The organic functional group in copper-containing amine oxidases

Inactivation of bovine plasma amine oxidase by 1,1,1-trihalo-3-aminopropanes

In this paper, we report the inactivation of copper containing bovine plasma amine oxidase (BPAO) by a series of saturated alkylamines containing halogen atoms at γ-position, which are 1,1,1-trihalo-3-aminopropane, 1,1,1-trifluoro-2-hydroxy-3-aminopropane, 1,1,1-trichloro-2-hydroxy-3-aminopropane, and 1,1,1-trichloro-2-(2-phenethyloxy)-3-aminopropane. The trihalo-2-hydroxypropylamine analogs exhibited a time-dependent inactivation behavior of BPAO, with 1,1,1-trifluoro-2-hydroxy-3-aminopropane as the most efficient inactivator. The incorporation of a OH group at β-position increased inactivation efficiency by 10-fold within the trifluoro analogs, and the incorporation of a phenethyloxy group at β-position exhibited a higher efficiency by 3-fold within the trichloro analogs based on I₇₅ values. All four compounds were found to be irreversible inactivators for BPAO.

Inactivation of bovine plasma amine oxidase by haloallylamines

Various 2- and 3-haloallylamines were synthesized and evaluated as inhibitors of the quinone-dependent bovine plasma amine oxidase (BPAO). 3-Haloallylamines, which were previously found to be good inhibitors of the flavin-dependent mitochondrial monoamine oxidase (MAO), exhibited a time-dependent inactivation of BPAO, with the 2-phenyl analogs being more potent than the 2-methyl analogs. No plateau of enzyme activity loss was observed, suggestive of a lack of competitive partitioning to normal turnover. The (E)- and (Z)-2-phenyl-3-fluoro analogs were the most potent (low microM IC(50)s), with the corresponding 3-bromo and 3-chloro analogs being >10-fold less potent. In each case, the Z-isomers were more potent than the E-isomers, the reverse of the configurational inhibitory preference observed with MAO. In contrast to the 2-phenyl analogs, 3-phenyl-2(or 3)-chloroallylamines displayed a partitioning behavior, consistent with these being both substrates and inactivators of BPAO.

Gene organization and molecular modeling of copper amine oxidase from Aspergillus niger: re-evaluation of the cofactor structure

Amine oxidase AO-I from Aspergillus niger AKU 3302 has been reported to contain topa quinone (TPQ) as a cofactor; however, analysis of the p-nitrophenylhydrazine-derivatized enzyme and purified active site peptides showed the presence of a carboxylate ester linkage of TPQ to a glutamate. The catalytic functionality of such a cross-linked cofactor has recently been shown unlikely by spectroscopic and voltammetric studies on synthesized model compounds. We have obtained resonance Raman spectra of native and substrate-reduced AO-I demonstrating that the catalytically active cofactor is unmodified TPQ. The primary structure of the enzyme (GenBank acc. no. U31869) has been reviewed and updated by repeated isolation and sequencing of AO-I cDNA. This allowed rectification of several errors that account for previously reported low homology to other amine oxidases in the regions around copper binding histididyl residues. The results were confirmed by cloning the ao-1 structural gene (GenBank acc. no. AF362473). Analysis of the gene 5'-upstream region of the gene revealed potential binding sites for an analog of NIT2, the nitrogen metabolism regulatory protein found in Neurospora crassa and other fungi. The molecular structure of AO-I was modeled by a comparative method using published crystal structures of amine oxidases as templates.

A comparison of the mechanism for the reductive half-reaction between pea seedling and other copper amine oxidases (CAOs)

In a previous DFT study a mechanism for the reductive half-reaction of pea seedling amine oxidase (PSAO) was suggested. In many of the suggested steps a lysine at the active site plays an important role. However, this lysine is not found in other amine oxidases. The primary aim of the present DFT study is therefore to investigate alternative mechanisms for those amine oxidases (CAO) where the lysine residue is not present. One of the most important roles suggested for the lysine in PSAO was to protonate the O2-site of TPQ before the critical Cbond;H bond cleavage of the substrate. In the absence of lysine the O2-site of TPQ is now suggested to be protonated by a water ligand on the copper metal complex, in line with experimental suggestions. In other steps the role of lysine is taken over by an asparagine. All results are compared with experimental observations and good agreement is generally found.

