Mechanistic Studies on Bioinspired Aerobic C-H Oxidation of Amines with an ortho-Quinone Catalyst

We report herein our mechanistic studies of the ortho-quinone-catalyzed aerobic oxidation of primary, secondary, and tertiary amines. Two different catalytic pathways were discovered for the reductive half reactions: for primary amines, the reaction was found to proceed via a transamination pathway, while the reactions with secondary amines and tertiary amines proceeded via hydride transfer. We also found that the amine substrates could significantly promote the regeneration of the ortho-quinone catalyst in the oxidative half reaction, in which a proton transfer occurs between the amine substrates and catechol derivatives (the reduced form of the ortho-quinone catalyst).

Characterization of recombinant human diamine oxidase (rhDAO) produced in Chinese Hamster Ovary (CHO) cells

Human diamine oxidase (hDAO) efficiently degrades polyamines and histamine. Reduced enzyme activities might cause complications during pregnancy and be involved in histamine intolerance. So far hDAO has been characterized after isolation from either native sources or the heterologous production in insect cells. Accessibility to human enzyme is limited and insect cells produce non-human glycosylation patterns that may alter its biochemical properties. We present the heterologous expression of hDAO in Chinese Hamster Ovary (CHO) cells and a three step purification protocol. Analysis of metal content using ICP-MS revealed that 93% of the active sites were occupied by copper. Topaquinone (TPQ) cofactor content was determined using phenylhydrazine titration. Ninety-four percent of DAO molecules contained TPQ and therefore the copper content at the active site was indirectly confirmed. Mass spectrometric analysis was conducted to verify sequence integrity of the protein and to assess the glycosylation profile. Electronic circular dichroism and UV-vis spectra data were used to characterize structural properties. The substrate preference and kinetic parameters were in accordance with previous publications. The establishment of a recombinant production system for hDAO enables us to generate decent amounts of protein with negligible impurities to address new scientific questions.

Bioinspired organocatalytic aerobic C-H oxidation of amines with an ortho-quinone catalyst

A simple bioinspired ortho-quinone catalyst for the aerobic oxidative dehydrogenation of amines to imines is reported. Without any metal cocatalysts, the identified optimal ortho-quinone catalyst enables the oxidations of α-branched primary amines and cyclic secondary amines. Mechanistic studies have disclosed the origins of different performances of ortho-quinone vs para-quinone in biomimetic amine oxidations.

Production and radioprotective effects of pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ) was produced by fermentation of the Methylovorus sp. MP688 strain and purified by ion-exchange chromatography, crystallization and recrystallization. The yield of PQQ reached approximately 125 mg/L and highly pure PQQ was obtained. To determine the optimum dose of PQQ for radioprotection, three doses (2 mg/kg, 4 mg/kg, 8 mg/kg) of PQQ were orally administrated to the experimental animals subjected to a lethal dose of 8.0 Gy in survival test. Survival of mice in the irradiation + PQQ (4 mg/kg) group was found to be significantly higher in comparison with the irradiation and irradiation + nilestriol (10 mg/kg) groups. The numbers of hematocytes and bone marrow cells were measured for 21 days after sublethal 4 Gy gamma-ray irradiation with per os of 4 mg/kg of PQQ. The recovery of white blood cells, reticulocytes and bone marrow cells in the irradiation + PQQ group was faster than that in the irradiation group. Furthermore, the recovery of bone marrow cell in the irradiation + PQQ group was superior to that in irradiation + nilestriol group. Our results clearly indicate favourable effects on survival under higher lethal radiation doses and the ability of pyrroloquinoline quinine to enhance haemopoietic recovery after sublethal radiation exposure.

Strategy for the isolation of native dehydrogenases with potential for biosensor development from the organism Hyphomicrobium zavarzinii ZV580

Dehydrogenases are interesting candidates for the development of electrochemical biosensors. Most dehydrogenases are characterised by a comparatively broad substrate spectrum, yet highly specific enzymes exist as well. A specific formaldehyde dehydrogenase has, e.g., been described for the organism Hyphomicrobium zavarzinii ZV580. Isolation of enzymes from their natural source instead of a recombinant expression renders the isolation more challenging, as common tools such as affinity tags are no longer available. In this contribution, we develop chromatographic procedures for such isolation tasks. The previously described formaldehyde dehydrogenase was isolated by two procedures, one based on affinity chromatography, the other on hydroxyapatite. Neither procedure yielded an active enzyme. In addition two dehydrogenases, a formaldehyde and a methylamine dehydrogenase, were found in the cell free extract, which had not been described previously. Both enzymes could be isolated to near purity by a sequence of hydroxyapatite and anion exchange chromatography. The new formaldehyde dehydrogenase requires reconstitution with calcium and pyrroloquinoline quinone in order to become active. The enzyme shows no cross-reactivity with methylamine or methanol. The methylamine dehydrogenase catalyses the conversion of methylamine into formaldehyde, hence it could become a technical catalyst for the inverse reaction. This enzyme consists of two types of subunit and may be one of the rare alpha,beta-methylamine dehydrogenases.

