Kynurenine 3-monooxygenase from Pseudomonas fluorescens: substrate-like inhibitors both stimulate flavin reduction and stabilize the flavin-peroxo intermediate yet result in the production of hydrogen peroxide

Kynurenine 3-monooxygenase (KMO) is a flavin-dependent hydroxylase that catalyzes the conversion of l-kynurenine (l-Kyn) to 3-hydroxykynurenine (3OHKyn) in the pathway for tryptophan catabolism. KMO inhibition has been widely suggested as an early treatment for stroke and other neurological disorders that involve ischemia. We have investigated the reductive and the oxidative half-reactions of a stable form of KMO from Pseudomonas fluorescens (KMO). The binding of l-Kyn by the enzyme is relatively slow and involves at least two reversible steps. The rate constant for reduction of the flavin cofactor by NADPH increases by a factor of approximately 2.5 x 10(3) when l-Kyn is bound. The rate of reduction of the KMO.l-Kyn complex is 160 s(-1), and the K(d) for the NADPH complex is 200 microM with charge-transfer absorption bands for the KMO(RED).l-Kyn.NADP(+) complex accumulating after reduction. The reduction potential of KMO is -188 mV and is unresponsive to the addition of l-Kyn or other inhibitory ligands. KMO inhibitors whose structures are reminiscent of l-Kyn such as m-nitrobenzoylalanine and benzoylalanine also stimulate reduction of flavin by NADPH and, in the presence of dioxygen, result in the stoichiometric liberation of hydrogen peroxide, diminishing the perceived therapeutic potential of inhibitors of this type. In the presence of the native substrate, the oxidative half-reaction exhibits triphasic absorbance data. A spectrum consistent with that of a peroxyflavin species accumulates and then decays to yield the oxidized enzyme. This species then undergoes minor spectral changes that, based on flavin difference spectra defined in the presence of 3OHKyn, can be correlated with product release. The oxidative half-reaction observed in the presence of saturating benzoylalanine or m-nitrobenzoylalanine also shows the accumulation of a peroxyflavin species that then decays to yield hydrogen peroxide without hydroxylation.

The Interaction of Hydroxymandelate Synthase with the 4-Hydroxyphenylpyruvate Dioxygenase Inhibitor: NTBC

Hydroxymandelate synthase (HMS) catalyzes the committed step in the formation of para-hydroxyphenylglycine, a recurrent substructure of polycyclic non-ribosomal peptide antibiotics such as vancomycin. HMS uses the same substrates as 4-hydroxyphenylpyruvate dioxygenase (HPPD), 4-hydroxyphenylpyruvate (HPP) and O(2), and also conducts a dioxygenation reaction. The difference between the two lies in the insertion of the second oxygen atom, HMS directing this atom onto the benzylic carbon of the substrate while HPPD hydroxylates the aromatic C1 carbon. We have shown that HMS will bind NTBC, a herbicide/therapeutic whose mode of action is based on the inhibition of HPPD. This occurs despite the difference in residues at the active site of HMS from those known to contact the inhibitor in HPPD. Moreover, the minimal kinetic mechanism for association of NTBC to HMS differs only slightly from that observed with HPPD. The primary difference is that three charge-transfer species are observed to accumulate during association. The first reversible complex forms with a weak dissociation constant of 520 microM, the subsequent two charge-transfer complexes form with rate constants of 2.7 s(-1) and 0.67 s(-1). As was the case for HPPD, the final complex has the most intense charge-transfer, is not observed to dissociate, and is unreactive towards dioxygen.

The diverse and pervasive chemistries of the alpha-keto acid dependent enzymes

The number of identified and confirmed alpha-keto acid dependent oxygenases is increasing rapidly. All of these enzymes have a relatively simple liganding arrangement for a single ferrous ion but collectively conduct a highly diverse set of chemistries. While hydroxylations and a variety of oxidation reactions have been most commonly observed, new reactions involving dealkylations, epimerizations and halogenations have recently been discovered. In this minireview we present what is known of the alpha-keto acid dependent enzymes and offer an argument that the chemistry that is unique to each enzyme occurs only after the production of a pivotal ferryl-oxo intermediate.

Spectroscopic and electronic structure studies of the role of active site interactions in the decarboxylation reaction of alpha-keto acid-dependent dioxygenases

The alpha-ketoglutate (alpha-KG)-dependent dioxygenases are a large class of mononuclear non-heme iron enzymes that require Fe(II), alpha-KG and dioxygen for catalysis, with the alpha-KG cosubstrate supplying the two additional electrons required for dioxygen activation. A sub-class of these enzymes exists in which the alpha-keto acid is covalently attached to the substrate, including (4-hydroxy)mandelate synthase (HmaS) and (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) which utilize the same substrate but exhibit two different general reactivities (H-atom abstraction and electrophilic attack). Previous kinetic studies of Streptomyces avermitilis HPPD have shown that the substrate analog phenylpyruvate (PPA), which only differs from the normal substrate (4-hydroxyphenyl)pyruvate (HPP) by the absence of a para-hydroxyl group on the aromatic ring, does not induce a reaction with dioxygen. While an Fe(IV)O intermediate is proposed to be the reactive species in converting substrate to product, the key step utilizing O(2) to generate this species is the decarboxylation of the alpha-keto acid. It has been generally proposed that the two requirements for decarboxylation are bidentate coordination of the alpha-keto acid to Fe(II) and the presence of a 5C Fe(II) site for the O(2) reaction. Circular dichroism and magnetic circular dichroism studies have been performed and indicate that both enzyme complexes with PPA are similar with bidentate alpha-KG coordination and a 5C Fe(II) site. However, kinetic studies indicate that while HmaS reacts with PPA in a coupled reaction similar to the reaction with HPP, HPPD reacts with PPA in an uncoupled reaction at an approximately 10(5)-fold decreased rate compared to the reaction with HPP. A key difference is spectroscopically observed in the n-->pi( *) transition of the HPPD/Fe(II)/PPA complex which, based upon correlation to density functional theory calculations, is suggested to result from H-bonding between a nearby residue and the carboxylate group of the alpha-keto acid. Such an interaction would disfavor the decarboxylation reaction by stabilizing electron density on the carboxylate group such that the oxidative cleavage to yield CO(2) is disfavored.

