of the stoichiometry of copper bound to dopamine beta-hydroxylase and the number of copper atoms required for maximal activity was addressed in this study. Incubation of tetrameric enzyme from bovine adrenal medulla with 64Cu2+ followed by rapid gel filtration yielded an enzyme containing 8.3-8.9 mol of Cu/mol of tetramer. An identical stoichiometry was obtained by analysis of bound copper by atomic absorption methods. NMR and EPR were used to monitor titrations of the enzyme with Cu2+ and showed that the longitudinal relaxation rate of solvent water protons and the amplitude of the signal at g approximately 2 increased linearly up to a copper to protein ratio of approximately 8. Additional titrations also indicate that an enzyme-Cu2+-tyramine-CN- inhibitory complex was formed when 8 mol of Cu2+ are bound per mol of enzyme. The rate of inactivation of dopamine beta-hydroxylase by the mechanism-based inhibitor 2-Br-3-(p-hydroxyphenyl)-1-propene was measured and used as a method to follow enzymatic catalysis. An increase in rate was observed with increasing Cu2+ up to a protein to Cu2+ ratio of 8 Cu/tetramer. The rate becomes constant after this ratio is achieved. These data indicate that dopamine beta-hydroxylase specifically binds 8 mol of Cu/tetramer and that this stoichiometry is required for maximal activity.

Alternate substrates of dopamine beta-hydroxylase. III. Stoichiometry of the inactivation reaction with benzyl cyanides and spectroscopic investigations

Dopamine beta-hydroxylase was incubated with p-hydroxybenzyl cyanide, ascorbate, and O2 and the products of the hydroxylation reaction were monitored by high performance liquid chromatography. At early times, p-hydroxymandelonitrile was the sole product but this compound slowly decomposed to p-hydroxybenzaldehyde and cyanide. Dopamine beta-hydroxylase was also inhibited under these reaction conditions but the amount of cyanide produced was insufficient to account for the extent of enzyme inhibition. Incubation of dopamine beta-hydroxylase with [ring-3H]p-hydroxybenzyl cyanide, ascorbate, and O2 resulted in incorporation of detectable radiolabel into enzyme only when 100% O2 (0.4 X Km) was present. The amount of incorporated radiolabel was substoichiometric with respect to enzyme subunits (approximately 30%) and also did not correlate with the total amount of inhibited dopamine beta-hydroxylase. These data suggest two different modes of inhibition: covalent-adduct formation at early times and inhibition due to cyanide generated at the active site. Separate K14CN binding studies with dopamine beta-hydroxylase corroborated the latter suggestion since 2 mol of cyanide were bound per tetramer. Tyramine stabilized [14C]cyanide binding to dopamine beta-hydroxylase, consistent with the lack of enzyme reactivation observed under certain conditions (Colombo, G., Giedroc, D. P., Rajashekhar, B., and Villafranca, J. J. 1984) J. Biol. Chem. 259, 1601-1606). EPR data for dopamine beta-hydroxylase-bound Cu2+ directly demonstrated that a quaternary complex with tyramine and cyanide was formed, since the spectrum of dopamine beta-hydroxylase-Cu2+-tyramine-cyanide is distinct from that of dopamine beta-hydroxylase-Cu2+-cyanide and dopamine beta-hydroxylase-Cu2+-tyramine. A comprehensive mechanism for dopamine beta-hydroxylase inhibition by benzyl cyanides is presented.

ATP-dependent phosphorylation of alpha-substituted carboxylic acids catalyzed by pyruvate kinase