Inhibition of six copper-containing amine oxidases by the antidepressant drug tranylcypromine

Potential inhibitory effects of the clinically utilized monoamine oxidase inhibitor tranylcypromine (TCP) on mammalian, plant, bacterial, and fungal copper-containing amine oxidases have been examined. The following enzymes have been investigated: human kidney diamine oxidase (HKAO), bovine plasma amine oxidase (BPAO), equine plasma amine oxidase (EPAO), pea seedling amine oxidase (PSAO), Arthrobacter globiformis amine oxidase (AGAO), and Pichia pastoris lysyl oxidase (PPLO). Only BPAO, EPAO, and AGAO were found to lose significant levels of activity when incubated with varying amounts of TCP. Inhibition of BPAO was completely reversible, with dialysis restoring full activity. TCP inhibition of AGAO was also found to be ultimately reversible; however, dialysis did not remove all bound compounds. Chemical displacement with either substrate or a substrate analogue successfully removed all bound TCP, indicating that this compound has a high affinity for the active site of AGAO. The notable lack of TCP inhibition on HKAO argues against the inhibition of diamine oxidase as a potential source for some of the deleterious side effects occurring in patients treated with this antidepressant. The marked differences observed in behavior among these enzymes speaks to the importance of intrinsic structural differences between the active sites of copper amine oxidases (CAO) which affect reactivity with a given inhibitor.

Cloning, sequence analysis, and characterization of the 'lysyl oxidase' from Pichia pastoris

Lysyl oxidase from Pichia pastoris has been successfully overexpressed. EPR and resonance Raman experiments have shown that copper and TPQ are present, respectively. Lysyl oxidase from P. pastoris has a similar substrate specificity to the mammalian enzyme (both have been shown to oxidize peptidyl lysine residues) and is 30% identical to the human kidney diamine oxidase (the highest of any non-mammalian source). This enzyme also has a relatively broad substrate specificity compared to other amine oxidases. Molecular modeling data suggest that the substrate channel in lysyl oxidase from P. pastoris permits greater active site access than observed in structurally-characterized amine oxidases. This larger channel may account for the diversity of substrates that are turned over by this enzyme.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

CuI-semiquinone radical species in plant copper-amine oxidases

The intermediate CuI-semiquinone radical species in the catalytic mechanism of copper-amine oxidase from Lens esculenta and Pisum sativum seedlings has been studied by optical, Raman resonance and ESR spectroscopies and by stopped-flow and temperature-jump measurements. Treatment of highly purified enzyme preparations with good, poor or suicide substrates, under anaerobic and aerobic conditions, at different pH values and temperatures, makes it possible to generate, detect and characterize this free radical intermediate.

Inhibitors of plant copper amine oxidases

In this review, inhibitors of plant copper amine oxidases from Lens esculenta seedlings, Pisum sativum seedlings, and Euphorbia characias latex are described. Reversible competitive inhibitors and non-competitive inhibitors, irreversible active-site directed inhibitors and mechanism-based inactivators are reviewed in regard to their mechanisms of action.

Characterization of Euphorbia characias latex amine oxidase

A copper-containing amine oxidase from the latex of Euphorbia characias was purified to homogeneity and the copper-free enzyme obtained by a ligand-exchange procedure. The interactions of highly purified apo- and holoenzyme with several substrates, carbonyl reagents, and copper ligands were investigated by optical spectroscopy under both aerobic and anaerobic conditions. The extinction coefficients at 278 and 490 nm were determined as 3.78 x 10(5) M-1 cm-1 and 6000 M-1 cm-1, respectively. Active-site titration of highly purified enzyme with substrates and carbonyl reagents showed the presence of one cofactor at each enzyme subunit. In anaerobiosis the native enzyme oxidized one equivalent substrate and released one equivalent aldehyde per enzyme subunit. The apoenzyme gave exactly the same 1:1:1 stoichiometry in anaerobiosis and in aerobiosis. These findings demonstrate unequivocally that copper-free amine oxidase can oxidize substrates with a single half-catalytic cycle. The DNA-derived protein sequence shows a characteristic hexapeptide present in most 6-hydroxydopa quinone-containing amine oxidases. This hexapeptide contains the tyrosinyl residue that can be modified into the cofactor 6-hydroxydopa quinone.