Spectroscopic and electronic structure studies of aromatic electrophilic attack and hydrogen-atom abstraction by non-heme iron enzymes

(4-Hydroxy)mandelate synthase (HmaS) and (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) are two alpha-keto acid dependent mononuclear non-heme iron enzymes that use the same substrate, (4-hydroxyphenyl)pyruvate, but exhibit two different general reactivities. HmaS performs hydrogen-atom abstraction to yield benzylic hydroxylated product (S)-(4-hydroxy)mandelate, whereas HPPD utilizes an electrophilic attack mechanism that results in aromatic hydroxylated product homogentisate. These enzymes provide a unique opportunity to directly evaluate the similarities and differences in the reaction pathways used for these two reactivities. An Fe(II) methodology using CD, magnetic CD, and variable-temperature, variable-field magnetic CD spectroscopies was applied to HmaS and compared with that for HPPD to evaluate the factors that affect substrate interactions at the active site and to correlate these to the different reactivities exhibited by HmaS and HPPD to the same substrate. Combined with density functional theory calculations, we found that HmaS and HPPD have similar substrate-bound complexes and that the role of the protein pocket in determining the different reactivities exhibited by these enzymes (hydrogen-atom abstraction vs. aromatic electrophilic attack) is to properly orient the substrate, allowing for ligand field geometric changes along the reaction coordinate. Elongation of the Fe(IV) O bond in the transition state leads to dominant Fe(III) O(*-) character, which significantly contributes to the reactivity with either the aromatic pi-system or the C H sigma-bond.

Heterologous expression and purification of kynurenine-3-monooxygenase from Pseudomonas fluorescens strain 17400

Kynurenine 3-monooxygenase (KMO) is an NADPH-dependent flavoprotein hydroxylase that catalyzes the conversion of l-Kynurenine (L-Kyn) to 3-hydroxykynurenine (3OHKyn). The reaction is central to the tryptophan degradative pathway and takes place within microglial cells defining cellular concentrations of the N-methyl-d-aspatate (NMDA) receptor agonist quinolinate and antagonist kynurenate. The influence over the cellular concentrations of these NMDA receptor effectors makes KMO an attractive target for the treatment of ischemic stroke. Pseudomonas fluorescens str 17400, expresses five activities of tryptophan catabolism including that of KMO. The KMO gene from P. fluorescens was cloned into the pET-17b plasmid using incorporated NdeI and XhoI restriction sites. This construct yielded PfKMO to 20% of total cell protein after 12h of expression at 22 degrees C without induction by isopropyl-beta-thiogalactopyranoside (IPTG). The enzyme could be readily purified using ammonium sulfate fractionation and ion exchange chromatography, resulting in pure KMO with a turnover number of 5.0 s(-1). PfKMO activity was dependent on the reduction state of the enzyme. Preparation and storage benefited from the presence of a reductant such as dithiothreitol or beta-mercaptoethanol. The loss of activity was found to be directly related to the oxidation of thiols as measured by dinitrothiobenzoate assay. Steady-state assays monitoring the consumption of dioxygen were used to measure apparent kinetic parameters and ligand perturbation of flavin fluorescence was used to determine a Kd value for both L-Kyn and the inhibitor m-nitrobenzoylalanine. PfKMO is offered as prototypical bacterial form of the enzyme to serve as a viable platform on which to base future KMO studies.

Catalytic, noncatalytic, and inhibitory phenomena: kinetic analysis of (4-hydroxyphenyl)pyruvate dioxygenase from Arabidopsis thaliana

(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) incorporates both atoms of molecular oxygen into 4-hydroxyphenylpyruvate (HPP) to form homogentisate (HG). This reaction has direct relevance in both medicine and agriculture. In humans, the specific inhibition of HPPD alleviates the symptoms of diseases that arise from tyrosine catabolism defects. However, in plants, the inhibition of HPPD bleaches, stunts, and ultimately kills the organism. The reason for this is that in mammalian metabolism the product HG does not feed into other pathways, whereas in plants it is the precursor for the redox active portion of tocopherols and plastoquinones. There are a number of commercially available herbicides that directly target the inhibition of the HPPD reaction. Plant HPPD however is largely uncharacterized in terms of its catalysis and inhibition reactions. In this study, we examine the catalysis and inhibition of HPPD from Arabidopsis thaliana (AtHPPD). We have expressed AtHPPD and purified the enzyme to high specific activity. This form of HPPD accumulates two transient species in single turnover reactions with the native substrate HPP. These transients appear to be equivalent to intermediates I and III observed in the enzyme from Streptomyces (Johnson-Winters et al. (2005), Biochemistry, 44, 7189-7199). The first intermediate is a relatively strongly absorbing species with maxima at 380 and 490 nm. This species decays to a second intermediate that is fluorescent and has been assigned as the complex of the enzyme with the product, HG. The decay of this intermediate is rate-determining in multiple turnover reactions. The reaction of the enzyme with the analogue of the substrate, phenylpyruvate (PPA), is noncatalytic. A single turnover reaction is observed with this ligand that renders the enzyme oxidized to the ferric form, consumes a stoichiometric amount of dioxygen, and yields 66% phenylacetate as a product. Additional absorbance features at 365 and 670 nm accumulate during inactivation and give the inactivated enzyme a green color but has the same molecular mass as the active enzyme as determined by mass spectrometry.

