β-Phenylethylamine and benzylamine as substrates for human monoamine oxidase A: A source of some anomalies?

Exploring Highly Functionalized Tetrahydropyridine as a Dual Inhibitor of Monoamine Oxidase A and B: Synthesis, Structural Analysis, Single Crystal XRD, Supramolecular Assembly Exploration by Hirshfeld Surface Analysis, and Computational Studies

Ethyl 4-(4-fluorophenylamino)-2,6-bis(4-(trifluoromethyl)phenyl)-1-(4-fluoro-phenyl)-1,2,5,6-tetrahydropyridine-3-carboxylate (FTEAA) has been synthesized efficiently in an iodine-catalyzed five-component reaction of 4-fluoroaniline, 4-trifluoromethyl benzaldehyde, and ethyl acetoacetate in methanol at 55 °C for 12 h. Various spectro-analytical techniques such as ¹H and ¹³C NMR and Fourier-transform infrared spectroscopy have validated the structure of FTEAA. Further confirmation of the structure of FTEAA has been established on the basis of single-crystal X-ray diffraction analysis. The supramolecular assembly of FTEAA in terms of strong and comparatively weak noncovalent interactions is fully investigated by Hirshfeld surface analysis, the interaction energy between pairs of molecules, and energy frameworks. The void analysis is conducted to explore the strength and stability of the crystal structure. Furthermore, molecular docking analysis was computationally performed to see the potential intermolecular interactions between the selected proteins and FTEAA. The binding interaction energies are found to be -8.8 and -9.6 kcal/mol for the proteins MAO-B (PDB ID: 2V5Z) and MAO-A (PDB ID: 2Z5X), respectively. These reasonably good binding energies (more negative values) indicate the efficient associations between the FTEAA and target proteins. The proteins and FTEAA were also analyzed for intermolecular interactions. FTEAA and proteins interact in a variety of ways, like conventional hydrogen bonds, carbon-hydrogen bonds, alkyl, π-alkyl, and halide interactions.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, "general chemicals," natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10-15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Profiling of Tryptophan Metabolic Pathways in the Rat Fetoplacental Unit During Gestation

Placental homeostasis of tryptophan is essential for fetal development and programming. The two main metabolic pathways (serotonin and kynurenine) produce bioactive metabolites with immunosuppressive, neurotoxic, or neuroprotective properties and their concentrations in the fetoplacental unit must be tightly regulated throughout gestation. Here, we investigated the expression/function of key enzymes/transporters involved in tryptophan pathways during mid-to-late gestation in rat placenta and fetal organs. Quantitative PCR and heatmap analysis revealed the differential expression of several genes involved in serotonin and kynurenine pathways. To identify the flux of substrates through these pathways, Droplet Digital PCR, western blot, and functional analyses were carried out for the rate-limiting enzymes and transporters. Our findings show that placental tryptophan metabolism to serotonin is crucial in mid-gestation, with a subsequent switch to fetal serotonin synthesis. Concurrently, at term, the close interplay between transporters and metabolizing enzymes of both placenta and fetal organs orchestrates serotonin homeostasis and prevents hyper/hypo-serotonemia. On the other hand, the placental production of kynurenine increases during pregnancy, with a low contribution of fetal organs throughout gestation. Any external insult to this tightly regulated harmony of transporters and enzymes within the fetoplacental unit may affect optimal in utero conditions and have a negative impact on fetal programming.

Potential Salivary mRNA Biomarkers for Early Detection of Oral Cancer

We evaluated potential biomarkers in human whole saliva for the early diagnosis of oral squamous cell carcinoma (OSCC). We selected 30 candidate genes with relevance to cancer from recent reports in PubMed. Saliva samples were obtained from 34 non-tumor control and 33 OSCC patients. Real-time PCR was performed, and mRNA levels were compared. Normalized mRNA levels of six genes (NGFI-A binding protein 2 (NAB2), cytochrome P450, family 27, subfamily A, polypeptide 1 (CYP27A1), nuclear pore complex interacting protein family, member B4 (NPIPB4), monoamine oxidase B (MAOB), sialic acid acetyltransferase (SIAE), and collagen, type III, alpha 1 (COL3A1)) were significantly lower in saliva of OSCC patients. Receiver operating characteristics (ROC) analysis was used to individually evaluate the predictive power of the potential biomarkers for OSCC diagnosis. The area under the curve (AUC) values were evaluated for the OSCC vs. non-tumor groups via univariate ROC analyses, as well as multivariate ROC analyses of combinations of multiple potential biomarkers. The combination of CYP27A1 + SIAE showed a favorable AUC value of 0.84. When we divided saliva samples into two groups according to age using a 60-year cut-off, with OSCC patients and controls evaluated together, the AUC of MAOB-NAB2 was more predictive of OSCC in the under-60 group (AUC, 0.91; sensitivity, 0.92; and specificity, 0.86) than any other gene combination. These results are expected to aid the early diagnosis of OSCC, especially in patients under 60 years of age. While more studies with larger numbers of patients are necessary, our result suggest that salivary mRNA would be a potent biomarker for early OSCC diagnosis.