Identification of the quinone cofactor in a lysyl oxidase from Pichia pastoris

A copper amine oxidase from Pichia pastoris is the only known non-mammalian lysyl oxidase [Tur, S.S. and Lerch, K. (1988) FEBS Lett. 238, 74-76]. Recently, the cofactor in mammalian lysyl oxidase has been identified as a novel lysine tyrosylquinone moiety [Wang, S.X., Mure, M., Medzihradszky, K.F., Burlingame, A.L., Brown, D.E., Dooley, D.M., Smith, A.J., Kagan, H.M. and Klinman, J.P. (1996) Science 273, 1078-1084]. In order to identify the cofactor in P. pastoris lysyl oxidase, we have isolated the phenylhydrazone-derivative of the active-site peptide. This peptide has the active-site sequence conserved among topa quinone containing amine oxidases. The resonance Raman spectra of the phenylhydrazone derivatives of the enzyme, active-site peptide, and a topa quinone model compound are essentially identical. Collectively, these results establish that P. pastoris lysyl oxidase is a topa quinone enzyme.

Two amine oxidases from Aspergillus niger AKU 3302 contain topa quinone as the cofactor: unusual cofactor link to the glutamyl residue occurs only at one of the enzymes

Amine oxidases (EC 1.4.3.6) from Aspergillus niger, AO-I (2 x 75 kDa) and AO-II (80 kDa), were examined to determine the cofactor structure. Inactivated with p-nitrophenylhydrazine, they showed absorption and fluorescence spectra similar to those published for other copper amine oxidases and to topa hydantoin p-nitrophenylhydrazone. After digestion by thermolysin and pronase, cofactor peptides were purified by HPLC and sequenced. For thermolytic peptides, a typical topa consensus sequence, Asn-X-Glu-Tyr, was obtained for AO-II, although in case of AO-I it overlapped with Val-Val-Ile-Glu-Pro-Tyr-Gly. For pronase peptides of AO-I, only the latter sequence was obtained. NMR and mass spectroscopy confirmed the residue X as topa p-nitrophenylhydrazone in AO-II and revealed the presence of a residue Z attached to the Glu in the peptide Val-Val-Ile-Glu(Z)-Pro of AO-I. This residue was separated from the peptide by hydrolysis and identified as a product derived from topa quinone. The data, together with amino-acid sequence of AO-I, confer strong evidence for topa quinone as the cofactor, bound in the typical consensus sequence. Raman spectra of the p-nitrophenylhydrazone derivative of AO-I and its pronase peptide showed essentially the same peaks matching to a model compound for topa p-nitrophenylhydrazone. However, there may exist an unusual ester link between the topa-404 and Glu-145 in the native enzyme.

Amine oxidases from Aspergillus niger: identification of a novel flavin-dependent enzyme

Upon induction with various amine sources, two different amine oxidases are expressed in the filamentous fungus Aspergillus niger. The enzymes which can be separated by anion exchange chromatography exhibit a similar substrate specificity pattern. From cofactor and inhibitor analysis it was found that one amine oxidase is identical to the earlier reported copper-containing amine oxidase (Yamada, H., Adachi, O. and Ogata, K. (1965) Agric. Biol. Chem. 29, 912-917) with 6-hydroxydopa (TOPA) quinone as the active site cofactor. The second form is a hitherto unknown flavoprotein of 55 kDa, which shows many of the characteristic properties of the mammalian monoamine oxidases (MAO). From substrate specificity and inhibitor susceptibility, it is suggested that the monoamine oxidase from A. niger (MAO-N) is a prototype of the two mammalian enzymes, MAO-A and MAO-B. A partial cDNA clone which encodes an amino-terminal peptide of 53 amino acid residues was identified by lambda gt11 immunoscreening. The consensus sequence of the putative flavin adenine dinucleotide (FAD) binding site is found within this sequence.