Spectroscopic and computational studies of NTBC bound to the non-heme iron enzyme (4-hydroxyphenyl)pyruvate dioxygenase: Active site contributions to drug inhibition

(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is an alpha-keto-acid-dependent dioxygenase which catalyzes the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate as part of tyrosine catabolism. While several di- and tri-ketone alkaloids are known as inhibitors of HPPD and used commercially as herbicides, one such inhibitor, [2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione (NTBC), has also been used therapeutically to treat type I tyrosinemia and alkaptonuria in humans. To gain further insight into the mechanism of inhibition by NTBC, a combination of CD/MCD spectroscopy and DFT calculations of HPPD/Fe(II)/NTBC has been performed to evaluate the contribution of the Fe(II)-NTBC bonding interaction to the high affinity of this drug for the enzyme. The results indicate that the bonding of NTBC to Fe(II) is very similar to that for HPP, both involving similar pi-backbonding interactions between NTBC/HPP and Fe(II). Combined with the result that the calculated binding energy of NTBC is, in fact, approximately 3 kcal/mol less than that for HPP, the bidentate coordination of NTBC to Fe(II) is not solely responsible for its extremely high affinity for the enzyme. Thus, the pi-stacking interactions between the aromatic rings of NTBC and two phenyalanine residues, as observed in the crystallography of the HPPD/Fe(II)/NTBC complex, appear to be responsible for the observed high affinity of drug binding.

Accumulation of multiple intermediates in the catalytic cycle of (4-hydroxyphenyl)pyruvate dioxygenase from Streptomyces avermitilis

(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) catalyzes the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate (HG). This reaction involves decarboxylation, substituent migration, and aromatic oxygenation in a single catalytic cycle. HPPD is a unique member of the alpha-keto acid dependent oxygenases that require Fe(II) and an alpha-keto acid substrate to oxygenate or oxidize an organic molecule. We have examined the reaction coordinate of HPPD from Streptomyces avermitilis using rapid mixing pre-steady-state methods in conjunction with steady-state kinetic analyses. Acid quench reactions and product analysis of homogentisate indicate that HPPD as isolated is fully active and that experiments limited in dioxygen concentration with respect to that of the enzyme do involve a single turnover. These experiments indicate that during the course of one turnover the concentration of homogentisate is stoichiometric with enzyme concentration by approximately 200 ms, well before the completion of the catalytic cycle. Subsequent single turnover reactions were monitored spectrophotometrically under pseudo-first-order and matched concentration reactant conditions. Three spectrophotometrically distinct intermediates are observed to accumulate. The first of these is a relatively strongly absorbing species with maxima at 380 and 480 nm that forms with a rate constant (k(1)) of 7.4 x 10(4) M(-)(1) s(-)(1) and then decays to a second intermediate with a rate constant (k(2)) of 74 s(-)(1). The rate constant for the decay of the second intermediate (k(3)) is 13 s(-)(1) and is concomitant with the formation of the product, homogentisate, based on rapid quench and pre-steady-state fluorescence measurements. The rate constant for this process decreases to 7.6 s(-)(1) when deuterons are substituted for protons in the aromatic ring of the substrate. The release of product from the enzyme is rate limiting and occurs at 1.6 s(-)(1). This final event exhibits a kinetic isotope effect of 2 with deuterium oxide as the solvent, consistent with a solvent isotope effect on V(max) of 2.6 observed in steady-state experiments.

4-Hydroxyphenylpyruvate dioxygenase

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is an Fe(II)-dependent, non-heme oxygenase that catalyzes the conversion of 4-hydroxyphenylpyruvate to homogentisate. This reaction involves decarboxylation, substituent migration and aromatic oxygenation in a single catalytic cycle. HPPD is a member of the alpha-keto acid dependent oxygenases that typically require an alpha-keto acid (almost exclusively alpha-ketoglutarate) and molecular oxygen to either oxygenate or oxidize a third molecule. As an exception in this class of enzymes HPPD has only two substrates, does not use alpha-ketoglutarate, and incorporates both atoms of dioxygen into the aromatic product, homogentisate. The tertiary structure of the enzyme would suggest that its mechanism converged with that of other alpha-keto acid enzymes from an extradiol dioxygenase progenitor. The transformation catalyzed by HPPD has both agricultural and therapeutic significance. HPPD catalyzes the second step in the pathway for the catabolism of tyrosine, that is common to essentially all aerobic forms of life. In plants this pathway has an anabolic branch from homogentisate that forms essential isoprenoid redox cofactors such as plastoquinone and tocopherol. Naturally occurring multi-ketone molecules act as allelopathic agents by inhibiting HPPD and preventing the production of homogentisate and hence required redox cofactors. This has been the basis for the development of a range of very effective herbicides that are currently used commercially. In humans, deficiencies of specific enzymes of the tyrosine catabolism pathway give rise to a number of severe metabolic disorders. Interestingly, HPPD inhibitor/herbicide molecules act also as therapeutic agents for a number of debilitating and lethal inborn defects in tyrosine catabolism by preventing the accumulation of toxic metabolites.