Mechanistic studies on the lysine-induced N-formylation of 2,5-dimethyl-p-benzoquinonediimine

2,5-Dimethyl- p-benzoquinonediimine was used as a model to study the reactivity of p-benzoquinonediimines, the first oxidation intermediates of allergenic p-amino aromatic compounds, toward lysine, as it has been suggested that this amino acid could play a key role in the induction mechanism of allergic contact dermatitis for a number of chemicals. The use of 6-[ (13)C]lysine and Nalpha-acetyl-6-[ (13)C]lysine, in association with (13)C NMR and HPLC in tandem with mass spectrometry techniques, allowed the identification of 4-amino-2,5-dimethylformanilide, 4-amino-2,5-dimethyl[ (13)C]formanilide, and the derivative containing the amino acid covalently bound at the para position. While enzymatic N-acetylation of p-phenylenediamine (PPD) has been described in the literature, in human skin for example, to our knowledge this was the first time that N-formylation of a PPD derivative induced by the reaction with an amino acid such as lysine was observed in solution, together with the formation of an adduct with the amino acid. To afford an explanation for the lysine-induced N-formylation,we undertook mechanistic studies, and they showed that 2,5-dimethyl- p-benzoquinonediimine was involved in an oxido reduction process that is capable of deaminating the alpha-NH 2 group, even when N-acetylated, and the epsilon-NH 2 groups of lysine in an oxidative way, forming the real reactive intermediates for N-formylation. This initially unexpected behavior should be considered when investigating the reactivity of such compounds with lysine-containing peptides or proteins in the context of hapten-protein binding studies.

Molecular Dynamics Study of the Active Site of Methylamine Dehydrogenase

We have obtained AMBER94 force-field parameters for the TTQ cofactor of the enzyme methylamine dehydrogenase (MADH). This enzyme catalyzes the oxidation of methylamine to produce formaldehyde and ammonia. In the rate-determining step of the catalyzed reaction, a proton is transferred from the methyl group of the substrate to residue Asp76. We used the new parameters to perform molecular dynamics simulations of MADH in order to characterize the dynamics of the active site prior to the proton-transfer step. We found that only one of the oxygen atoms of Asp76 can act as an acceptor of the proton. The other oxygen interacts with Thr122 via a strong hydrogen bond. In contrast, because of the rotation the methyl group of the substrate, the three methyl hydrogen atoms are alternately in position to be transferred. The distance that the proton has to travel presents a broad distribution with a peak between 1.0 and 1.1 A and reaches values as short as 0.8 A. The fluctuation of the distance between the donor and the acceptor has the largest frequency component at 50 cm(-1), but the spectrum presents a rich structure between 10 and 400 cm(-1). The more important peaks appear below 250 cm(-1).

A Dopaquinone Model That Mimics the Water Addition Step of Cofactor Biogenesis in Copper Amine Oxidases

The consensus mechanism for biogenesis of the 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor in copper amine oxidases involves a key water addition to the dopaquinone intermediate. Although hydration of o-quinones seems straightforward and was implicated previously in aqueous autoxidation of catechols to give ultimately hydroxyquinones, a recent study (Mandal, S.; Lee, Y.; Purdy, M. M.; Sayre, L. M. J. Am. Chem. Soc. 2000, 122, 3574-3584) showed that the observed hydroxyquinones arise not from hydration, but from addition to the o-quinones of H(2)O(2) generated during autoxidation of the catechols. In the enzyme case, hydration of dopaquinone is proposed to be mediated by the active site Cu(II). To establish precedent for this mechanism, we engineered a catechol tethered to a Cu(II)-coordinating unit, such that the corresponding o-quinone could be generated in situ by oxidation with periodate (to avoid generation of H(2)O(2)). Thus, coordination of 4-((2-(bis(2-pyridylmethyl)amino)ethylamino)methyl)-1,2-benzenediol (1) to Cu(II) and subsequent addition of periodate resulted in rapid formation of the TPQ-like corresponding hydroxyquinone. Hydroxyquinone formation was seen also using Zn(II) and Ni(II), but not in the absence of M(II). Under the same conditions, periodate oxidation of the simple catechol 4-tert-butylcatechol does not give hydroxyquinone in the presence or absence of Cu(II). M(II)OH(2) pK(a) data for the Cu(II), Zn(II), and Ni(II) complexes with the pendant tetradentate ligand in the masked (dimethyl ether) catechol form, and kinetic pH-rate profiles of the metal-dependent hydroxyquinone formation from periodate oxidation of catechol 1, suggested a rate-limiting addition step of the ligand-coordinated M(II)OH to the o-quinone intermediate. This study represents the first chemical demonstration of a true o-quinone hydration, which occurs in cofactor biogenesis in copper amine oxidases.