Effect of polyphosphates on the activity of amine oxidases

The interaction between polyphosphates and polyamines was investigated by 31P-NMR spectroscopy and by amine oxidase activity measurements. An apparent competition between negatively charged polyphosphates (ATP, ADP, AMP, tripolyphosphate and pyrophosphate) and positively charged polyamine, for the active site of bovine serum and soybean seedling amine oxidases, was observed by activity measurements. This behavior was explained by formation of polyamine-polyphosphate complexes and the stability constants of these complexes were calculated by 31P NMR. However, at a given concentration of polyphosphate, the amine oxidase activity was found higher than that expected on the basis of the free amine concentration calculated according to the NMR stability constant. This fact, and the different extent of inhibition of the spermidine oxidase activity of soybean seedling and of bovine serum amine oxidases observed in the presence of a given polyphosphate, suggest that amine oxidases may be active also on the polyamine-polyphosphate complexes. This hypothesis was supported by the strong dependence of the kcat/Km of bovine serum amine oxidase on ionic strength, indicating an electrostatic interaction between the charged amine and the active site, while no effect of ionic strength on kcat/Km was observed in the presence of ATP. A kinetic model of this behavior was found to fit the experimental data.

Copper/topa quinone-containing histamine oxidase from Arthrobacter globiformis. Molecular cloning and sequencing, overproduction of precursor enzyme, and generation of topa quinone cofactor

The gene coding for histamine oxidase has been cloned and sequenced from a Coryneform bacterium Arthrobacter globiformis. The deduced amino acid sequence consists of 684 residues with a calculated molecular mass of 75,109 daltons and shows a high overall identity (58%) with that of phenethylamine oxidase derived from the same bacterial strain. Although the sequence similarities are rather low when compared with copper amine oxidases from other organisms, the consensus Asn-Tyr-Asp/Glu sequence, in which the middle Tyr is the precursor to the quinone cofactor (the quinone of 2,4,5-trihydroxyphenylalanine, topa) covalently bound to this class of enzymes, is also conserved in the histamine oxidase sequence. To identify the quinone cofactor, an overexpression plasmid has been constructed for the recombinant histamine oxidase. The inactive enzyme purified from the transformed Escherichia coli cells grown in a copper-depleted medium gained maximal activity upon stoichiometric binding of cupric ions. Concomitantly with the enzyme activation by copper, a brownish pink compound was generated in the enzyme, which was identified as the quinone of topa by absorption and resonance Raman spectroscopies of the p-nitrophenylhydrazine-derivatized enzyme and found at the position corresponding to the precursor Tyr (Tyr-402). Therefore, the copper-dependent autoxidation of a specific tyrosyl residue operates on the formation of the topa quinone cofactor in this enzyme, as recently demonstrated with the precursor form of phenethylamine oxidase (Matsuzaki, R., Fukui, T., Sato, H., Ozaki, Y., and Tanizawa, K. (1994) FEBS Lett. 351, 360-364).

Identification by gas chromatography-mass spectrometry of an adduct between pure pig plasma benzylamine oxidase and the inhibitor 3,5-diethoxy-4-aminomethylpyridine

3,5-Diethoxy-4-aminomethylpyridine (B24) interacts with pure pig plasma benzylamine oxidase (BAO), giving a Schiff base with the carbonyl active site. This Schiff base was reduced, isolated by chemical hydrolysis of the enzyme, purified by HPLC and identified by gas chromatography-mass spectrometry (GC-MS) after derivatization. The isolated B24 adduct had the same absorption spectrum, retention time on HPLC and GC and the same mass spectrum as B24-pyridoxamine. B24, which is a reversible enzyme inhibitor, is also a weak substrate and competes with benzylamine, which is the best substrate, for the active site. These results further indicate the presence of pyridoxal-phosphate covalently linked to the pig plasma benzylamine oxidase and involved in the active site of this enzyme.

Polyamine oxidase, properties and functions

Polyamine oxidase (PAO) is a FAD-dependent enzyme with a molecular mass of about 62 kDa, present with high activity in most tissues of vertebrates. Structural requirements of a substrate for PAO are two positively charged amino groups, separated by a short carbon chain and an alkyl substituent on one or both nitrogen atoms. Spermine and the monoacetyl derivatives N1-acetylspermine and N1-acetylspermidine appear to be the natural substrates. Spermidine is only poorly oxidized by PAO. Using O2, the substrates are oxidatively cleaved by PAO to form equimolar amounts of an amine, an aldehyde and hydrogen peroxide. PAO is an integral part of the polyamine interconversion cycle, a major intracellular regulatory system, which contributes to the maintenance of polyamine homeostasis in non-proliferating cells, including brain cells. Selective inactivators were used as tools in the elucidation of the functions of PAO. Interestingly, even long-term inactivation of PAO did not provoke behavioral changes in experimental animals, despite considerable changes in polyamine metabolism. PAO inactivation, however, improves the growth-inhibitory effects of inhibitors of polyamine biosynthetic enzymes and the antitumoral effects of some structural analogs of the polyamines.

