Copper reduction by substrate in diamine oxidase

Is the catalytic mechanism of bacteria, plant, and mammal copper-TPQ amine oxidases identical?

This short review is mostly concerned with the work carried out in Rome on the copper amine oxidase from bovine serum (BSAO). The first target was the copper oxidation state and its relationship with the organic cofactor. It was found that copper is not reduced on reaction with amines under anaerobic conditions or along the catalytic cycle and that it is not within bonding distance of the quinone cofactor. The copper stability in the oxidised state was supported by BSAO ability to oxidise benzylhydrazine, a slow substrate, in the presence of N,N-diethyldithiocarbamate (DDC) and by the substantial catalytic activity of Co(2+)-substituted BSAO. Parallel work established that only one subunit of the dimeric enzyme readily binds reagents of the carbonyl group. Flexible hydrazides with a long aromatic tail were found to be highly specific inhibitors, suggesting the presence of an extended hydrophobic region at the catalytic site. A study by stopped-flow transient spectroscopy and steady state kinetics led to the formulation of a simplified, yet complete and consistent, catalytic mechanism for BSAO that was compared with that available for lentil seedling amine oxidase (LSAO). As in other copper amine oxidases, BSAO is inactivated by H(2)O(2) produced in the catalytic reaction, while the cofactor is stabilised in its reduced state. A conserved tyrosine hydrogen-bonded to the cofactor might be oxidised.

Inhibition of copper amine oxidase by haloamines: a killer product mechanism

The observation that the alkylamines 2-Br-ethylamine and 2-C1-ethylamine and 1,2-diaminoethane, the shortest diamine, are irreversible inhibitors of several copper amine oxidases led to the investigation of the mechanism by which these compounds react with the highly active amine oxidase from lentil seedlings. 1,2-Diaminoethane, 2-Br-ethylamine, and 2-C1-ethylamine were found to be both poor substrates and irreversible inhibitors of lentil amine oxidase; inactivation took place in both the presence and absence of oxygen. All three compounds strongly affected the spectrum of the enzyme, leading to the formation of a stable band at 336 nm both in anaerobiosis and in aerobiosis, consistent with an interaction with the enzyme cofactor 6-hydroxydopa. On the contrary, the corresponding propylamine compounds 1,3-diaminopropane, 3-Br-propylamine, and 3-C1-propylamine were reversible inhibitors of lentil amine oxidase. Inhibition was shown to be due to the aldehyde oxidation products rather than the short chain amines themselves; a reaction mechanism is presented which involves attack of the aldehyde on the 6-hydroxydopa-derived free radical catalytic intermediate. With 1,2-diaminoethane, 2-Br-ethylamine, and 2-C1-ethylamine, the complex produced will form a stable 6-membered ring, causing irreversible inhibition of the enzyme.

A substrate-cofactor free radical intermediate in the reaction mechanism of copper amine oxidase

Reduction of copper amine oxidase with substrate led to the appearance of a free radical which can be detected in anaerobiosis by ESR and optical spectroscopy. The origin of this radical was examined through studies of the semiquinones of 6-hydroxydopamine, an analogue of the recently identified cofactor 6-hydroxydopa. The ESR spectrum of the 6-hydroxydopamine radical was too narrow to account for the enzyme radical signal; however, after spontaneous reaction with primary amines the hyperfine splittings and spectral width obtained by modulation broadening became very similar to those observed for the oxidase radical species. This effect was ascribed to covalent binding of a nitrogen atom directly to the aromatic ring structure, suggesting that the amine oxidase radical is an amino-6-hydroxydopa semiquinone. Identical ESR spectra were obtained using the amines putrescine, cadaverine, p-[(dimethylamino)methyl]benzylamine, and ethylenediamine; these oxidase substrates gave identical enzyme radical spectra as well. The interaction between cofactor and substrate was proved unambiguously by the technique of isotopic labeling: addition of [15N2]ethylenediamine instead of the normal 14N-labeled compound changed the ESR spectra of both the enzyme radical and its 6-hydroxydopamine counterpart. The results were confirmed by optical spectroscopy measurements; 6-hydroxydopamine and oxidized 6-hydroxydopamine gave spectra identical to those of reduced and oxidized amine oxidase, respectively. The 6-hydroxydopamine radical showed a sharp peak at 440 nm; upon addition of amines the maximum shifted to 460 nm, as found for the enzyme. It is proposed that copper amine oxidase represents the first example of a mixed substrate-cofactor radical within the family of tyrosine radical enzymes.