Evaluation of dose-response curve analysis in delineating shared or different molecular sites of action for osteolathyrogens

Single-chemical and mixture concentration-response curves generated using a frog embryo model were examined for value in assessing whether chemicals exert toxic effects at the same or at different molecular sites of action. Toxicity tests were conducted on a series of osteolathyrogens, i.e. chemicals that inhibit cross-linking of developing connective tissue fibers. Induction of osteolathyrism, which manifests as lesions in the notochord of exposed tadpoles, has several possible molecular sites of action, including agent-cofactor reactivity during the enzyme-mediated cross-linking process. UV-VIS spectrophotometry of osteolathyrogen-cofactor reactivity (i.e. in vitro analysis) was coupled with the 96-h frog embryo mixture toxicity assay (i.e. in vivo toxicity) to compare molecular sites of action for several osteolathyrogens with the combined osteolathyritic effects of the agents. Single-chemical concentration-response curves were used to calculate theoretical curves for the dose-addition model of combined effect. Slope and EC(50) values for both theoretical and experimental mixture curves were then generated to statistically examine the hypothesis that agents with shared sites of action have dose-response curve (DRC) slopes that are similar when given alone and in combination, and slope and EC(50) values that, when administered together, are consistent with those calculated for dose-addition. For combinations of cofactor-binding agents (semicarbazide, thiosemicarbazide, aminoacetonitrile), slope values were generally similar with additivity quotients near 1.0 (1.0=dose-additive) and combined osteolathyritic effects that were consistent with dose-addition. None of these were true for combinations that included agents that did not show rapid cofactor binding (β-aminopropionitrile, methyleneaminoacetonitrile). The results suggest that DRC analysis could be a useful tool for delineating common or different molecular sites of toxic action and that the approaches used warrant further study for evaluating the mechanistic basis for combined effects of toxicants.

Regioselectivity for condensation reactions of quinonoid models of tryptophan tryptophylquinone: a density functional theory study

The model compounds of tryptophan tryptophylquinone (TTQ), o-benzoquinone (OBQ), 3-methyl-6,7-dihydro-1H-6,7-indoledione (MIQ), and 3-methyl-4-(3-methyl-1H-2-indolyl)-6,7-dihydro-1H-6,7-indoledione (IIQ), all of which are characteristic of o-quinone groups, have been studied with density functional theory. The dihedral angle of the two indole rings (chi) of IIQ is calculated to be 49.6 degrees for the global minimum. Another local minimum, 0.74 kcal/mol higher in energy, with a chi value of 123.5 degrees is also fully optimized. The transition state connecting the two minima, with a chi value of 97.9 degrees, has been located and the rotation barrier is 1.71 kcal/mol. A scan of the potential energy surface along this dihedral angle showed that the difference of the total energy was within 1.0 kcal/mol at a range of the dihedral angle from 30 degrees to 75 degrees. Hence, IIQ is flexible for the rotation of inter-indole rings. The origin of regioselectivity for the condensation reactions of the models MIQ and IIQ with NH(3) has been elucidated. It is shown that the energy difference between the two different types of carbinolamine intermediates (Delta E) and their corresponding transition structures (Delta E(++)) should be responsible for the regioselectivity. To assess the effect of the fused ring on regioselectivity of the condensation reaction, a series of models were designed. A good linear correlation has been found between the energy difference of the two different carbinolamine intermediates (Delta E) and that of the corresponding transition states (Delta E(++)), suggesting that the factors that stabilize the carbinolamine intermediate also favor the stability of the corresponding transition structure. The pair, 6-amino-6-hydroxy-8-methyl-6H-quinolin-5-one and 5-amino-5-hydroxy-8-methyl-5H-quinolin-6-one (7/8), deviates from the correlation and represents some anomalous behavior, which may be due to their structural particularity. It also has been shown that the tricyclic models, which consist of OBQ and two fused heterocyclic rings, represent more regioselectivity in contrast to the bicyclic systems. Moreover, the fused electron-donating pyrrole and the fused electron-withdrawing pyridine or pyrimidine show a somewhat synergistic effect on each other via the medial OBQ molecule. The barrier of the condensation reaction for pyrrolo[2,3-f]quinoline-4,5-dione is calculated to be ca. 22 kcal/mol. This is lower than that for MIQ (ca. 33 kcal/mol) and IIQ (ca. 32 kcal/mol) by as much as 10.0 kcal/mol, explaining reasonably the larger catalytic effect of pyrroloquinolinequinone (PQQ) relative to TTQ.