Purification and active-site characterization of equine plasma amine oxidase

An improved purification scheme for an amine oxidase from equine plasma (EPAO), a nonruminant source, is described and the protein's active-site is characterized. EPAO is dimeric and contains one Type-2 Cu(II) ion per monomer. The EPAO Cu(II) site is spectroscopically very similar to the Cu(II) sites in other amine oxidases. Unlike the extensively investigated nonruminant amine oxidase from porcine plasma, EPAO does not display half-of-the-sites reactivity; titrations with p-nitrophenylhydrazine and phenylhydrazine indicate two active cofactors per dimer. This cofactor is determined to be the same as that of other copper-containing amine oxidases, 6-hydroxydopa quinone (topa quinone). Upon anaerobic reduction with substrate at ambient temperature, the EPR spectrum of EPAO exhibits a sharp signal at g congruent to 2, attributable to the topa semiquinone. Equine plasma amine oxidase possesses novel in vitro substrate specificity; while other mammalian amine oxidases oxidize norepinephrine only slowly or not at all, EPAO displays significant activity toward this biogenic amine.

Active-site covalent modifications of quinoprotein amine oxidases from Aspergillus niger. Evidence for binding of the mechanism-based inhibitor, 1,4-diamino-2-butyne, to residue Lys356 involved in the catalytic cycle

Interactions of two distinct quinoprotein amine oxidases from Aspergillus niger, AO-I and AO-II, with active-site covalent modifiers have been investigated. Both enzymes are inhibited similarly by phenylhydrazine or p-nitrophenylhydrazine, forming an orange Schiff base with a carbonyl group of topaquinone cofactor. Modification of histidyl and tyrosyl residues by diethylpyrocarbonate and sulfhydryl groups by 5,5'-dithio-bis-(2-nitrobenzoic acid) and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole have been described. A substrate analog, 1,4-diamino-2-butyne, was found to function as a mechanism-based inhibitor. It shows both substrate saturation kinetics and time-dependent irreversible inhibition caused by formation of pyrrole bound to the active site. The pyrrole formation was confirmed spectrophotometrically by reaction with Ehrlich's reagent at 525 nm. Inhibition by 1,4-diamino-2-butyne produces a new maximum in the absorption spectra of AO-I and AO-II at 310 nm and 306 nm, respectively. Inactivated AO-I was digested by proteases; labeled peptides were purified by C18 HPLC and sequenced by Edman degradation. Data reveal the evidence that 1,4-diamino-2-butyne reacts with the epsilon-amino group of the Lys356 residue in the sequence Lys-Met-Pro-Asn-Ala of Aspergillus niger amine oxidase AO-I.

Generation of the topa quinone cofactor in bacterial monoamine oxidase by cupric ion-dependent autooxidation of a specific tyrosyl residue

The quinone of 2,4,5-trihydroxyphenylalanine (topa), recently identified as the covalently bound redox cofactor in copper amine oxidases, is encoded by a specific tyrosine codon. To elucidate the mechanism of its formation, the recombinant phenylethylamine oxidase of Arthrobacter globiformis has been overproduced in Escherichia coli and purified in a Cu(2+)-deficient form. The inactive precursor enzyme thus obtained was dramatically activated upon incubation with Cu2+, concomitantly with the formation of the topa quinone at the position corresponding to Tyr382, occurring in the tetrapeptide sequence highly conserved in this class of enzymes. The topa quinone was produced only under aerobic conditions, but its formation required no external enzymatic systems. These findings demonstrate the Cu(2+)-dependent autooxidation of a specific tyrosyl residue to generate the topa quinone cofactor.