Structure of the ferrous form of (4-hydroxyphenyl)pyruvate dioxygenase from Streptomyces avermitilis in complex with the therapeutic herbicide, NTBC

Di- and triketone inhibitors of (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) are both effective herbicides and therapeutics. The inhibitory activity is used to halt the production of lipophilic redox cofactors in plants and also in humans to prevent accumulation of toxic metabolic byproducts that arise from specific inborn defects of tyrosine catabolism. The three-dimensional structure of the Fe(II) form of HPPD from Streptomyces avermitilis in complex with the inhibitor 2-[2-nitro-4-(triflouromethyl)benzoyl]-1,3-cyclohexanedione (NTBC) has been determined at a resolution of 2.5 A. NTBC coordinates to the active site metal ion, located at the bottom of a wide solvent-accessible cavity in the C-terminal domain of the protein. The iron is liganded in a predominantly five-coordinate, distorted square-pyramidal arrangement in which Glu349, His187, and His270 are protein-derived ligands and two other ligands are from the 5' and 7' oxygens of NTBC. There is a low-occupancy water molecule in the sixth coordination site in one of the protomers. The distance to His270 is unusually long at 2.5 A, and its orientation is somewhat distorted from ideal ligand geometry to within 2.8 A of the inhibitor nitro group. In contrast to the tetrameric quartenary structure observed for HPPD from other bacterial sources, the asymmetric unit is composed of two weakly associated protomers with a buried surface area of 1266 A(2) and a total of 12 hydrogen-bonding and no electrostatic interactions. The overall tertiary structure is similar to that of HPPD from Pseudomonas fluorescens (Serre et al., (1999) Structure 7, 977-988), although the position of the C-terminal alpha-helix is dramatically shifted. This C-terminal alpha-helix provides Phe364, which in combination with Phe336 sandwiches the phenyl ring of the bound NTBC; no other significant hydrogen-bonding or charge-pairing interactions are observed. Moreover, the structure reveals that, with the exception of Val189, NTBC makes contacts to only fully conserved amino acids. The combination of bidentate metal-ion coordination and pi-stacked aromatic rings is suggestive of a binding mode for the substrate and/or a transition state, which may be the origin of the exceedingly high affinity these inhibitors have for HPPD.

CD and MCD studies of the non-heme ferrous active site in (4-hydroxyphenyl)pyruvate dioxygenase: correlation between oxygen activation in the extradiol and alpha-KG-dependent dioxygenases

(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is an unusual alpha-keto acid-dependent non-heme iron dioxygenase as it incorporates both atoms of dioxygen into a single substrate, paralleling the extradiol dioxygenases. CD/MCD studies of the catalytically active ferrous site and its interaction with substrate reveal a geometic and electronic structure and mechanistic approach to oxygen activation which bridges those of the alpha-KG-dependent and the extradiol dioxygenases.

Modeling1H exchange: An estimate of the error introduced in MRI by assuming the fast exchange limit in bolus tracking

A simulation is presented which calculates the MRI signal expected from a model tissue for a given pulse sequence after a bolus injection of a contrast agent. The calculation assumes two physiologic compartments only, the intravascular and extravascular spaces. The determination of the concentration of contrast in each compartment as a function of time and position has been outlined in a previous publication (Moran and Prato, Magn Reson Med 2001;45:42-45). These contrast agent concentrations are used here to determine the NMR relaxation times as a function of time and position within the tissue. Knowledge of this simulated tissue 'map' of relaxation times as a function of time provides the information required to determine whether the proton exchange rate is fast or slow on the NMR timescale. Since with a bolus injection the concentration of contrast and hence the relaxation time may vary with position along the capillary, some segments of the capillary are allowed to be in fast exchange with the extravascular space, while others may be in slow exchange. Using this information, and parameters specific to a given tissue, the MRI signal for a given pulse sequence is constructed which correctly accounts for differences in proton exchange across the length of the capillary. It is shown that extravascular contrast agents show less signal dependence on water exchange, and thus may be more appropriate for quantitative imaging when using fast exchange assumptions. It is also shown that nondistributed compartment models can incorrectly estimate the water exchange that is occurring at the capillary level if exchange-minimizing pulse sequences are not used.

Kinetic analysis of human homogentisate 1,2-dioxygenase

Homogentisate 1,2-dioxygenase (HGD) is a mononuclear Fe(II)-dependent oxygenase that catalyzes the third step in the pathway for the catabolism of tyrosine, the conversion of homogentisate (HG) to maleylacetoacetate (MAA). We have heterologously expressed and purified native human HGD in the apo form. Steady-state analysis varying the concentration of both HG and molecular oxygen shows that the purified enzyme has a turnover number of 16 s(-1). Our data suggest that HG binds to the apo-enzyme and that the apo-HGD.HG complex does not bind Fe(II) and dissociates slowly at approximately 0.028 s(-1). The rate constant for the dissociation of Fe(II) from the holo-enzyme as measured under anaerobic conditions is 0.00004 s(-1) and indicates that this process is not relevant in steady-state turnover. The addition of HG and molecular oxygen to the holo-enzyme is formally random as the holo-enzyme reduces molecular oxygen at a rate of 1.35x10(3) M(-1) s(-1) at 4 degrees C. The term ordered with respect to the addition of substrates is most descriptive as the rate of reduction of molecular oxygen must increase in the presence of HG to sustain the observed turnover number.