Biochemical and toxicological evaluation of agent-cofactor reactivity as a mechanism of action for osteolathyrism

In vitro reactivity for each of four osteolathyrogens with a model compound for the lysyl oxidase (LO) cofactor was evaluated and coupled with mixture toxicity testing to evaluate agent-cofactor reactivity as a potential mechanism of action for osteolathyrism. Reactivity of the model cofactor (mLTQ: 4-butylamino-5-methyl-o-quinone), with each of two ureides, semicarbazide (SC) and thiosemicarbazide (TSC), and each of two aminonitriles, aminoacetonitrile (AAN) and beta-aminopropionitrile (betaAPN), was assessed using UV-vis spectrophotometry; both in the absence and presence of Cu(II)-bipyridine (bipy) complex. Two sets of mixture toxicity experiments were conducted using a frog embryo assay that assessed the incidence of osteolathyrism in the notochord of tadpoles after 96-h exposure. The resulting concentration-response curves for each set were evaluated (chi(2) goodness-of-fit test) against theoretical curves for two combined effects models: dose-addition and independence, to determine the combined effect of each osteolathyrogen combination. The agents SC, TSC and AAN each showed rapid, irreversible reactivity with mLTQ, both in the absence and presence of Cu(II)-bipy complex, as indicated by bleaching of the mLTQ peak (504 nm) and formation of an adduct at 350 nm. betaAPN showed no apparent reactivity in the absence of prolonged incubation with mLTQ, whether Cu(II)-bipy complex was present or not. After prolonged incubation (24-144 h) a new peak formed at 350 nm, suggesting that betaAPN reacts weakly with the cofactor, but in a manner different from the other agents examined. The toxicity tests indicated a dose-additive combined effect for the SC:TSC, AAN:SC and AAN:SC:TSC mixtures (0.1<P<0.9; with slope and EC(50) additivity quotient values between 0.93-1.14), suggesting these agents induce osteolathyrism via a common mechanism. The betaAPN:SC and betaAPN: SC:TSC mixtures showed a greater-than dose-additive combined effect (P<0.001; slope and EC(50) additivity quotient values from 0.74 to 0.89), suggesting a different or additional mechanism of osteolathyrism for betaAPN. Taken together the results indicate that SC, TSC and AAN induce osteolathyrism primarily, if not completely, by binding to the LO cofactor, thereby inhibiting proper connective tissue fiber cross-linking. While betaAPN also has the potential to react with the cofactor, the nature of the reactivity observed suggests that betaAPN-cofactor binding is, at best, a secondary mechanism for induction of osteolathyrism in vivo.

Copper-promoted overall transformation of 4-tert-butylphenol to its para-hydroxyquinonic derivative, 2-hydroxy-5-tert-butyl-1,4-benzoquinone. Biomimetic studies on the generation of topaquinone in copper amine oxidases

Topaquinone (TPQ) is a cofactor present at the active site of copper amine oxidases, derived from a Tyr residue inserted in the polypeptide chain through a copper-dependent but otherwise largely unknown mechanism. A simple model system was developed that permits to obtain the overall transformation of 4-tert-butylphenol, chosen as a model for Tyr, into a TPQ-like, para-hydroxyquinonic structure in the presence of Cu(II)-imidazole mononuclear complexes.

Physiological importance of quinoenzymes and the O-quinone family of cofactors

O-quinone cofactors derived from tyrosine and tryptophan are involved in novel biological reactions that range from oxidative deaminations to free-radical redox reactions. The formation of each of these cofactors appears to involve post-translational modifications of either tyrosine or tryptophan residues. The modifications result in cofactors, such as topaquinone (TPQ), tryptophan tryptophylquinone (TTQ), lysine tyrosylquinone (LTQ) or the copper-complexed cysteinyl-tyrosyl radical from metal-catalyzed reactions. Pyrroloquinoline quinone (PQQ) appears to be formed from the annulation of peptidyl glutamic acid and tyrosine residues stemming from their modification as components of a precursor peptide substrate. PQQ, a primary focus of this review, has invoked considerable interest because of its presence in foods, antioxidant properties and role as a growth-promoting factor. Although no enzymes in animals have been identified that exclusively utilize PQQ, oral supplementation of PQQ in nanomolar amounts increases the responsiveness of B- and T-cells to mitogens and improves neurologic function and reproductive outcome in rodents. Regarding TPQ and LTQ, a case may be made that the formation of TPQ and LTQ is also influenced by nutritional status, specifically dietary copper. For at least one of the amine oxidases, lysyl oxidase, enzymatic activity correlates directly with copper intake. TPQ and LTQ are generated following the incorporation of copper by a process that involves the two-step oxidation of a specified tyrosyl residue to first peptidyl dopa and then peptidyl topaquinone to generate active enzymes, generally classed as "quinoenzymes." Limited attention is also paid to TTQ and the copper-complexed cysteinyl-tyrosyl radical, cofactors important to fungal and bacterial redox processes.