Models and molecular orbital semiempirical calculations in the study of the spectroscopic properties of bovine serum amine oxidase quinone cofactor

The electronic properties of 2,4,5-trihydroxyphenylalanine quinone (TPQ), the cofactor of bovine serum amine oxidase [Janes, S. M., Mu, D., Wemmer, D., Smith, A. J., Kaur, S., Maltby, D., Burlingame, A. L., & Klinman, J. P. (1990) Science 248, 981-987], and some adducts with hydrazines were investigated by means of low-molecular-weight models and semiempirical molecular orbital calculation methods. The enzyme visible band was assigned to the first pi-->pi* transition of the cofactor in p-quinonic form, with the C-4 hydroxyl ionized and hydrogen bonded either to a water molecule or to a basic protein residue. The spectra of the protein adducts with some substituted hydrazines were well accounted for by assuming the inhibitor bound to the C-5 carbonyl, usually in azo form. The adduct with the unsubstituted hydrazine was instead assigned an o-quinone hydrazone form, stabilized by an internal hydrogen bond between the amino group and the ortho carbonyl oxygen, a larger electron delocalization, and formation of a hydrogen bond at the C-6 ionized hydroxyl. On the basis of these assignments, the reaction of the protein with benzylhydrazine [Morpurgo, L., Agostinelli, E., Muccigrosso, J., Martini, F., Mondovi, B., & Avigliano, L. (1989) Biochem. J. 260, 19-25] was rewritten. All examined electronic transitions, though highly sensitive to cofactor ionization and hydrogen bonding, could be accounted for without introducing perturbations due to copper. This confirms that copper is not within bonding distance of the oxidized cofactor.

Cloning, sequencing, expression, and regulation of the structural gene for the copper/topa quinone-containing methylamine oxidase from Arthrobacter strain P1, a gram-positive facultative methylotroph

Deoxyoligonucleotides corresponding to amino acid sequences of methylamine oxidase and polyclonal anti-methylamine oxidase antibodies were used to probe Arthrobacter strain P1 plasmid and chromosomal DNA libraries. Two open reading frames, maoxI and maoxII, which are greater than 99% homologous, were cloned from the chromosomal library. The deduced amino acid sequences of the coding regions are identical except for two residues near the C termini. On the other hand, the 5'- and 3'-flanking regions of maoxI and maoxII are quite different. While either gene could code for methylamine oxidase, the dissimilarity in the 5'-flanking regions indicates that the genes are differently regulated. It was determined that maoxII alone encodes methylamine oxidase. The tyrosyl residue which is converted to topa quinone in the mature enzyme was located by comparison with amino acid sequences at the cofactor sites in other copper/topa quinone-containing amine oxidase. Transcriptional start sites and possible regulatory elements were identified in the 5' region of maoxI and maoxII, and stem-loop structures were found in the 3'-flanking regions. High levels of methylamine oxidase are produced when Arthrobacter strain P1 is grown on methylamine alone or on glucose plus methylamine, but growth on LB medium plus methylamine resulted in very low production of the enzyme. Expression of maoxII from its own promoter in Escherichia coli grown on glucose or LB medium with or without methylamine gave the same level of production of methylamine oxidase.

Characterization of topa quinone cofactor

Electrochemical characterization of topa quinone (6-hydroxydopa quinone), the organic cofactor of copper-containing amine oxidases, has been performed with the aid of spectroscopy and ab initio energy minimization technique. Topa quinone exhibits a totally reversible cyclic voltammogram at a mercury electrode, which is ascribed to a two-step one-electron conversion between topa quinone and topa via topa semiquinone intermediate. Digital simulation of the reversible wave has afforded the separated estimation of each one-electron redox potential. The acid-dissociation constants of the phenolic hydroxyl groups of topa quinone, topa semiquinone and topa have been evaluated electrochemically and supported by electronic and electron spin resonance spectra. At pH 7.0, topa quinone is acid-dissociated and has a two-electron redox potential of 0.079 V vs. NHE coupled with a three-proton transfer. Redox catalytic activity of topa quinone for the oxidation of amines and NADH was not observed over conventional voltammetric time periods. Energy minimization calculation of acid-dissociated topa quinone anion indicates an intermediate electronic structure between the p- and o-quinone types with three almost equivalent carbonyl groups. The lack of the redox catalytic activity of free topa quinone appears to be attributable to the partial contribution of the p-quinone-type structure.

Protein-radical enzymes

Protein-radical enzymes use a free radical located on an intrinsic amino acid residue as a cofactor. The amino acid involved can be a tyrosine (ribonucleotide reductase, photosystem II, prostaglandin H synthase), a modified tyrosine (amine oxidase, galactose oxidase), a tryptophan (cytochrome c peroxidase), a modified tryptophan (methylamine dehydrogenase) or a glycine (ribonucleotide reductase, pyruvate formate lyase). The mechanistic role of these radicals appears to be that of a one-electron gate, allowing the separation of single reducing equivalents in time and space.

