Inhibitors of plant copper amine oxidases

More Than a Diamine Oxidase Inhibitor: L-Aminoguanidine Modulates Polyamine-Related Abiotic Stress Responses of Plants

L-aminoguanidine (AG) is an inhibitor frequently used for investigating plant abiotic stress responses; however, its exact mode of action is not well understood. Many studies used this compound as a specific diamine oxidase inhibitor, whereas other studies used it for reducing nitric oxide (NO) production. Recent studies suggest its antiglycation effect; however, this remains elusive in plants. This review summarises our current knowledge about different targets of AG in plants. Our recommendation is to use AG as a modulator of polyamine-related mechanisms rather than a specific inhibitor. In the future overall investigation is needed to decipher the exact mechanisms of AG. More careful application of AG could give more insight into plant abiotic stress responses.

The Copper Amine Oxidase AtCuAOδ Participates in Abscisic Acid-Induced Stomatal Closure in Arabidopsis

Plant copper amine oxidases (CuAOs) are involved in wound healing, defense against pathogens, methyl-jasmonate-induced protoxylem differentiation, and abscisic acid (ABA)-induced stomatal closure. In the present study, we investigated the role of the Arabidopsis thaliana CuAOδ (AtCuAOδ; At4g12290) in the ABA-mediated stomatal closure by genetic and pharmacological approaches. Obtained data show that AtCuAOδ is up-regulated by ABA and that two Atcuaoδ T-DNA insertional mutants are less responsive to this hormone, showing reduced ABA-mediated stomatal closure and H₂O₂ accumulation in guard cells as compared to the wild-type (WT) plants. Furthermore, CuAO inhibitors, as well as the hydrogen peroxide (H₂O₂) scavenger N,N¹-dimethylthiourea, reversed most of the ABA-induced stomatal closure in WT plants. Consistently, AtCuAOδ over-expressing transgenic plants display a constitutively increased stomatal closure and increased H₂O₂ production compared to WT plants. Our data suggest that AtCuAOδ is involved in the H₂O₂ production related to ABA-induced stomatal closure.

Inhibition of plant amine oxidases by a novel series of diamine derivatives

A series of N,N'-bis(2-pyridinylmethyl)diamines was synthesized and characterized for their inhibition effects towards plant copper-containing amine oxidase (EC 1.4.3.6) and polyamine oxidase (EC 1.5.3.11), which mediate the catabolic regulation of cellular polyamines. Even though these enzymes catalyze related reactions and, among others, act upon two common substrates (spermidine and spermine), their molecular and kinetic properties are different. They also show a different spectrum of inhibitors. It is therefore of interest to look for compounds providing a dual inhibition (i.e. inhibiting both enzymes with the same inhibition potency), which would be useful in physiological studies involving modulations of polyamine catabolism. The synthesized diamine derivatives comprised from two to eight carbon atoms in the alkyl spacer chain. Kinetic measurements with pea (Pisum sativum) diamine oxidase and oat (Avena sativa) polyamine oxidase demonstrated reversible binding of the compounds at the active sites of the enzymes as they were almost exclusively competitive inhibitors with K(i) values ranging from 10(-5) to 10(-3)M. In case of oat polyamine oxidase, the K(i) values were significantly influenced by the number of methylene groups in the inhibitor molecule. The measured inhibition data are discussed with respect to enzyme structure. For that reason, the oat enzyme was analyzed by de novo peptide sequencing using mass spectrometry and shown to be homologous to polyamine oxidases from barley (isoform 1) and maize. We conclude that some of the studied N,N'-bis(2-pyridinylmethyl)diamines might have a potential to be starting structures in design of metabolic modulators targeted to both types of amine oxidases.

Mechanism-based inactivators of plant copper/quinone containing amine oxidases

Copper/quinone amine oxidases contain Cu(II) and the quinone of 2,4,5-trihydroxyphenylalanine (topaquinone; TPQ) as cofactors. TPQ is derived by post-translational modification of a conserved tyrosine residue in the protein chain. Major advances have been made during the last decade toward understanding the structure/function relationships of the active site in Cu/TPQ amine oxidases using specific inhibitors. Mechanism-based inactivators are substrate analogues that bind to the active site of an enzyme being accepted and processed by the normal catalytic mechanism of the enzyme. During the reaction a covalent modification of the enzyme occurs leading to irreversible inactivation. In this review mechanism-based inactivators of plant Cu/TPQ amine oxidases from the pulses lentil (Lens esculenta), pea (Pisum sativum), grass pea (Lathyrus sativus) and sainfoin (Onobrychis viciifolia,) are described. Substrates forming, in aerobiotic and in anaerobiotic conditions, killer products that covalently bound to the quinone cofactor or to a specific amino acid residue of the target enzyme are all reviewed.

