Rabbit Serum Monoamine Oxidase

A pilot study of cerebral metabolism and serotonin 5-HT2A receptor occupancy in rats treated with the psychedelic tryptamine DMT in conjunction with the MAO inhibitor harmine

Rationale: The psychedelic effects of the traditional Amazonian botanical decoction known as ayahuasca are often attributed to agonism at brain serotonin 5-HT2A receptors by N,N-dimethyltryptamine (DMT). To reduce first pass metabolism of oral DMT, ayahuasca preparations additionally contain reversible monoamine oxidase A (MAO-A) inhibitors, namely β-carboline alkaloids such as harmine. However, there is lacking biochemical evidence to substantiate this pharmacokinetic potentiation of DMT in brain via systemic MAO-A inhibition. Objectives: We measured the pharmacokinetic profile of harmine and/or DMT in rat brain, and tested for pharmacodynamic effects on brain glucose metabolism and DMT occupancy at brain serotonin 5-HT2A receptors. Methods: We first measured brain concentrations of harmine and DMT after treatment with harmine and/or DMT at low sub-cutaneous doses (1 mg/kg each) or harmine plus DMT at moderate doses (3 mg/kg each). In the same groups of rats, we also measured ex vivo the effects of these treatments on the availability of serotonin 5-HT2A receptors in frontal cortex. Finally, we explored effects of DMT and/or harmine (1 mg/kg each) on brain glucose metabolism with [18F]FDG-PET. Results: Results confirmed that co-administration of harmine inhibited the formation of the DMT metabolite indole-3-acetic acid (3-IAA) in brain, while correspondingly increasing the cerebral availability of DMT. However, we were unable to detect any significant occupancy by DMT at 5-HT2A receptors measured ex vivo, despite brain DMT concentrations as high as 11.3 µM. We did not observe significant effects of low dose DMT and/or harmine on cerebral [18F]FDG-PET uptake. Conclusion: These preliminary results call for further experiments to establish the dose-dependent effects of harmine/DMT on serotonin receptor occupancy and cerebral metabolism.

Variation in semicarbazide-sensitive amine oxidase activity in plasma and tissues of mammals

Semicarbazide-sensitive amine oxidase (SSAO) (E.C. 1.4.3.6) is a group of enzymes with as yet poorly understood function which is widely present in nature. The variation in methodology for determination of activity, differences in substrates used and in nomenclature have made it difficult to compare SSAO in different species and tissues. Since SSAO is implicated in the pathophysiology of diabetes mellitus and congestive heart failure, our aim was to analyse the importance and abundance of SSAO in human plasma and tissues compared to other mammals. In plasma of ten different mammals, Vmax values were found to vary more than 10,000-fold, while KM differed much less; in human plasma SSAO activity is relatively low. In some species more than one SSAO entity was present in plasma. SSAO activity was ubiquitous in tissues of human, rat and pig, but varied considerably, both between species and between tissues. In human tissues, SSAO activity is higher than in tissues from rat and pig. Relative to monoamine oxidase-B there is also wide variation in SSAO, with much higher relative activities in human than in rat and pig tissues. We conclude that in plasma, SSAO activity is highest in ruminants, while in tissues, SSAO activity is more prominently present in human than in rat and pig.

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.

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.

Semicarbazide-sensitive amine oxidases in sheep plasma: interactions with some substrates and inhibitors

The present study has examined the affinities of sheep plasma semicarbazide-sensitive amine oxidase (SSAO) enzymes for a range of aliphatic amines and also the effects of two inhibitory compounds, beta-aminopropionitrile (BAPN) and mexiletine. Two kinetically separable enzyme activities appeared to be responsible for the metabolism of amines containing 2-5 carbon atoms while the deamination of higher amines and methylamine and allylamine produced kinetic plots characteristic of only one enzyme activity. When benzylamine metabolism was used as an indication of enzyme activity, the two inhibitors had different effects. BAPN exhibited predominantly a mixed pattern of inhibition while the effects of low concentrations of mexiletine were largely competitive. These results present evidence confirming the presence of two kinetically separable SSAO activities in sheep plasma, although we must await the development of highly selective inhibitors before these two activities can be fully resolved.