Hydroxyl radical production in the reactions of copper-containing amine oxidases with substrates

Solutions of porcine kidney diamine oxidase, PKDAO, and bovine plasma amine oxidase, BPAO, were saturated with the spin-trapping agent alpha-phenyl-N-t-butylnitrone, PBN, and incubated with cadaverine or benzylamine substrate, respectively, under aerobic conditions. EPR spectra due to trapped hydroxyl radicals were obtained for both enzymes with no evidence of superoxide formation. Under anaerobic conditions, hydroxyl radicals were formed only when H2O2 was present as well as substrate. Catalase prevented hydroxyl radical formation by PKDAO but not BPAO. The results indicate that hydroxyl radical is produced in the reaction of the product H2O2 with the reduced enzymes and therefore may be important in turnover-related enzyme degradation, but is not a true reaction intermediate.

Identification of topaquinone and its consensus sequence in copper amine oxidases

The nature of the active site cofactor and the amino acid sequence flanking this structure have been determined in a range of copper amine oxidases. For enzymes from porcine plasma, porcine kidney, and pea seedlings, proteolytic digestion was performed on phenylhydrazone or p-nitrophenylhydrazone derivatives. Thermolysin treatment leads to relatively small active site peptides, which have been characterized by Edman degradation and by resonance Raman spectroscopy. Resonance Raman spectra of peptides show identical peak positions and intensities relative to each other and to a model p-nitrophenylhydrazone derivative of topaquinone hydantoin, establishing topaquinone as the cofactor in each instance. Edman degradation of peptides provides active site sequences for comparison to previous determinations with bovine serum and yeast amine oxidases. The available data establish a consensus sequence of Asn, Topa, Asp/Glu. Trypsin leads to significantly longer peptides, which reveal a high degree of sequence identity between plasma proteins from bovine and porcine sources (89%), with significantly decreased identity between the porcine serum and intracellular amine oxidases (56%). A lower degree of identity (45%) is observed between the pea seedling and mammalian enzymes. As an alternative to the isolation of active site peptides for topaquinone identification, visible spectra of intact proteins have been investigated. It is shown that p-nitrophenylhydrazone derivatives of native enzymes, active site-derived peptides, and a topaquinone model exhibit identical behavior, absorbing at 457-463 nm at neutral pH (pH 7.2) and at 575-587 nm in basic solution (1-2 M KOH). These spectral properties, which appear unique to topaquinone, provide a rapid and simple test for the presence of this cofactor in intact enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalytic oxidation of 2,4,5-trihydroxyphenylalanine by tyrosinase: identification and evolution of intermediates

The oxidation of 3,4-dihydroxyphenylalanine (dopa) by O2 catalyzed by tyrosinase yields 4-(2-carboxy-2-aminoethyl)-1,2-benzoquinone, with its amino group protonated (o-dopaquinone-H+). This evolves non-enzymatically through two branches (cyclization and/or hydroxylation), whose respective operations are determined by pH. The hydroxylation branch of o-dopaquinone-H+ only operates significantly at pH < or = 5.0 and involves the accumulation of 2,4,5-trihydroxyphenylalanine (topa), which has been detected by high-performance liquid chromatography (HPLC). This last compound is also a substrate of tyrosinase. The oxidation of topa by both tyrosinase and periodate yields 5-(2-carboxy-2-aminoethyl)-4-hydroxy-1,2-benzoquinone, with its amino group protonated (o-topaquinone-H+), which is red (RTQH) (lambda max 272-485 nm) at pH 7.0 and yellow (TTQH) (lambda max 265-390 nm) at pH 3.0. This is based on pKa 4.5 of the 2-OH group of the benzene ring of o-topaquinone-H+, as derived from spectrophotometric and HPLC assays. At physiological pH, RTQH undergoes deprotonation of the ammonium group of the side chain to yields RTQ, which cyclize into 2-carboxy-2,3-dihydroxyindolen-5,6-quinone (dopachrome), with a 1:1 stoichiometry and first-order kinetics. The evolution of RTQH has been analyzed by spectrophotometry, HPLC, cyclic voltammetry and constant potential electrolytic assays. From HPLC assays, the value of the first-order constant for the evolution of RTQH at pH 7.0 (kRTQHapp 4.83 x 10(-5) s-1), as well as of the rate constant for the cyclization step of RTQ (kRTQc 2.53 x 10(-3) s-1) were determined.

