Mechanism-based cofactor derivatization of a copper amine oxidase by a branched primary amine recruits the oxidase activity of the enzyme to turn inactivator into substrate

The copper amine oxidases (CAOs) have evolved to catalyze oxidative deamination of unbranchedprimary amines to aldehydes. We report that a branched primary amine bearing an aromatization-prone moiety, ethyl 4-amino-4,5-dihydrothiophene-2-carboxylate (1), is recognized enantioselectively (S > R) by bovine plasma amine oxidase (BPAO) both as a temporary inactivator and as a substrate. Substrate activity results from an O(2)-dependent turnover of the covalently modified enzyme, with release of 4-aminothiophene-2-carboxylate (2) as ultimate product. Interaction of (S)-1 with BPAO occurs within the enzyme active site with a dissociation constant of 0.76 microM. Evidence from kinetic and spectroscopic studies, and HPLC analysis of stoichiometric reactions of BPAO with (S)-1, combined with a model study using a quinone cofactor mimic, establishes that the enzyme metabolizes 1 according to a transamination mechanism. Following the initial isomerization of substrate Schiff base to product Schiff base, a facile aromatization of the latter results in a metastable N-aryl derivative of the reduced cofactor aminoresorcinol, which is catalytically inactive. The latter derivative is then slowly oxidized by O(2), apparently facilitated partially by the active-site Cu(II), to form a quinonimine of the native cofactor that releases 2 upon hydrolysis or transimination with substrate amine. Preferential metabolism of (S)-1 is consistent with the preferential removal of the pro-Salpha-proton in metabolism of benzylamine by BPAO. This study represents the first report of product identification in metabolism of a branched primary amine by a copper amine oxidase and suggests a novel type of reversible mechanism-based (covalent) inhibition where inhibition lifetime can be fine-tuned independently of inhibition potency.     

Michaelis-Menten analysis of bovine plasma amine oxidase by capillary electrophoresis using electrophoretically mediated microanalysis in a partially filled capillary

A method for determining bovine plasma amine oxidase (PAO; EC 1.4.3.6) activity with benzylamine (Bz) as substrate is described. Electrophoretically mediated microanalysis (EMMA) combined with micellar electrokinetic capillary chromatography (MEKC) was used to perform an on-capillary enzymatic reaction and to separate the generated benzaldehyde from the other reaction products. The capillary was only partially filled with the separation solution, since the enzyme was unstable in the presence of the applied surfactant. The initial reaction velocity of the enzyme-catalyzed reaction was estimated from the peak area of the enzyme product, benzaldehyde. An amplification step was introduced by means of an on-capillary incubation of 15 min, in order to accumulate enough reaction product to detect spectrophotometrically at 254 nm. This set-up resulted in a fully automated assay, which can be carried out in less then 35 min. Using the Lineweaver-Burk equation, an average Michaelis constant (K(M)) for PAO was calculated to be 0.74 mM +/- 0.05 mM, which is consistent with previously reported values.     

Copper/topaquinone-containing amine oxidase from lentil seedlings and bovine plasma: Catalytic mechanism and energetic domains

In this review the characteristics of the prosthetic group and the role of copper in amine oxidase purified from lentil seedlings are compared with the corresponding features of the amine oxidase isolated from bovine serum. Although both enzymes contain the same organic cofactor, the 6-hydroxydopa (2,4,5-trihydroxyphenethylamine) quinone, the catalytic cycle of lentil seedling amine oxidase operates through a Cu(I)-free-radical intermediate of the cofactor, whereas in bovine serum enzyme the radical form was not observed. The role of the metal in the catalytic mechanism of the two enzymes is also discussed. Moreover, the energetic domains and the effect of the temperature on activity, for both enzymes, are examined using differential scanning calorimetry.     

Kinetic determination of plasma amine oxidase photometric test with aldehyde dehydrogenase

Conclusions Of three different methods for the determination of plasma amine oxidase activity we found the method with coupled aldehyde dehydrogenase to show the least interferences and to give reliable results with unfractionated samples, e. g. human plasma. From our inhibitor studies we concluded that no flavine containing amine oxidase (MAO) could be detected in human plasma and that all detectable amine oxidase activity in plasma was attributable to Cu-amine oxidases.

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Kinetische Bestimmung der Plasmaaminoxidase; Photometrischer Test mit Aldehyd-Dehydrogenase

     

Activity staining of plasma amine oxidase after polyacrylamide gel electrophoresis and its application to natural inhibitor screening

Plasma amine oxidase (plasma AO, EC 1.4.3.6) is a copper-containing AO which converts benzylamine (BZ) to benzaldehyde, generating hydrogen peroxide and ammonia. The peroxidase was used as an ancillary enzyme to couple hydrogen peroxide to 3-amino-9-ethylcarbazole (AEC) to achieve plasma AO activity after electrophoresis on native polyacrylamide gels. It was confirmed that plasma AO is inhibited by semicarbazide but neither by clorgyline nor by deprenyl. We also used plasma AO activity staining for the screening of natural inhibitors. This fast and sensitive method can be used in the process of plasma AO purification, characterization, and inhibitor screening.     