Absence of plasma amine oxidase in some frequently used animal models

1. Plasma amine oxidase (PAO), an enzyme that oxidizes primary amines is generally assumed to be present in the sera of all mammals. 2. We studied plasma from eleven different mammals to determine if they could oxidize a group of amines of biomedical interests. 3. Our results indicate that PAO is not present in all mammals and also that substrate specificity of the enzyme varies within the species tested. Rodents which are frequently used as animal models in pharmacology and toxicology do not contain PAO. 4. The common polyamines, spermine, spermidine and putrecine were oxidized only by bovine plasma.

The metabolism of dopamine in the blood of ruminant animals: formation of 3,4-dihydroxyphenylacetaldehyde

A stable derivative of 3,4-dihydroxyphenylacetaldehyde (DOPAL) has been prepared that yields a solution of the parent aldehyde when dissolved in 1 M-hydrochloric acid. Dopamine, when injected into the bloodstream of a sheep, is metabolized to DOPAL which is then converted to 3,4-dihydroxyphenylethanol by aldehyde reductase activity associated with the cellular components of the blood, most probably the erythrocytes. In vitro, dopamine is metabolized by ruminant blood plasma to DOPAL, not to DOPET as previously reported. Dopamine is as good a substrate for sheep plasma amine oxidase as benzylamine. The plasma amine oxidase of ruminant animals could be a protection against the effects of dopamine released from the mast cells in these species.

Some properties of semicarbazide-sensitive amine oxidases

The semicarbazide-sensitive amine oxidases (SSAOs) comprise a substantial but diffuse group of enzymes separable from classical monoamine oxidase in several respects. Differences in cofactor requirement, molecular weight and subcellular distribution are crucial for such a separation. Differential sensitivity to enzyme inhibitors, characterized by resistance to inhibition by acetylenic MAO inhibitors coupled with sensitivity to semicarbazide and some related compounds are characteristic of these enzymes. SSAO enzymes have been found in the plasma of man, ox, pig and horse, for example as well as in the solid tissues of many species. Extensive studies have so far failed to produce any conclusive evidence to indicate what the precise functions of many of these enzymes may be. Indeed in most cases there is no clear idea as to the nature of the preferred physiological substrate, although many amines with pharmacological activity have been shown to be substrates. The actions of these amines may be potentiated following inhibition of SSAO, but as yet little is known whether or not these actions can be important in vivo. An attempt is made in this review to bring together some of the evidence to see if there are indications for future endeavours.

Monoamine oxidase in adult Ascaridia galli

Monoamine oxidase (MAO), catalysing oxidative deamination of biogenic monoamines, has been detected in adult Ascaridia galli. MAO was present in mitochondria and deaminated noradrenaline at the maximal rate, although serotonin, adrenaline, tyramine and dopamine were also degraded but more slowly. Of the organs studied, the body wall, female reproductive organ and intestine, the body wall (containing neuronal structures) showed highest MAO activity. Km value for chick ascarid mitochondrial MAO using tyramine as substrate was 1.66 X 10(-3) M and it was most active at 2.5 mM tyramine concentration, pH 7.5 and 40 degrees C. MAO of A. galli appeared to be thermolabile as nearly 80% of its activity was lost when the incubation temperature was increased 5 degrees above optimum.

The metabolism of dopamine in the blood of ruminant animals: an enzyme system to metabolize dopamine released from the mast cells in these species?

Following the i.v. administration of 3,4-dihydroxyphenylethylamine (dopamine) to sheep, 3,4-dihydroxyphenylethanol and 3,4-dihydroxyphenylacetic acid rapidly appeared in the blood. 3,4-Dihydroxyphenylethanol can be formed from dopamine by blood plasma of ruminants in vitro. It is suggested that this plasma enzyme system might serve to inactive dopamine released from mast cells in ruminant species.

Amine oxidase in human blood vessels and non-vascular smooth muscle

The pattern of amine oxidation was studied in human blood vessels at various stages of development, and in tissues with muscular layers made up predominantly or exclusively of smooth muscle. Specific activity of benzylamine oxidase, present in all vascular tissues examined, was higher in vessels than in other tissue; the enzyme, in organs rich in non-vascular smooth muscle, though lower than in blood vessels, was significantly higher than in striated muscle or connective tissue. The localization of benzylamine oxidase activity in smooth muscle, as opposed to connective tissue, may have important physiological implications.