Interaction of (4-hydroxyphenyl)pyruvate dioxygenase with the specific inhibitor 2-[2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione

(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is a non-heme Fe(II) enzyme that catalyzes the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate as part of the tyrosine catabolism pathway. Inhibition of HPPD by the triketone 2-[2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione (NTBC) is used to treat type I tyrosinemia, a rare but fatal defect in tyrosine catabolism. Although triketones have been used for many years as HPPD inhibitors for both medical and herbicidal purposes, the mechanism of inhibition is not well understood. The following work provides mechanistic insight into NTBC binding. The tautomeric population of NTBC in aqueous solution is dominated by a single enol as determined by NMR spectroscopy. NTBC preferentially binds to the complex of HPPD and FeII [HPPD.Fe(II)] as evidenced by a visible absorbance feature centered at 450 nm. The binding of NTBC to HPPD.Fe(II) was observed using a rapid mixing method and was shown to occur in two phases and comprise three steps. A hyperbolic dependence of the first observable process with NTBC concentration indicates a pre-equilibrium binding step followed by a limiting rate (K(1) = 1.25 +/- 0.08 mM, k(2) = 8.2 +/- 0.2 s(-1)), while the second phase (k(3) = 0.76 +/- 0.02 s(-1)) had no dependence on NTBC concentration. Neither K(1),k(2), nor k(3) was influenced by pH in the range of 6.0-8.0. Isotope effects on both k(2) and k(3) were observed when D(2)O is used as the solvent (for k(2), k(h)/k(d) = 1.3; for k(3), k(h)/k(d) = 3.2). It is therefore proposed that the bidentate association of NTBC with the active site metal ion (k(2)) precedes the Lewis acid-assisted conversion of the bound enol to the enolate (k(3)). Although the native enzyme without substrate reacts with molecular oxygen to form the oxidized holoenzyme, the HPPD.Fe(II).NTBC complex does not. When the complex is exposed to atmospheric oxygen, the absorbance feature associated with NTBC binding does not diminish over the course of 2 days. This means not only that the HPPD.Fe(II).NTBC complex does not oxidize but also that the dissociation rate constant for NTBC is essentially zero because any HPPD.Fe(II) that formed would readily oxidize in the presence of dioxygen. Consistent with this observation, EPR spectroscopy has shown that only 2% of the HPPD.Fe(II).NTBC complex forms an NO complex as compared to the holoenzyme.

An investigation of the toxicity of gadolinium based MRI contrast agents using neutron activation analysis

The toxicity of gadolinium (Gd) based MRI contrast agents, is based upon the amount of Gd that dissociates from its chelate and deposits in tissues. In this study, the toxicities of two contrast agents were tested using different injection strategies in two animal models. Following a bolus injection of 0.2 mmol/kg of Gd-DTPA in a pilot study with a single canine, Gd levels were as high as 2.05 +/- 0.17 ppm and 0.47 +/- 0.11 ppm 2 weeks post injection in the kidney and liver tissues, respectively. To evaluate the role that the injection strategy plays in toxicity, 0.8 mmol/kg of Gd-(HP-DO3A) was injected into rats, in a second study, via bolus and constant infusion techniques. Gd was only detected in the kidney in the bolus injected rats but in the lung as well in the constant infusion injected rats. Concentrations detected in the kidney for both strategies, were comparable within error: 1.37 +/- 0.46 ppm for the bolus and 1.24 +/- 0.39 ppm for the bolus/constant infusion strategy and 0.16 +/- 0.14 ppm in the lung for the constant infusion technique. The contrast infusion technique does not appear to present an increased risk of toxicity over the bolus technique except perhaps to a small degree in the lung.

(4-Hydroxyphenyl)pyruvate dioxygenase from Streptomyces avermitilis: the basis for ordered substrate addition

(4-hydroxyphenyl)pyruvate dioxygenase (HPPD) catalyzes the second step in the pathway for the catabolism of tyrosine, the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate (HG). This reaction involves decarboxylation, substituent migration, and aromatic oxygenation. HPPD is a member of the alpha-keto acid dependent oxygenases that require Fe(II) and an alpha-keto acid substrate to oxygenate an organic molecule. We have examined the binding of ligands to HPPD from Streptomyces avermitilis. Our data show that HPP binds to the apoenzyme and that the apo-HPPD.HPP complex does not bind Fe(II) to generate active holoenzyme. The binding of HPP, phenylpyruvate (PPA), and pyruvate to the holoenzyme produces a weak ligand charge-transfer band at approximately 500 nm that is indicative of bidentate binding of the 1-carboxylate and 2-keto pyruvate oxygen atoms to the active site metal ion. For HPPD from this organism the 4-hydroxyl group of (4-hydroxyphenyl)pyruvate is a requirement for catalysis; no turnover is observed in the presence of phenylpyruvate. The rate constant for the dissociation of Fe(II) from the holoenzyme is 0.0006 s(-)(1) and indicates that this phenomenon is not significantly relevant in steady-state turnover. The addition of HPP and molecular oxygen to the holoenzyme is formally random. The basis of the ordered bi bi steady-state kinetic mechanism previously observed by Rundgren (Rundgren, M. (1977) J. Biol. Chem. 252, 5094-9) is the 3600-fold increase in oxygen reactivity when holo-HPPD is in complex with HPP. This complex reacts with molecular oxygen with a second-order rate constant of 1.4 x 10(5) M(-)(1) s(-)(1) inducing the formation of an intermediate that decays at the catalytically relevant rate of 7.8 s(-)(1).