Reactions of plant copper/topaquinone amine oxidases with N6-aminoalkyl derivatives of adenine

Plant copper/topaquinone-containing amine oxidases (CAOs, EC 1.4.3.6) are enzymes oxidising various amines. Here we report a study on the reactions of CAOs from grass pea (Lathyrus sativus), lentil (Lens esculenta) and Euphorbia characias, a Mediterranean shrub, with N6-aminoalkyl adenines representing combined analogues of cytokinins and polyamines. The following compounds were synthesised: N6-(3-aminopropyl)adenine, N6-(4-aminobutyl)adenine, N6-(4-amino-trans-but-2-enyl) adenine, N6-(4-amino-cis-but-2-enyl) adenine and N6-(4-aminobut-2-ynyl) adenine. From these, N6-(4-aminobutyl) adenine and N6-(4-amino-trans-but-2-enyl)adenine were found to be substrates for all three enzymes (Km approximately 10(-4)M). Absorption spectroscopy demonstrated such an interaction with the cofactor topaquinone, which is typical for common diamine substrates. However, only the former compound provided a regular reaction stoichiometry. Anaerobic absorption spectra of N6-(3-aminopropyl)adenine, N6-(4-amino-cis-but-2-enyl)adenine and N6-(4-aminobut-2-ynyl)adenine reactions revealed a similar kind of initial interaction, although the compounds finally inhibited the enzymes. Kinetic measurements allowed the determination of both inhibition type and strength; N6-(3-aminopropyl)adenine and N6-(4-amino-cis-but-2-enyl)adenine produced reversible inhibition (Ki approximately 10(-5) - 10(-4) M) whereas, N6-(4-aminobut-2-ynyl)adenine could be considered a powerful inactivator.

1,5-Diamino-2-pentyne is both a substrate and inactivator of plant copper amine oxidases

1,5-diamino-2-pentyne (DAPY) was found to be a weak substrate of grass pea (Lathyrus sativus, GPAO) and sainfoin (Onobrychis viciifolia, OVAO) amine oxidases. Prolonged incubations, however, resulted in irreversible inhibition of both enzymes. For GPAO and OVAO, rates of inactivation of 0.1-0.3 min(-1) were determined, the apparent KI values (half-maximal inactivation) were of the order of 10(-5) m. DAPY was found to be a mechanism-based inhibitor of the enzymes because the substrate cadaverine significantly prevented irreversible inhibition. The N1-methyl and N5-methyl analogs of DAPY were tested with GPAO and were weaker inactivators (especially the N5-methyl) than DAPY. Prolonged incubations of GPAO or OVAO with DAPY resulted in the appearance of a yellow-brown chromophore (lambda(max) = 310-325 nm depending on the working buffer). Excitation at 310 nm was associated with emitted fluorescence with a maximum at 445 nm, suggestive of extended conjugation. After dialysis, the color intensity was substantially decreased, indicating the formation of a low molecular mass secondary product of turnover. The compound provided positive reactions with ninhydrin, 2-aminobenzaldehyde and Kovacs' reagents, suggesting the presence of an amino group and a nitrogen-containing heterocyclic structure. The secondary product was separated chromatographically and was found not to irreversibly inhibit GPAO. MS indicated an exact molecular mass (177.14 Da) and molecular formula (C10H15N3). Electrospray ionization- and MALDI-MS/MS analyses yielded fragment mass patterns consistent with the structure of a dihydropyridine derivative of DAPY. Finally, N-(2,3-dihydropyridinyl)-1,5-diamino-2-pentyne was identified by means of 1H- and 13C-NMR experiments. This structure suggests a lysine modification chemistry that could be responsible for the observed inactivation.

Highly potent propargylamine and allylamine inhibitors of bovine plasma amine oxidase

Propargylamine was reported many years ago to be a mechanism-based inhibitor of bovine plasma amine oxidase (BPAO), though the potency was modest and allylamine was a substrate. Herein, selected 3-substituted propargylamines and allylamines were found to be potent time-dependent inactivators of BPAO, exhibiting IC(50) values of 2-13 microM at 30 degrees C, making them the most potent BPAO inhibitors reported to date. The most potent compound, trans-3-chloroallylamine, was previously found not to inhibit the flavin-dependent monoamine oxidase (the cis isomer did), and thus appears to be a highly selective inhibitor.