Structure-Based Specific Detection and Inhibition of Monoamine Oxidases and Their Applications in Central Nervous System Diseases

Monoamine oxidases (MAOs) are the enzymes that catalyze the oxidation of monoamines, such as dopamine, norepinephrine, and serotonin, which serve as key neurotransmitters in the central nervous system (CNS). MAOs play important roles in maintaining the homeostasis of monoamines, and the aberrant expression or activation of MAOs underlies the pathogenesis of monoamine neurotransmitter disorders, including neuropsychiatric and neurodegenerative diseases. Clearly, detecting and inhibiting the activities of MAOs is of great value for the diagnosis and therapeutics of these diseases. Accordingly, many specific detection probes and inhibitors have been developed and substantially contributed to basic and clinical studies of these diseases. In this review, progress in the detecting and inhibiting of MAOs and their applications in mechanism exploration and treatment of neurotransmitter-related disorders is summarized. Notably, how the detection probes and inhibitors of MAOs were developed has been specifically addressed. It is hoped that this review will benefit the design of more effective and sensitive probes and inhibitors for MAOs, and eventually the treatment of monoamine neurotransmitter disorders.

Monoamine oxidase inhibitors: promising therapeutic agents for Alzheimer's disease (Review)

Activated monoamine oxidase (MAO) has a critical role in the pathogenesis of Alzheimer's disease (AD), including the formation of amyloid plaques from amyloid β peptide (Aβ) production and accumulation, formation of neurofibrillary tangles, and cognitive impairment via the destruction of cholinergic neurons and disorder of the cholinergic system. Several studies have indicated that MAO inhibitors improve cognitive deficits and reverse Aβ pathology by modulating proteolytic cleavage of amyloid precursor protein and decreasing Aβ protein fragments. Thus, MAO inhibitors may be considered as promising therapeutic agents for AD.

Current place of monoamine oxidase inhibitors in the treatment of depression

This paper reviews the discovery and history of the use of irreversible monoamine oxidase (MAO) inhibitors (MAOIs) such as phenelzine, tranylcypromine and isocarboxazid, as well as the second generation selective and reversible MAOIs such as the MAO-A inhibitor, moclobemide and the MAO-B inhibitor, selegiline. Data for review were identified from a literature search of OvidSP Medline and PsycInfo performed in July 2012, using the subject terms and keywords of 'monoamine oxidase inhibitors', 'major depression', 'depressive disorder' and 'depression (emotion)'. The search was limited to papers published in the English language and from 2007 onward only. Irreversible MAOIs have the potential to treat the most challenging mood disorder patients including those with treatment-resistant depression, atypical depression and bipolar depression. Unfortunately, the use of irreversible MAOIs has been declining sharply due to lack of marketing and the excessive fears of clinicians. Moreover, few clinicians now have any experience, let alone comfort, in prescribing this class of antidepressants. The newer MAOIs are available as another option for the treatment of major depression but have not replaced the irreversible MAOIs for the specific sub-types of depression for which they are now recommended in most consensus guidelines and treatment algorithms. The pharmacology, drug interactions and dietary recommendations associated with the use of MAOIs are reviewed. With the appropriate dietary restrictions and attention to potential drug interactions with serotonin and noradrenaline agents this class of drugs can be used effectively and safely. The MAOIs still represent an important element in our therapeutic armamentarium. Despite recommendations by opinion leaders and consensus guidelines for the use of MAOIs in specific sub-types of depression, the prescription rate of MAOIs is far less than expected and is decreasing. The "bad reputation" and the lack of industry support for this class of agents (especially the irreversible MAOIs) must be overcome in order to continue to provide a potentially useful treatment for a very vulnerable yet substantial sub-population of mood disorder patients.