The fate of biogenic monoamines in perfused rabbit lung

1. Inactivation of beta-phenylethylamine and several of its derivatives was studied in a preparation of rabbit lung perfused with Krebs physiological medium at 37 degrees C. Inactivation or removal of these compounds was calculated as the difference between the concentration of each amine in the perfusion medium and the effluent, collected separately from each lung. The extent of amine metabolic degradation was also measured, by column chromatography, in lung effluent. 2. With this technique the magnitude of amine removal as a function of concentration was determined and an apparent Km and Vmax of removal were calculated for each amine. 3. Percentage removal was highest with phenylethylamine (95%), and decreased, apparently in relation to increasing phenyl- and side chain-hydroxylation (and therefore likely increased hydrophilicity), and 5-hydroxytryptamine (64%), tyramine (53%), octopamine (35%), dopamine (32%) and noradrenaline (23%). 4. Inactivation of each amine could be accounted for by metabolic degradation to deaminated products, which appeared in lung effluent within 90 s of beginning amine perfusion. 5. When intrapulmonary metabolism of phenylethylamine was inhibited by simultaneous perfusion with semicarbazide (10 mM) and pargyline (10 micronM), the removal rate was unaltered, establishing that uptake of the amine from the vascular space is not dependent on metabolism at least for 4 min infusions.

Localization, purification and substrate specificity of monoamine oxidase

Bovine kidney monoamine oxidase (amine:oxygen oxidoreductase (deaminating) (flavin-containing), EC 1.4.3.4) has been purified to one band on disc electrophoresis, and is shown to be localized in the intra- and extramitochondrial membrane. Kinetic models have been used to determine the effect of different substances on the enzyme activity. This enzyme shows a very high substrate specificity. It is suggested that phenol ring and one hydrogen atom each on the methylene and amine groups are responsible for the enzyme activity. N-methylbenzylamine exhibits a homotropic negative cooperative effect which is also supported by the n and Rs values. Benzylhydrazine is apparently a good substrate unlike phenylhydrazine, semicarbazide, harmaline and alpha- and beta-naphthol which show an inhibitory effect on the enzyme activity. Methylamine has no effect. It is suggested that the enzyme may have different sites or different conformations for different substrates. The results of this communication demonstrate bovine kidney monoamine oxidase to be different from monoamine oxidase from other sources.

The essential histidine residues of bovine plasma amine oxidase

Ethoxyformic acid anhydride and photooxidation have been used to study the function of histidine residues in bovine plasma amine oxidase. Ethoxyformic acid anhydride at pH 6.1 reacted with nearly all of the histidine residues in the enzyme in 15 min but complete enzyme inactivation occurred in several minutes. The concentration of the reagent which caused 50% inhibition was 2.2-10(-5) M under the conditions of the experiment. The diamine oxidases, Aspergillus niger and pea seedling amine oxidases were also inhibited by ethoxyformic acid anhydride. The concentrations of reagent required for 50% inhibition were 6.6-10(-5) and 3.3-10(-4) M, respectively, for the two enzymes. NH2OH could not be used to regenerate the reacted histidine residues since NH2OH itself inhibited the enzyme. Photooxidation in the presence of 0.001% Rose Bengal at pH 7.0 also inactivated bovine plasma amine oxidase. Histidine was the only amino acid destroyed by photooxidation. About six histidine residues were destroyed but in the presence of the substrate kynuramine, two less histidine residues were destroyed. Since lysine which is neither a substrate nor inhibitor of the enzyme did not protect the enzyme from photooxidation, it was concluded that two histidine residues, one in each sub-unit of the enzyme are essential for activity.

A purification procedure for the isolation of homogeneous preparations of bovine aorta amine oxidase and a study of its lysyl oxidase activity

It has been reported that bovine aorta amine oxidase oxidizes lysine residues in tropoelastin to allysine (Rucker, R.B. and O'Dell, B.L. (1971) Biochim. Biophys. Acta 235, 32-43). Pure bovine aorta amine oxidase was isolate by DEAE-cellulose, hydroxylapatite, Bio-Gel A-1.5 m and concanavalin A-Sepharose 4B chromatography. Enzymatic, chromatographic and immunochemical tests disclosed that pure bovine aorta amine oxidase was not a lysyl oxidase capable of oxidizing the lysine residues of tropoelastin to allysine; The bovine aorta amine oxidase preparation used by Rucker and O'Dell appears to have been contaminated with lysyl oxidase which is the emzyme that oxidizes some of the lysine residues in tropoelastin and tropocollagen to allysine.