Pyruvate kinase from rabbit muscle catalyzes an ATP-dependent phosphorylation of glycolate to yield 2-phosphoglycolate (F. J. Kayne (1974) Biochem. Biophys. Res. Commun. 59, 8-13). An investigation of anologous reactions with other alpha-substituted carboxylic acids reveals several new substrates for such a phosphorylation reaction. Thus the alpha-hydroxy carboxylic acids L-lactate, D-lactate, DL-alpha-hydroxybutyrate, DL-alpha-hydroxyvalerate, L-glycerate, D-glycerate, DL-nitrolactate, and DL-beta-chlorolactate are phosphorylated on the alpha-hydroxy group to give the corresponding phosphoesters. Thioglycolate is also a slow substrate for phosphorylation of the thiol group to give the phosphothioglycolate, and DL-thiolactate is phosphorylated in a very slow reaction to give phosphothiolactate. beta-Hydroxypyruvate is a substrate; but, unlike the reaction with pyruvate, with beta-hydroxypyruvate the equilibrium for the reaction lies in favor of ADP and the phosphorylated product which appears from 31P NMR data to be tartronate-semialdehyde-2-phosphate. 31P NMR spectroscopy has been used to verify the identity of the products for all of the reactions. Steady-state kinetic constants have been obtained for some of the more rapid reactions. The reactions with glycolate, L-glycerate, and beta-hydroxypyruvate have kcat values that are close to that for phosphorylation of pyruvate in the reverse of the physiological reaction.

Pyruvate kinase catalyzed phosphorylation of 3-nitro-2-hydroxypropionate by ATP and concomitant inactivation of the enzyme

Pyruvate kinase catalyzes the phosphorylation of 3-nitro-2-hydroxypropionate by ATP. The product (3-nitro-2-phosphopropionate) decomposes nonenzymatically and inactivates the enzyme. The inactivation process is irreversible and the enzyme is completely protected against inactivation by mercaptoethanol or the competitive inhibitor oxalate. The Km for 3-nitro-2-hydroxypropionate (DL) is 40 mM and kcat = 5 s-1 at pH 8.6. In terms of kcat/km, 3-nitro-2-hydroxypropionate is a fourfold better substrate than glycolate.

Electron paramagnetic resonance studies of MN(II) complexes with myosin subfragment 1 and oxygen 17-labeled ligands

Ligands in the first coordination sphere of Mn(II) in the complex of MnADP with myosin subfragment 1 from rabbit skeletal muscle have been investigated. EPR spectroscopy was used to detect superhyperfine coupling between unpaired electrons of the metal ion and the nuclei of oxygen atoms specifically labeled with oxygen 17. The results show that ADP is a beta-monodentate ligand for Mn(II) and that there are probably two water oxygens directly bound to Mn(II). The inhibitory complex of vanadate with subfragment 1 . MnADP was also investigated. Vanadate-dependent changes in the EPR spectra for enzyme-bound Mn(II) indicate that the coordination sphere of MN(II) changes upon binding of vanadate. ADP remains a beta-monodenate ligand in the complex and experiments with 17O-labeled water indicate that two oxygen atoms originally in water are ligands in the complex. However, the oxygens of vanadate equilibrate with those of water during sample preparation so that one of these ligands may be a vanadate oxygen. Three additional ligands, probably from the protein, are required to complete the sextet of ligands to Mn(II) in both complexes studied.

EPR studies of the Mn(II) complex with elongation factor Tu and GDP Identification of oxygen ligands to Mn(II) by observation of 17O superhyperfine coupling

The coordination sphere of Mn(II) in the complex with GDP and elongation factor Tu from Escherichia coli has been probed by EPR spectroscopy with 17O-labeled ligands. Inhomogeneous broadening in the EPR signals for Mn(II) due to unresolved superhyperfine coupling to the 17O nucleus was used to identify directly bound oxygen ligands. Results with GDP selectively enriched with 17O either in the alpha-phosphate or in the beta-phosphate revealed that GDP was a beta-monodentate ligand for Mn(II) in the complex with the protein. Results with 17O-enriched water showed that two water molecules are coordinated to the Mn(II). The EPR spectrum for the complex is characteristic of octahedral coordination for Mn(II). Hence, three ligands from the protein are required to complete the sextet of ligands.