Mammalian plasma and tissue-bound semicarbazide-sensitive amine oxidases: biochemical, pharmacological and toxicological aspects

Mammalian plasma and tissues contain various soluble and membrane-bound enzymes which metabolize the synthetic amine benzylamine particularly well. The sensitivity of these enzymes to inhibition by semicarbazide and related compounds suggests that they contain a cofactor with a reactive carbonyl group, which has been proposed to be either pyridoxal phosphate, pyrroloquinoline quinone or (more recently) 6-hydroxydopa. It is not yet clear if all of these semicarbazide-sensitive amine oxidases (SSAOs) are copper-dependent enzymes. A variety of compounds have now been identified as relatively selective inhibitors to distinguish the SSAOs from other amine oxidases, in order to investigate the properties of SSAOs and their potential role in biogenic and xenobiotic amine metabolism in vivo. While plasma SSAO is soluble, most tissue SSAOs appear to be membrane-bound, probably plasmalemmal enzymes, which may be capable of metabolizing extracellular amines. Vascular (and non-vascular) smooth muscle cells have particularly high SSAO activity, although recently the enzyme has been found in other cell types (e.g. adipocytes, chondrocytes, odontoblasts) implying a functional importance not restricted solely to smooth muscle. The substrate specificity of plasma and tissue SSAOs shows considerable species-related variations. For example, while some endogenously-occurring aromatic amines such as tyramine and tryptamine are metabolized well by SSAO in homogenates of rat blood vessels, and also in vitro inhibition of SSAO can potentiate vasoconstrictor actions of these amines in rat vascular preparations, these amines are poor substrates for human SSAO, thus complicating attempts to generalize possible physiological roles for these enzymes. Vascular SSAO can metabolize the xenobiotic aliphatic amine, allylamine, to the cytotoxic aldehyde acrolein and this has been linked to the ability of allylamine administration to produce cardiovascular lesions in experimental animals, sometimes mimicking features of atherosclerotic disease. Recent studies showing that the endogenously-occurring aliphatic amines methylamine and aminoacetone are metabolized in vitro to formaldehyde and methylglyoxal, respectively, by SSAO in some animal (including human) tissues, suggest the possibility that toxicological consequences upon cellular function could result if such conversions occur in vivo.

Determination of monoamine oxidase B activity by high-performance liquid chromatography with electrochemical detection

A sensitive high-performance liquid chromatography method with electrochemical detection for measuring monoamine oxidase B activity in blood platelets is described. Dopamine is used as substrate and is incubated with isolated platelets and aldehyde dehydrogenase to convert dihydroxyphenylacetaldehyde to dihydroxyphenylacetic acid (DOPAC). The acid and the added internal standard hydrocaffeic acid are separated from dopamine and the incubation mixture by extraction with 5 ml of ethyl acetate-toluene (5:1, v/v). The organic phase is evaporated under nitrogen stream and the residue dissolved in 0.1 M citric acid. Dihydroxyphenylacetic acid and the internal standard dihydrocaffeic acid are then separated on a Eurosphere 100-C18 5 microns column. The mobile phase used was a mixture of sodium acetate, citric acid, and acetonitrile at pH 2.5. The standard curve was linear from 125 pg to 10 ng. Absolute recovery of DOPAC was 85 +/- 3.8% and of hydrocaffeic acid 87 +/- 4.1%. The method presented is sensitive (detection limit 8.0 pg of DOPAC injected) and reproducible (coefficient of variation 0.4-1%) with good accuracy (94-98%).

Comparative studies on semicarbazide-sensitive amine oxidase in heart and plasma of rats treated with hepatotoxin allyl formate

1. After allyl formate (AF) was administered to the rats, the existence of semicarbazide-sensitive amine oxidase (SSAO) in rat identified. 2. When the heart homogenate and plasma of AF-administered rat were pretreated with 10(-3) M clorgyline and deprenyl, the Km value for benzylamine of rat heart was same as the value of plasma. 3. The existence of SSAO in plasma of AF-administered rats were identified by IEF-gel electrophoresis. The pI values of SSAO in heart and plasma were a single peak of 5.0. 4. SSAO released from the rat heart in response to AF, although the other origins of this enzyme are unknown.