A Cu(I)-semiquinone state in substrate-reduced amine oxidases

The role of copper in copper-containing amine oxidases has long been a source of debate and uncertainty. Numerous electron paramagnetic resonance (EPR) experiments, including rapid freeze-quench studies, have failed to detect changes in the copper oxidation state in the presence of substrate amines. One suggestion that copper reduction might occur, has never been confirmed. Copper amine oxidases contain another cofactor, recently identified as 6-hydroxydopa quinone (topa quinone), which is reduced by substrates. Copper has been implicated in the reoxidation of the substrate-reduced enzyme, but the failure to detect any copper redox change has led to proposals that Cu(II) acts as a Lewis acid, that it has an indirect role in catalysis, or that it serves a structural role. We present evidence for the generation of a Cu(I)-semiquinone state by substrate reduction of several amine oxidases under anaerobic conditions, and suggest that the Cu(I)-semiquinone may be the catalytic intermediate that reacts directly with oxygen.

Evidence for methoxatin (pyrroloquinolinequinone) as the cofactor in bovine plasma amine oxidase from resonance Raman spectroscopy

Resonance Raman spectra of the 2,4-dinitrophenylhydrazine derivatives of bovine plasma amine oxidase [amine:oxygen oxidoreductase (deaminating) (copper-containing), EC 1.4.3.6] have been measured. Detailed comparisons to the spectra of the corresponding derivatives of methoxatin (pyrroloquinolinequinone), pyridoxal, and other aldehydes and diones provide further evidence that covalently bound methoxatin or a closely similar derivative is the organic cofactor in copper-containing amine oxidases.

Two copper-containing proteins from white and gray matter of brain

The procedure for the isolation of two water soluble copper-containing proteins from the white and gray matter of bovine brain is described. One of the proteins, cerebrocuprein I, is superoxide dismutase; and three molecular forms of this enzyme are to be found in brain. The other protein present in gray and white matter is devoid of superoxide dismutase and amine oxidase activities. The amino acid composition, molecular weight, isoelectric point and copper content of this protein were determined. The effect of some agents, pH and thermal treatment of the optical and EPR spectra of the protein were also studied. The copper of the protein may be removed and the holoprotein reconstituted again from apoprotein and copper. The results obtained led to the conclusion that in brain a new copper protein is discovered, which is named neurocuprein.

On the nature of chromophore in pig kidney diamine oxidase

The nature of the 500-nm chromophore in pig kidney diamine oxidase was investigated by absorption spectroscopy and fluorescence in the presence of various chelating or carbonyl-specific reagents. From the spectroscopic measurements the following conclusions can be drawn. First, the 500-nm absorption band is not due to copper, the reduction of which is not related to the disappearance of this band. Second, phenylhydrazine and cycloserine give rise, upon reaction with the enzyme, to absorptions very similar to those of a pyridoxal enzyme, aspartate aminotransferase. Third, these enzyme derivatives are unexpectedly non-fluorescent. Copper removal, obtained after prolonged incubation of cycloserine-treated enzyme in the presence of reducing and chelating agents, leads to a fluorescence similar to that of cycloserine-aspartate transminase. It is proposed that copper is coordinated to the postulated pyridoxal phosphate of diamine oxidase through the pyridine nitrogen.

Kinetics of the diamine oxidase reaction

1. The oxidation of p-dimethylaminomethylbenzylamine was followed spectrophotometrically by measuring the change in E(250) caused by the p-dimethylaminomethylbenzaldehyde produced under a wide variety of experimental conditions. 2. The effect of variations in concentrations of both substrates (amine and oxygen) and all products (aminoaldehyde, hydrogen peroxide and ammonia) on this reaction was studied and the results used to develop a formal mechanism. 3. The nature of the rate-limiting step was elucidated by studying the effects of alterations in ionic strength, dielectric constant and deuterium substitution on the velocity of the forward reaction. 4. Thermodynamic activation energy parameters were obtained at several pH values from the effects of temperature on the reaction.

Inhibition of pig kidney diamine oxidase by substrate analogues

1. The oxidation of p-dimethylaminomethylbenzylamine was followed spectrophotometrically by measuring the change in E(250) caused by the p-dimethylaminomethylbenzaldehyde produced. 2. This reaction was inhibited by substrate analogues such as isothiouronium, guanidino, dimethylsulphonium and trimethylammonium compounds. 3. The inhibition by both mono- and bis-onium compounds has been studied and a comprehensive theory is developed to explain both the type and degree of inhibition produced.

Synthesis and oxidation of aminoalkyl-onium compounds by pig kidney diamine oxidase

1. The preparation of a series of compounds derived from diamines by replacing one amino group by a dimethylsulphonium, isothiuronium, trimethylammonium, NN'-dimethylimidazolium or N-methylpyridinium species is described. 2. The behaviour of these compounds as substrates of pig kidney diamine oxidase is reported. All but the trimethylammonium compounds proved to be substrates. 3. Many of these compounds showed potent inhibition at high substrate concentration and this was studied. 4. On the basis of these and other observations a scheme for enzyme-substrate interaction is suggested.