Conformational lock and dissociative thermal inactivation of lentil seedling amine oxidase

The kinetics of thermal inactivation of copper-containing amine oxidase from lentil seedlings were studied in a 100 mM potassium phosphate buffer, pH 7, using putrescine as the substrate. The temperature range was between 47-60 degrees C. The thermal inactivation curves were not linear at 52 and 57 degrees C; three linear phases were shown. The first phase gave some information about the number of dimeric forms of the enzyme that were induced by the higher temperatures using the "conformational lock" pertaining theory to oligomeric enzyme. The "conformational lock" caused two additional dimeric forms of the enzyme when the temperature increased to 57 degrees C. The second and third phases were interpreted according to a dissociative thermal inactivation model. These phases showed that lentil amine oxidase was reversibly-dissociated before the irreversible thermal inactivation. Although lentil amine oxidase is not a thermostable enzyme, its dimeric structure can form "conformational lock," conferring a structural tolerance to the enzyme against heat stress.

Inhibition of copper amine oxidases by pyridine-derived aldoximes and ketoximes

The reactions of pea diamine oxidase (PSAO) and 2-phenylethylamine oxidase from Arthrobacter globiformis (AGAO) with pyridine-derived oximes were studied. Pyridine carbaldoximes and alkyl pyridyl ketoximes act as strong non-competitive inhibitors of the enzymes. The inhibition constants K(i) of these compounds vary between 10(-4) and 10(-5) M, for AGAO and some of the studied oximes were found even micromolar K(i) values. The presence of pyridine moiety in the studied compounds has remarkable influence on the inhibition potency. Elementary oximes lacking the heterocyclic ring, i.e., aliphatic (acetone oxime), alicyclic (cyclohexanone oxime) and aromatic (benzaldoxime), are considerably weaker non-competitive inhibitors (K(i) similar to 10(-3) or 10(-2) M). The position of the pyridine ring substitution by -C(R)=NOH group does not play a significant role for the inhibition potency of the studied oxime compounds. If the pyridine nitrogen is quaternised (in hydroxyiminomethyl-1-methylpyridinium iodides), the compound looses its inhibitory properties. Extended length of alkyl substituents on the ketoxime group of alkyl pyridyl ketoximes increases the K(i) value. The enzyme-bound copper represents one of possible target sites for pyridine-derived oxime inhibitors. The addition of an alkyl pyridyl ketoxime or a pyridine carbaldoxime to a native PSAO sample perturbs the absorption spectrum of the enzyme (by an absorption increase in the region 300-400 nm) that is not observed in the spectrum of reacted PSAO apoenzyme. However, an additional formation of hydrogen bonds with amino acid side-chains at the active site should be considered, namely for 3- and 4-substituted pyridine derivatives.

Electrooxidation mechanism of biogenic amines at amine oxidase modified graphite electrode

Amine oxidase (AO, EC. 1.4.3.6) was previously shown to be a very efficient biological recognition element of amperometric biosensors for monitoring biogenic amines. The enzyme was effectively working in both mono- and bienzyme electrode designs, based on either a direct or a mediated electron-transfer pathway. This work focuses on the elucidation of the electron-transfer mechanism of the monoenzymatic unmediated AO-modified biosensor. The observed unmediated catalytic currents were assumed to be caused by (i) a direct electron-transfer process, (ii) the electrooxidation of the formed product, or (iii) their combination. Experiments supporting these assumptions are discussed in detail.

A study on the reactions of plant copper amine oxidase with C3 and C4 aliphatic diamines

The paper reports a study on the reactions of grass pea (Lathyrus sativus) amine oxidase (GPAO) with several aliphatic diamines. The influence of the chain length and of unsaturations in the molecules was examined. Kinetic measurements confirmed that trans-, i.e., (E)-2-butene-1,4-diamine (TDABE) and cis-, i.e., (Z)-2-butene-1,4-diamine (CDABE) could be classified as good substrates. Propane-1,3-diamine (DAP) and propene-1,3-diamine (DAPE) were only weakly oxidized, whereas 1,3-diamino-2-propanol (DAPL) was not utilized as a substrate. Contrary to the inactivator 2-butyne-1,4-diamine (DABI), DAPE was shown to be only a competitive inhibitor. DAP itself did not inhibit the catalytic activity. Irreversible inhibition of the activity occurred only after the incubation of GPAO with DABI; other diamines were without this effect. Differential pulse polarography and chromatofocusing confirmed that the aminoaldehyde product of DABI oxidation binds to the enzyme. Activity assay of pea aminoaldehyde dehydrogenase enabled us to detect the products of the oxidation of TDABE, CDABE, and DAP by GPAO. As the product of DAP oxidation, 3-amino-propanal (APAL) was detected by mass spectrometry and confirmed to be a potent noncompetitive inhibitor of GPAO. The absorption changes that occurred in the course of the reaction of GPAO with the diamines were investigated using rapid-scanning spectrophotometry. DABI, TDABE, CDABE, DAP, and DAPE reacted with GPAO providing characteristic maxima of the Cu(I)-semiquinolamine species that is formed in the catalytic cycle. The results presented here confirm that with the exception of DAPL, all the studied diamines could be classified as GPAO substrates, but only DABI can be considered as a mechanism-based inhibitor.