Myocardial viability imaging using Gd-DTPA: physiological modeling of infarcted myocardium, and impact on injection strategy and imaging time

Results of simulations are shown which illustrate how the concentration-time curves of an extravascular extracellular (EVEC) contrast agent, such as Gd-DTPA, vary in myocardial tissue. The simulations show that the variable permeability of dead myocytes within a recent myocardial infarction will significantly alter delayed enhancement patterns following a bolus injection, invariably reducing the sensitivity of this technique for the detection of permanently damaged tissue. It is further predicted that if the bolus injection is followed by a suitably selected constant infusion, the infarct size and infarct volume of distribution may be more accurately determined, even though the degree of enhancement of an infarcted region (with normal flow) above normal tissue is slightly higher for the bolus technique within the first 30 min following the injection. The degree of enhancement of an infarcted region (with normal flow) above normal tissue was comparable between the two techniques at the point in the constant infusion at which the volume of contrast injected was the same as in the bolus case, i.e., at approximately 30 min after the bolus injection. The constant infusion approach became superior thereafter as overall tissue concentrations became greater in both normal and infarcted tissue, and these concentrations remained more stable with the constant infusion approach. Preliminary experimental results in a canine model of infarction/reperfusion illustrated a delayed wash-in of contrast agent in infarcted tissue, which may be explained by a physiological model in which dead myocytes in infarcted myocardium have non-infinite permeability.

Role of tryptophan hydroxylase phe313 in determining substrate specificity

The active site residue phenylalanine 313 is conserved in the sequences of all known tryptophan hydroxylases. The tryptophan hydroxylase F313W mutant protein no longer shows a preference for tryptophan over phenylalanine as a substrate, consistent with a role of this residue in substrate specificity. A tryptophan residue occupies the homologous position in tyrosine hydroxylase. The tyrosine hydroxylase W372F mutant enzyme does not show an increased preference for tryptophan over tyrosine or phenylalanine, so that this residue cannot be considered the dominant factor in substrate specificity in this family of enzymes.

Shielding, but not zeroing of the ambient magnetic field reduces stress-induced analgesia in mice

Magnetic field exposure was consistently found to affect pain inhibition (i.e. analgesia). Recently, we showed that an extreme reduction of the ambient magnetic and electric environment, by mu-metal shielding, also affected stress-induced analgesia (SIA) in C57 mice. Using CD1 mice, we report here the same findings from replication studies performed independently in Pisa, Italy and London, ON, Canada. Also, neither selective vector nulling of the static component of the ambient magnetic field with Helmholtz coils, nor copper shielding of only the ambient electric field, affected SIA in mice. We further show that a pre-stress exposure to the mu-metal box is necessary for the anti-analgesic effects to occur. The differential effects of the two near-zero magnetic conditions may depend on the elimination (obtained only by mu-metal shielding) of the extremely weak time-varying component of the magnetic environment. This would provide the first direct and repeatable evidence for a behavioural and physiological effect of very weak time-varying magnetic fields, suggesting the existence of a very sensitive magnetic discrimination in the endogenous mechanisms that underlie SIA. This has important implications for other reported effects of exposures to very weak magnetic fields and for the theoretical work that considers the mechanisms underlying the biological detection of weak magnetic fields.

Two-dimensional time correlation relaxometry of skeletal muscle in vivo at 3 Tesla

A hybrid two-dimensional relaxometry (2DR) sequence was used to simultaneously measure both the spin-spin (R2) and spin-lattice relaxation rates (R1) of skeletal muscle in vivo. The 2DR sequence involved a 180 degrees inversion pulse followed by a variable delay time (30 values from 40 to 7000 ms); a projection presaturation (PP) scheme to localize a 16-ml cylindrical voxel; and a CPMG sequence (950 even echoes, effective echo spacing = 1.2 ms, equilibrium time = 12 s). The 2DR data were collected at 3.0 Tesla from the flexor digitorum profundus of eight healthy males, 26 +/- 2 years old. Analysis was performed with a 2D version of the non-negative least-squares algorithm and a one-way ANOVA. All subjects exhibited at least three spin-groups (R2 < 200 s(-1)), designated B, C, and D, with R2 values of 42.7, 26.5, and 8.1 s(-1), and fractional volumes of 52, 35, and 7%, respectively. The R1 values of B and C were similar, congruent with0.7 s(-1), but different from that of D (P < 0.001), which had an R1 of 1.0 s(-1). The results suggest that exchange between B and C ranges from 0.7-16.2 s(-1), while exchange between either of these spin-groups with D is slower. If the data are interpreted with a compartment model, in which spin-groups with short and long R2 values are attributed to extra- and intracellular fluid, respectively, the exchange of water across the cell membrane in living skeletal muscle is slow or intermediate relative to both R1 and R2.