Decreased plasma benzylamine oxidase in a subgroup of schizophrenia

Plasma benzylamine oxidase (BzAO) activity was analysed in a group of 40 schizophrenic patients and compared with that of healthy controls. Although plasma BzAO was significantly decreased in schizophrenics as a whole, when patients were divided into type I (favourable course) and type II (poor course) it was found that the reduced BzAO activity was only observed in type I. Several clinical features were associated with a low BzAO level. The clinical and biological significance of the identification of a biochemical alteration in a subgroup of schizophrenic patients is discussed, and we suggest that the possibility of BzAO being a marker for prognosis in schizophrenia should be further investigated.

The oxidation of tryptamine by the two forms of monoamine oxidase in human tissues

The selective monoamine oxidase inhibitors clorgyline and (-)-deprenyl have been used to determine the activities of monoamine oxidase-A and -B towards tryptamine in several human tissues. The results were compared with those obtained with the A-form-selective substrate 5-hydroxytryptamine, the B-form-selective substrate 2-phenethylamine and the common substrate tyramine. Tryptamine was found to be a substrate for both forms of the enzyme in human liver, kidney cortex and medulla and in seven different brain regions. The Km values of the two forms towards this substrate were similar in all the human tissues examined but the maximum velocities differed. Thus the A-form would contribute approximately 50% of the total monoamine oxidase activity towards this substrate in human cerebral cortex, whereas it would contribute about 60% in kidney cortex and medulla and 75% in liver. These results suggest that both forms of monoamine oxidase would contribute to the metabolism of tryptamine in human tissues and are difficult to reconcile with suggestions that tryptamine excretion may provide a simple index of monoamine oxidase-A inhibition.

Properties of monoamine oxidase in monkey heart

The activities of monoamine oxidase (MAO) in a homogenate and the mitochondrial fraction of monkey (Macaca fascicularis) heart were measured with labelled benzylamine, serotonin (5-HT), tyramine and beta-phenylethylamine (PEA) as substrates. The effects of clorgyline, deprenyl and semicarbazide on MAO were also investigated. Benzylamine deamination was more sensitive to deprenyl than to clorgyline, although it was not completely inhibited by a high concentration of either, and the remaining activity was completely inhibited by pretreatment with semicarbazide. The activities of 5-HT and tyramine were both more sensitive to clorgyline than to deprenyl, and both inhibitors gave single-sigmoidal inhibition curves. Experiments with increasing concentrations of PEA as substrate and clorgyline and deprenyl as inhibitors indicated that at higher concentrations, PEA was a substrate for MAO-A as well as MAO-B in monkey heart. These results suggest that monkey heart mitochondria contain not only MAO-A and MAO-B, but also clorgyline resistant amine oxidase (CRAO). The ratio of MAO-A, MAO-B and CRAO was determined from plots of inhibitions with each substrate. The kinetic constants of MAO in monkey heart were compared with those of MAO in hearts of other animals. The enzymic properties of MAO in monkey heart were discussed on the basis of these results.

Transfer and metabolism of norepinephrine studied from maternal-to-fetal and fetal-to-maternal sides in the in vitro perfused human placental lobe

An in vitro system for the dual perfusion of an isolated lobe of human placenta was used to study transfer and metabolism of 3H-labeled norepinephrine at two different concentrations. At 39 mumol/L, the transfer of total radioactivity from the maternal to the fetal side was 11.60% +/- 0.60% and significantly higher (p less than 0.05) than that in the opposite direction (6.50% +/- 0.37%). In both directions almost 50% of tritium was transferred as norepinephrine while the rest was metabolized and appeared on the other side as normetanephrine, dihydroxymandelic acid, and vanillylmandelic acid. When a reduced concentration (870 nmol/L) was infused into either the maternal or the fetal compartment, transfer and metabolism were similar to those determined before. Analysis of the tissue showed 50% of total radioactivity as normetanephrine, dihydroxymandelic acid, and vanillylmandelic acid and 17% as conjugates. Inclusion of the monoamine oxidase inhibitor clorgyline in the perfusate significantly reduced fetal-to-maternal transfer but not transfer in the reverse direction. In addition, inhibition of placental monoamine oxidase resulted in a considerable reduction in dihydroxymandelic acid and vanillylmandelic acid while the normetanephrine fraction increased. Norepinephrine significantly increased glucose utilization and lactate production. A possible physiologic role of norepinephrine transferred from the fetal into the maternal circulation during labor is discussed.