Manganese(II) and substrate interaction with unadenylylated glutamine synthetase (Escherichia coli w). I. Temperature and frequency dependent nuclear magnetic resonance studies

A comprehensive study of solvent interaction with unadenylylated glutamine synthetase (E1.7) has been conducted using the enzyme isolated from Escherichia coli W. The longitudinal, (1/T1p)b, and transverse, (1/T2p)b, proton relaxation rates were measured with various enzyme samples as a function of frequency (6-48 MHZ) and temperature (1-40 degrees C). With Mn(II) bound at the "tight" metal ion site approximately two water molecules are rapidly exchanging with bulk solvent. This number is reduced to approximately one in the presence of glutamine. All data were successfully analyzed according to the Solomon-Bloembergen-Morgan (SBM) scheme for dipolar relaxation of water protons interacting with enzyme-bound Mn(II). The correlation time for this process varies from 1 to 3 X 10(-9) for the complexes described above. Significant contributions to the correlation time arise from both 1/taum, the exchange rate for water molecules bound at the metal site, and from 1/taus, the electron spin relaxation rate for Mn(II) with the latter rate showing a frequency dependence at the magnetic field strengths used in this study. A study of Mn(II) binding to E1.7 at 25 degrees C revealed two classes of metal ion sites, a "tight" set of one per subunit with KD=5.0 X 10(-7) M and a "weak" set of one per subunit with KD=4.5 X10(-5)M. In the presence of glutamine the affinity of the first site for Mn(II) was unchanged but the KD value for the weak site changed to 3 X 10(-6)M. In E1.7 samples with Mn(II) bound at both the tight and weak metal ion sites the data are interpretable with two rapidly exchanging water molecules interacting with each bound Mn(II)ion. With saturating amounts of glutamine or of ADP or of glutamine plus ADP plus arsenate, the proton relaxation rates progressively decreased suggesting that the substrates or inhibitors used were interacting with the bound Mn(II) ions resulting in diminished solvent accessibility to these bound ions. These results are interpretable in terms of ligand substitution into the coordination sphere of the bound Mn(II) ions. Indeed this is probably the case for Mn(II) at the weak metal ion site since Hunt et al. ((1975), Arch. Biochem. Biophys. 166, 102) showed that Mn(II) can bind as the Mn(II)-ADP complex to the second metal ion site. Results of proton relaxation rate data on E1.7 with Mn(II) bound at both the tight and weak metal ion sites led to the conclusion that these metal ion sites are greater than 6 A apart. In comparison with proton relaxation rate data on fully adenylylated glutamine synthetase (E11.8) as studied by Villafranca and Wedler ((1974), Biochemistry 13, 3286), the first "tight" metal ion site in E11.8 has three rapidly exchanging water molecules. Mn(II) has a weaker binding constant to E11.8 (KD approximately 5 X 10(-6)M) at the pH value used in both studies and a suggestion is made that an additional protein ligand is binding to Mn(II) in glutamine synthetase when the subunits are not adenylylated.

Manganese (II) and substrate interaction with unadenylylated glutamine synthetase (Escherichia coli w). II. Electron paramagnetic resonance and nuclear magnetic resonance studies of enzyme-bound manganese(II) with substrates and a potential transition-state analogue, methionine sulfoximine

The enhancement of the longitudinal proton relaxation rate of solvent water protons which occurs when Mn(II) is bound to the "tight" metal ion site of unadenylylated glutamine synthetase (GS) was used to determine the binding constant of L-methionine (SR)-sulfoximine to GS-Mn(II) complexes. The binary enhancement for GS-Mn(II) is 22 at 24 MHz, 25 degrees C. The enhancement is lowered in the presence of the sulfoximine and the computed dissociation constant is 30 muM with epsilont, the enhancement for the ternary complex, equal to 3.0. Titration curves for the sulfoximine were also obtained in the presence of Mg-ADP, Mg-ADP plus Pi, and Mg-ATP. The dissociation constants were 9, 5, and 0.8 muM, respectively. The progressive tightening of the dissociation constants is symptomatic of conformational changes at the active site as the total subsite occupied by ATP is filled. The number of rapidly exchanging water molecules drops from 2 to approximately 0.1 when saturating concentrations of L-methionine (SR)-sulfoximine and nucleotide are present. The kinetically determined KI value of approximately 4 muM for the sulfoximine is about three orders of magnitude tighter than thee Km' value of approximately 3 mM for L-glutamate. The previously mentioned dissociation constants obtained by enhancement titrations are also orders of magnitude tighter than Km'. These data suggest that L-methionine (SR)-sulfoximine is a "transition-state" analogue for the glutamine synthetase reaction. ...