1,4-Diamino-2-butyne as the mechanism-based pea diamine oxidase inhibitor

1,4-Diamino-2-butyne is a mechanism-based inhibitor of diamine oxidase (EC 1.4.3.6) from pea cotyledons. It shows saturation kinetics Km = 1 mM like a substrate, but its interaction leads to time-dependent loss of enzyme activity which is not restored by gel filtration. The substrate 1,4-diaminobutane and the competitive inhibitor 1,4-diamino-2-butanone protect the enzyme against inactivation. Changes in the enzyme electronic spectra with 1,4-diamino-2-butyne were found. The mechanism of the interaction involves an intermediate aminoallenic compound, which is formed with covalent bound pyrrole in the reaction of the nucleophile with the enzyme. The presence of a pyrrole in the inactivated enzyme was confirmed by reaction with Ehrlich's reagent. The kinetic data obtained in this study indicate that 1,4-diamino-2-butyne is a mechanism-based inactivator with number of turnovers, r = 17 and characteristic constants K' = 0.32 mM and k(in) = 4.89 min-1.

Active-site directed irreversible inhibition of diamine oxidase by a homologous series of aziridinylalkylamines

Three electrophilic homologous aminoalkylaziridine analogues of putrescine, cadaverine, and 1,3-diaminopropane were synthesized and found to represent a mechanistically distinct class of irreversible inhibitors of diamine oxidase. The putrescine analogue, N-(4-aminobutyl)aziridine gave the lowest calculated IC50 value, whereas N-(3-aminopropyl)aziridine, an analogue of the poorest substrate of the series, showed the highest IC50. The findings suggest that the aziridinylalkylamines tested are site-directed agents that form irreversible complexes at the active site of diamine oxidase. Affinity of the inhibitors for the active site appeared to be dependent on alkyl chain length, suggesting that binding promotes the reactivity of the aziridinyl group.

The pyrroloquinoline quinone (PQQ) coenzymes: a case of mistaken identity

Recent evidence has failed to support the claim for a pyrroloquinoline quinone (PQQ) cofactor in mammalian enzymes previously reported to have PQQ. The validity of the original analysis now has been questioned, and a second cofactor, topa quinone, has been identified in at least one enzyme.

Stereochemical course of tyramine oxidation by semicarbazide-sensitive amine oxidase

Two semicarbazide-sensitive amine oxidases (SSAO's) from bovine and porcine aortic tissue were partially purified and characterized, and the stereochemical course of amine oxidation was evaluated. The porcine and bovine SSAO's were membrane bound glycoproteins, with Km values for benzylamine of 8 and 16 microM, respectively. The reactivity of SSAO with semicarbazide and phenylhydrazine suggests that the cofactor is a carbonyl type molecule. The stereochemical course of the bovine and porcine aortic semicarbazide-sensitive amine oxidase reaction was investigated using chiral tyramines, deuterated at C-1 and C-2, and 1H-NMR spectroscopy to establish the loss or retention of deuterium in product p-hydroxyphenethyl alcohols. The preferred mode of tyramine oxidation was found to occur with the loss of pro-S proton at C-1, coupled with solvent exchange into C-2, a pattern which has not been observed for any copper amine oxidase examined to date. The solvent exchange reaction also occurred stereospecifically, with loss from and reprotonation to the pro-R position, suggesting that these two processes occur from the same face of the enamine double bond.

cDNA-derived amino-acid sequence of lentil seedlings' amine oxidase

Lentil seedlings' amine oxidase (LSAO) cDNAs were identified in a cDNA lambda gt 10 library by plaques hybridization. The nucleotide sequence of a 2111 bp clone was determined. It contains part of a signal peptide, the complete sequence coding for the mature protein and the 3'-untranslated region of the mRNA. The deduced protein sequence shows that the mature protein is composed of 569 amino acids with a molecular mass of 67 kDa, also taking into account the glucidic component. The LSAO cDNA was identified by sequencing the N-terminal part of the protein and several tryptic peptides. The protein sequence shows a characteristic hexapeptide present in amine oxidases containing 6-hydroxydopa as the organic cofactor. Three conserved histidines might be the ligands of copper bound to the enzyme.