Modeling tissue contrast agent concentration: a solution to the tissue homogeneity model using a simulated arterial input function

The tissue homogeneity model, which describes tissue in terms of two compartments, one intravascular (iv) and one extravascular (ev), is solved by Laplace transformation of two coupled differential equations. By assuming a functional form for the arterial input function (AIF), or by fitting to an experimentally determined AIF, this function is introduced into the solution as a boundary condition describing the time dependent input to the capillary. The solution to the tissue homogeneity model equations in Laplace space are numerically inverted to obtain the concentration of tracer in the ev space as a function of time and in the iv space as a function of both position and time. Magn Reson Med 45:42-45, 2001.

A dielectric analysis of liquid and glassy solid glucose/water solutions

Dielectric relaxation data covering a temperature range from above room temperature to below the glass transition for 40% (w/w) and 75% (w/w) glucose/water solutions in the frequency range between 5 and 13 MHz are presented. These data are used to obtain correlation times for the dielectric relaxation in the viscous liquid and the glass and are compared with correlation times determined from deuterium nuclear spin relaxation times [J. Chem. Phys., 110 (1999) 3472-3483]. The two sets of results have the same temperature dependence, but differ in magnitude by a factor of 3, implying that the relaxation is a small-step rotational diffusion. Both the structural relaxation (alpha process) and the slow beta process are present. In the 40% glucose/water sample, there is a dielectric relaxation attributable to the ice that forms at low temperature. It is shown that the reciprocal of the viscosity, the correlation time derived from the dielectric relaxation, and the dc conductivity have a similar dependence on temperature.

Influence of steric bulk and electrostatics on the hydroxylation regiospecificity of tryptophan hydroxylase: characterization of methyltryptophans and azatryptophans as substrates

Tryptophan hydroxylase is a pterin-dependent amino acid hydroxylase that catalyzes the incorporation of one atom of molecular oxygen into tryptophan to form 5-hydroxytryptophan. The substrate specificity and hydroxylation regiospecificity of tryptophan hydroxylase have been investigated using tryptophan analogues that have methyl substituents or nitrogens incorporated into the indole ring. The products of the reactions show that the regiospecificity of tryptophan hydroxylase is stringent. Hydroxylation does not occur at the 4 or 6 carbon in response to changes in substrate topology or atomic charge. 5-Hydroxymethyltryptophan and 5-hydroxy-4-methyltryptophan are the products from 5-methyltryptophan. These products establish that the hydroxylating intermediate is sufficiently potent to hydroxylate benzylic carbons and that the direction of the NIH shift in tryptophan hydroxylase is from carbon 5 to carbon 4. The effects on the V/K values for the amino acids indicate that the enzyme is most sensitive to changes at position 5 of the indole ring. The V(max) values for amino acid hydroxylation differ at most by a factor of 3 from that observed for tryptophan, while the efficiencies of hydroxylation with respect to tetrahydropterin consumption vary 6-fold, consistent with oxygen transfer to the amino acid being partially or fully rate limiting in productive catalysis.

Mechanistic insights into p-hydroxybenzoate hydroxylase from studies of the mutant Ser212Ala

In the crystal structure of native p-hydroxybenzoate hydroxylase, Ser212 is within hydrogen bonding distance (2.7 A) of one of the carboxylic oxygens of p-hydroxybenzoate. In this study, we have mutated residue 212 to alanine to study the importance of the serine hydrogen bond to enzyme function. Comparisons between mutant and wild type (WT) enzymes with the natural substrate p-hydroxybenzoate showed that this residue contributes to substrate binding. The dissociation constant for this substrate is 1 order of magnitude higher than that of WT, but the catalytic process is otherwise unchanged. When the alternate substrate, 2,4-dihydroxybenzoate, is used, two products are formed (2,3,4-trihydroxybenzoate and 2,4, 5-trihydroxybenzoate), which demonstrates that this substrate can be bound in two orientations. Kinetic studies provide evidence that the intermediate with a high extinction coefficient previously observed in the oxidative half-reaction of the WT enzyme with this substrate is composed of contributions from both the dienone form of the product and the C4a-hydroxyflavin. During the reduction of the enzyme-2,4-dihydroxybenzoate complex by NADPH with 2, 4-dihydroxybenzoate, a rapid transient increase in flavin absorbance is observed prior to hydride transfer from NADPH to FAD. This is direct evidence for movement of the flavin before reduction occurs.

Substrate recognition by "password" in p-hydroxybenzoate hydroxylase

The flavin of p-hydroxybenzoate hydroxylase (PHBH) adopts two conformations [Gatti, D. L., Palfey, B. A., Lah, M.-S., Entsch, B., Massey, V., Ballou, D. P., and Ludwig, M. L. (1994) Science 266, 110-114; Schreuder, H. A., Mattevi, A., Obmolova, G., Kalk, K. H., Hol, W. G. J., van der Bolt, F. J. T., and van Berkel, W. J. H. (1994) Biochemistry 33, 10161-10170]. Kinetic studies detected the movement of the flavin from the buried conformation to the exposed conformation caused by the binding of NADPH prior to its reaction with the flavin. The pH dependence of the rate constant for flavin reduction in wild-type PHBH and the His72Asn mutant indicates that the deprotonation of bound p-hydroxybenzoate is also required for flavin movement, and is accomplished by the same internal proton transport network previously found to be involved in substrate oxidation. The linkage of substrate deprotonation to flavin movement constitutes a novel mode of molecular recognition in which the enzyme tests the suitability of aromatic substrates before committing to the catalytic cycle.

