Transient kinetics of benzaldehyde formation during the catalytic action of pig-plasma benzylamine oxidase

Superoxide anion radical generation during the oxidation of various amines by diamine oxidase

Diamine oxidase (DAO) or histaminase is an enzyme which deaminates histamine and several aliphatic amines to their corresponding aldehydes. Hydrogen peroxide and ammonia are side products of this reaction. The purpose of the present work was to evaluate if determination of produced hydrogen peroxide reflects DAO activity or if intermediate formation of the superoxide radical could be a reason for lack of correspondence between oxygen uptake and hydrogen peroxide production at different pH. Superoxide radical formation was determined by cytochrome c reduction in the presence and absence of superoxide dismutase (SOD). Oxygen uptake was measured with an oxygen electrode and hydrogen peroxide production by a spectrophotometric method. At pH 6.6 there was no superoxide production, but at pH 7.4 there was some, and it increased markedly at pH 9.5. Oxygen uptake also increased with increasing pH, especially with histamine as substrate. These results lead us to suggest that the mechanism of action of DAO involves the intermediate generation of superoxide radicals.

Semicarbazide-sensitive amine oxidases: some biochemical properties and general considerations

Semicarbazide-sensitive amine oxidases with a high affinity for benzylamine (Bz.SSAO) (E.C.1.4.3.6) have been biochemically described in many mammalian tissues (adipose tissue, lung, heart, blood vessels). The enzymic activity appears to be expressed by mesenchymal cells (fibroblasts, adipocytes, smooth muscles). Although the physiological role of this enzymic activity is still unclear, some possible physiological substrates such as histamine are discussed. Some enzymes of this class (SSAO) have been purified. They share many similarities, among which are that they contain copper and a carbonyl active site. The nature of the organic cofactor of these enzymes is discussed and data are presented which have identified pyridoxal in pig kidney diamine oxidase and in pig plasma benzylamine oxidase by gas chromatography-mass spectrometry.

Active-site titration of pig-plasma benzylamine oxidase

1. Titration of benzylamine oxidase with benzylamine under anaerobic conditions shows that full reduction of the enzymic 470-nm chromophore is obtained on the addition of one mole of substrate per mole of enzyme. Concomitantly, one mole of benzaldehyde per mole of enzyme is produced. 2. A single prosthetic group interacting with carbonyl reagents can be detected on titration of benzylamine oxidase with phenylhydrazine. Titration data reported to indicate a higher content of prosthetic groups were obtained under conditions where equilibration between enzyme and phenylhydrazine is insufficiently complete. 3. It is concluded that pig-plasma benzylamine oxidase contains a single catalytically active site. This means that the two copper atoms present in the enzyme may be structurally or functionally different.

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.

The kinetics of reoxidation of reduced benzylamine oxidase

1. The mechanism of reoxidation of reduced benzylamine oxidase has been investigated at different pH between 6 and 10 by steady-state and transient-state kinetic methods. 2. The reoxidation process involves minimally a second-order interaction between reduced enzyme and oxygen leading to the formation of a spectrally modified enzyme intermediate, and a subsequent first-order step converting this intermediate into free enzyme. The variation with pH of rate constants according to such a reaction scheme is reported. 3. Under aerobic conditions the oxygen-independent reaction represents the main rate-limiting step in the catalytic process at alkaline pH. At neutral or acid pH the interaction between reduced enzyme and oxygen becomes mainly rate-limiting, indicating that the concentration of oxygen may be a critical factor controlling enzyme activity under physiological conditions. 4. The spectrally modified intermediate formed during the reoxidation process exhibits a difference-absorption band centered around 290 nm in comparison to free enzyme, and an additional difference-absorption band at 470 nm in comparison to reduced enzyme. These data indicate that formation of the intermediate, besides leading to a reappearance of the 470-nm absorption band disappearing on reduction of the enzyme, results in a spectral perturbation of one or several aromatic amino-acid residues in the protein. This perturbation could possibly reflect a conformational change of the enzymes.