Primary amine, thiourea-based dual catalysis motif for synthesis of stereogenic, all-carbon quaternary center-containing cycloalkanones

The enantioselective synthesis of α,α-disubstituted cycloalkanones has been developed using a primary amine, thiourea-based dual catalysis pathway. A range of electrophiles and ring sizes are tolerated under the reaction conditions. A possible catalytic cycle is presented to explain the reactivity.     

The bicyclic n,n'-diacylaminal: a new motif for molecular self-assembly

A cage-shaped N,N'-diacylaminal crystallizes from some aromatic solvents as "supramolecular chair cyclohexanes", squat cylindrical hexamers with approximate D3d symmetry containing two arene molecules, and from other aromatic and nonaromatic solvents as infinite tapes. A homologous diacylaminal crystallizes only as an infinite tape. The hexamers represent the first examples of cyclic hexamers held together by %@mt;sys@%%@bold@%R%@rsf@%%@sx@%2%@be@%2%@sxx@%%@fn;(;vis;full;auto@%8%@fnx;);vis;full@%-type%@mx@% hydrogen bonds in which the hydrogen-bonded atoms are not coplanar. The diacylaminal represents a new supramolecular synthon, one perhaps more suited to the design of three-dimensional architectures.     

Selective amine recognition: development of a chemosensor for dopamine and norepinephrine

[reaction: see text] A boronic acid-containing coumarin aldehyde was designed and synthesized. The sensor binds to catecholamines such as dopamine and norepinephrine by forming an iminium ion with the amine as well as a boronate ester with the catechol. An internal hydrogen bond produces a colorimetric response to these analytes with good selectivity for catecholamines over simple amines. The fluorescence of the sensor is quenched by the catechol.     

Selective kinase inhibition by exploiting differential pathway sensitivity

Protein kinase inhibitors are optimized to have high affinity for their intended target(s) to elicit the desired cellular effects. Here, we asked whether differences in inhibitory sensitivity between two kinase signaling pathways, controlled by the cyclin-dependent kinases Cdk1 and Pho85, can be sufficient to allow for selective targeting of one pathway over the other. We show the oxindole inhibitor GW297361 elicits a Pho85-selective response in cells despite having a 20-fold greater biochemical potency for Cdk1 in vitro. We provide evidence that partial inhibition of Pho85 is sufficient to activate Pho85-dependent signaling, but partial inhibition of Cdk1 is not sufficient to block Cdk1-dependent cell proliferation. Identification of highly sensitive kinases may provide a means to achieve selective perturbation of kinase signaling pathways complementary to efforts to achieve maximal differences between in vitro IC50 values.     

Ternifolide A, a new diterpenoid possessing a rare macrolide motif from Isodon ternifolius

Ternifolide A (1), a new diterpenoid featuring a unique 10-membered lactone ring formed between C-6 and C-15, along with ternifolide B (2), a nor-diterpenoid, and ternifolide C (3) were isolated from the leaves of Isodon ternifolius. Both H-8 and H-9 being α-orientations in compound 1 were found for the first time. The absolute configurations of 1 and 3 were confirmed by X-ray diffraction study. Compounds 1 and 3 were evaluated for their cytotoxicity.     

Selective etching of graphene edges by hydrogen plasma

We devised a controlled hydrogen plasma reaction at 300 °C to etch graphene and graphene nanoribbons (GNRs) selectively at the edges over the basal plane. Atomic force microscope imaging showed that the etching rates for single-layer and few-layer (≥2 layers) graphene are 0.27 ± 0.05 nm/min and 0.10 ± 0.03 nm/min, respectively. Meanwhile, Raman spectroscopic mapping revealed no D band in the planes of single-layer or few-layer graphene after the plasma reaction, suggesting selective etching at the graphene edges without introducing defects in the basal plane. We found that hydrogen plasma at lower temperature (room temperature) or a higher temperature (500 °C) could hydrogenate the basal plane or introduce defects in the basal plane. Using the hydrogen plasma reaction at the intermediate temperature (300 °C), we obtained narrow, presumably hydrogen terminated GNRs (sub-5 nm) by etching of wide GNRs derived from unzipping of multiwalled carbon nanotubes. Such GNRs exhibited semiconducting characteristics with high on/off ratios (～1000) in GNR field effect transistor devices at room temperature.     

24-Thiacycloartanol, a potent mechanism-based inactivator of plant sterol methyltransferase

The rationally designed substrate mimic of cycloartenol, 24-thiacycloartanol (24-TC) 4A, and its corresponding sulfonium salt 5A were tested against the recombinant sterol methyltransferase (SMT) from Glycine max (soybean). Analog 4A was found to irreversibly inactivate the enzyme generating competitive- and time-dependent inhibition of activity accompanied by a K_i value of 2 μM and k_inact of 0.3 min⁻¹. Analog 5A, a presumptive high-energy intermediate of the SMT catalyzed reaction, was found to be a reversible, non-competitive-type inhibitor generating a K_i value of 55 nM.