A continuous fluorescence assay for tryptophan hydroxylase

A continuous fluorometric assay for tryptophan hydroxylase activity based on the different spectral characteristics of tryptophan and 5-hydroxytryptophan is presented. Hydroxylation of tryptophan at the 5-position results in a large increase in the fluorescence of the molecule. The assay selectively monitors the fluorescence yield of 5-hydroxytryptophan by exciting the reaction mix at 300 nm. The rate of increase of the emission signal was found to be directly proportional to the enzyme concentration. Inner filter effects due to quinonoid dihydropterin accumulation were eliminated by the inclusion of a thiol reductant. Activity measured using this assay method was found to be the same as that determined by established discontinuous HPLC assay methods. The application of the assay to routine activity measurements and to steady-state determinations with the substrates tryptophan and tetrahydropterin is described.

Expression and characterization of the catalytic core of tryptophan hydroxylase

Wild type rabbit tryptophan hydroxylase (TRH) and two truncated mutant proteins have been expressed in Escherichia coli. The wild type protein was only expressed at low levels, whereas the mutant protein lacking the 101 amino-terminal regulatory domain was predominantly found in inclusion bodies. The protein that also lacked the carboxyl-terminal 28 amino acids, TRH102-416, was expressed as 30% of total cell protein. Analytical ultracentrifugation showed that TRH102-416 was predominantly a monomer in solution. The enzyme exhibited an absolute requirement for iron (ferrous or ferric) for activity and did not turn over in the presence of cobalt or copper. With either phenylalanine or tryptophan as substrate, stoichiometric formation of the 4a-hydroxypterin was found. Steady state kinetic parameters were determined with both of these amino acids using both tetrahydrobiopterin and 6-methyltetrahydropterin.

Electrostatic effects on substrate activation in para-hydroxybenzoate hydroxylase: studies of the mutant lysine 297 methionine

p-Hydroxybenzoate hydroxylase (EC 1.14.13.2) is a flavoprotein monooxygenase that catalyzes the incorporation of one atom of molecular oxygen into p-hydroxybenzoate to form 3,4-dihydroxybenzoate. The enzyme activates the substrate at the 3 position to electrophilic substitution by lowering the pKa of the phenolic oxygen. The results presented here indicate that regions of positive potential in the active site facilitate this substrate activation, which is necessary for rapid hydroxylation. We have neutralized a positive point charge by mutating lysine 297 to methionine (K297M). This mutation changes an amino acid near the active site, but not directly in contact with the flavin or the substrate. A variety of transient state kinetic and static parameters have been determined with two substrates. The results indicate that the K297M mutant does not activate the substrate through phenolic ionization to the same extent as wild-type (WT) and yet remains a competent hydroxylase. However, catalysis by the mutant is slow compared to that of WT, particularly in the oxidative half-reaction. Thus, normally quite labile oxygenated flavin intermediates encountered in the hydroxylation pathway of WT p-hydroxybenzoate hydroxylase are stabilized and their decay is rate limiting in the K297M turnover. Electrostatic potential calculations offer an explanation for the lack of substrate activation. The stability of the oxidative reaction intermediates seems to be related to a lower degree of substrate activation.

Evidence for flavin movement in the function of p-hydroxybenzoate hydroxylase from studies of the mutant Arg220Lys

The isoalloxazine ring system of the FAD cofactor of p-hydroxybenzoate hydroxylase must be secluded from solvent at specific stages of catalysis in order to form and stabilize a flavin C4a-hydroperoxide. This species may then react with the activated phenolate of p-hydroxybenzoate. A number of crystal structures of the enzyme with alterations to active site substituents or complexes with analogue benzoates have revealed an alternate position for the isoalloxazine (Gatti et al. (1994) Science 266, 110-114; Schreuder et al. (1994) Biochemistry 33, 10161-10170). This new flavin conformation is 7 A "out" toward solvent and may open a passage for substrate entry to the active site. Arginine 220 is one of the few residues in the structure to demonstrate conformational changes when the flavin is "out". In this study we have made the Arg220Lys mutant to test the significance of this residue in flavin movement. The R220K mutation has brought about dramatic alterations to all aspects of catalysis. Stopped flow kinetic characterization of the mutant has revealed that, while the effector role for the substrate is maintained, there exists an order of magnitude decrease in the limiting rate of reduction, even though there is 40-fold increase in association with NADPH. The mutant enzyme has only a fraction of its reductive half-reaction coupled to product formation, and the hydroxylation process is slow. This occurs despite a higher proportion of the more activated substrate phenolate in the active site. Many of the observed changes can be attributed to a decrease in the stability of the "in" conformation of the flavin during the catalysis and indicate a role for flavin conformational states in many of the catalytic processes of the enzyme.

Plasmid mutagenesis by PCR for high-level expression of para-hydroxybenzoate hydroxylase

We report a PCR deletion mutagenesis method for the exact positioning of a foreign gene (pobA) in the lac operon of an expression plasmid in place of the lacZ protein code. This method requires the synthesis of four oligonucleotides and three PCR reactions to delete unwanted bases and retain the nucleotide sequence naturally found between the lac promoter and the protein code. The engineered plasmid results in the production of at least 40% of the cellular protein as the foreign polypeptide. In the example presented the expression of the protein is high even with a substantial difference in codon usage between the host (Escherichia coli) and a foreign gene from Pseudomonas aeruginosa. Some of the polypeptide produced has the ame properties as native protein and is easily purified. The remainder is present as insoluble inclusion bodies. This method of plasmid refinement may be applicable to the expression of many proteins.