Stopped-flow spectrophotometric characterization of enzymic reaction intermediates in the anaerobic reduction of pig-plasma benzylamine oxidase by amine substrates

Reduction of benzylamine oxidase by p-methoxybenzylamine under anaerobic conditions leads to biphasic absorbance changes at 470 nm. These reflect the intermediate formation of an enzyme substrate complex with spectral properties different from those of native enzyme and fully reduced enzyme. The spectrally modified enzyme-substrate complex exhibits a broad difference absorption band centered around 360 nm. The transient accumulation of this intermediate during reaction can be conveniently followed by stopped-flow techniques at wavelengths between 320 and 360 nm, where contributions from the subsequent reduction of the enzymic 470-nm chromophore are of minor significance. 2. Analogous intermediates exhibiting similar absorption spectra seem to be formed on reduction of the enzyme by benzylamine and other amine substrates which were tested. Substitution of benzylamine as the reducing substrate by [alpha, alpha-2H]benzylamine results in a decreased accumulation of the spectrally modified intermediate. This indicates that its formation is preceded by deprotonation of the alpha-carbon of the amine substrate. 3. Circular dichroism spectra of benzylamine oxidase exhibit a positive band at 360 nm, lending support to the previous conclusion that benzylamine oxidase is a pyridoxal enzyme. Formation of the spectrally modified enzyme-substrate complex then most likely reflects the prototropic shift converting an amine-pyridoxal Schiff-base obtained by rapid pre-equilibration between enzyme and substrate into an aldehyde-pyridoxamine Schiff-base.

Kinetic isotope effects on the catalytic activity of pig-plasma benzylamine oxidase

1. Isotope effects on the catalytic activity of benzylamine oxidase at pH 7 and 9 have been studied by steady-state and transient-state kinetics methods, using [alpha,alpha-2H]benzylamine as the substrate. 2. Replacement of the alpha-hydrogen atoms in benzylamine by deuterium has no significant effect on substrate-binding to benzylamine oxidase, neither does it affect the rate of reoxidation of the reduced form of the enzyme. Conversion of the primarily formed enzyme-substrate complex into the reduced enzyme species, however, exhibits an isotope effect of about 3. 3. The data obtained are consistent with a mechanism in which reduction of benzylamine oxidase takes place by a rapid pre-equilibration between enzyme and substrate to form an amine-pyridoxal Schiff-base, which is then tautomerized by a comparatively slow prototropic shift to an amino aldehyde-pyridoxamine Schiff-base from which there is a rapid hydrolytic release of the aldehyde product corresponding to the amine substrate. Proton abstraction from the alpha-carbon of the amine moiety in the primary Schiff-base appears to be at least partially rate-limiting for the tautomerization step, and hence for the entire process of enzyme reduction.

Effect of pH on the transient reduction of pig-plasma benzylamine oxidase by benzylamine derivatives

1. The transient kinetics of reduction of the 470-nm absorption band in benzylamine oxidase by substrate at different pH values between 6 and 10 have been studied by stopped-flow techniques, and substituent effects on kinetic parameters for the reduction process have been examined using a series of ring-substituted benzylamine derivatives as the substrates. 2. Reduction of the enzyme by substrate takes place in two kinetically distinguishable steps, with the intermediate formation of an enzyme-substrate complex in which the substrate appears to be covalently bound through its amino group to the prosthetic group of the enzyme, possibly in the form of an amine-pyridoxal Schiff-base. 3. The apparent stability of the enzyme-substrate complex shows no obvious dependence on the electronic properties of the amine substrates, but is strongly pH-dependent in a way suggesting that substrate-binding involves the non-protonated amines, exclusively, and requires the presence of the acid form of an ionizing group in the enzyme with apparent pKa of 8.8. 4. Reduction of the enzymatic 470-nm chromophore and release of the aldehyde product of the catalytic process are rate-limited by the same monomolecular reaction step involving the enzyme-substrate complex. Rate constants for the rate-limiting reaction exhibit no significant dependence on pH between 6 and 10, but correlate with Hammett sigma-values for the ring-substituted benzylamine derivatives tested, yielding a phi-value of + 0.3.