beta-Carbolines as selective monoamine oxidase inhibitors: in vivo implications

The inhibitory action of a range of beta-carbolines on human and rat monoamine oxidase (MAO) A and B has been studied. Concentrations of 5-hydroxytryptamine and phenylethylamine, approximately at their Km values, were used as substrates for MAO A and B respectively. A wide variation in selectivity was found, with harmaline being 10,000 times more potent an inhibitor of A than B whereas, using tetrahydro-beta-carboline and harmane, the difference was nearer to ten-fold. Of the carbolines which have been found endogenously, tetrahydro-beta-carboline, 6-methoxytetrahydro-beta-carboline and harmane are all sufficiently potent inhibitors of human MAO A, with I50 values of 5 X 10(-6), 10(-6), 5 X 10(-7) M respectively, for this property to be of possible physiological significance. Harmane, with an I50 of 5 X 10(-6) M, might also play a role as an inhibitor of MAO B.

Monoamine-oxidizing enzymes in human pregnancy

The activity of benzylamine oxidase (BzAO) was investigated in human maternal blood at all stages of gestation, including parturition, as well as in the puerperium. In addition, BzAO and monoamine oxidase (MAO) A and B activities were assayed in amniotic fluid, placenta, placental vessels and umbilical vessels. No correlation was found between BzAO values in maternal blood and fetal growth. Highly significant variations in maternal plasma BzAO activity were seen by the end of the first trimester, at parturition and at 6-72 h post-partum. The predominance of MAO A in placenta was again confirmed, whereas in vascular tissue and amniotic fluid, BzAO was clearly preponderant; in the latter, no MAO A activity could be detected. Placental vessels showed significantly higher MAO A activity than umbilical vessels. BzAO and what appears to be a true, soluble MAO B were demonstrated in amniotic fluid. The physiological implications of these findings are discussed.

Human liver MAO-A and MAO-B separated by immunoaffinity chromatography with MAO-B-specific monoclonal antibody

A monoclonal antibody was used to prepare immunoaffinity columns that efficiently bind monoamine oxidase B activity but not monoamine oxidase A activity from detergent extracts of human liver mitochondria. The only discrete polypeptide component that eluted from affinity columns with potassium thiocyanate migrated in sodium dodecyl sulfate-polyacrylamide gels with an apparent molecular weight of 59,000, as expected for human monoamine oxidase B. These results support the hypothesis that there is an intrinsic structural difference between monoamine oxidase A and B and demonstrate that immunoaffinity chromatography can physically resolve the two enzyme species in liver extracts.

Characteristics and specificity of phenelzine and benserazide as inhibitors of benzylamine oxidase and monoamine oxidase

The selectivity of benserazide and phenelzine toward inhibition of benzylamine oxidase (BzAO) and monoamine oxidases (MAO-A and MAO-B) was studied in homogenates of rat skull and lung. In addition, the kinetic interaction and reversibility of BzAO inhibition were assessed. Both drugs inhibited BzAO but only phenelzine inhibited MAO, whether tested in vitro or in vivo. Neither compound acted as an irreversible inhibitor of BzAO. Benserazide was found to be a noncompetitive inhibitor. Phenelzine acted as a substrate for BzAO followed by product-induced noncompetitive inhibition which was labile at 37 degrees but not at 4 degrees. A reversible component in phenelzine-induced inhibition of MAO-A and -B is also suggested from in vivo studies.

Some factors influencing the metabolism of benzylamine by type A and B monoamine oxidase in rat heart and liver

The ability of MAO-A and MAO-B to metabolize benzylamine in vitro has been investigated in mitochondrial preparations from rat liver and heart. Although under normal circumstances benzylamine appeared to be metabolized exclusively by MAO-B in the rat liver, a contribution by both MAO-A and a clorgyline-resistant enzyme component was revealed when the MAO-B activity was much reduced by pretreatment of the mitochondria with appropriate concentrations of deprenyl. These three enzyme activities also contributed to benzylamine deamination in rat heart mitochondria. However, binding studies with [3H]pargyline, which provided an estimate of the respective concentrations of MAO-A and MAO-B active centres in heart mitochondria, indicated a ratio between MAO-A and MAO-B, markedly different from that shown by plots of inhibition of benzylamine metabolism by various concentrations of clorgyline. The interpretation of these clorgyline plots is discussed in terms of the kinetic constants of both MAO-A and MAO-B, and the relative amounts of each enzyme. It is proposed that although the turnover rate constant for benzylamine metabolism by MAO-A is much smaller than that shown by MAO-B, in those tissues containing a large ratio of MAO-A:MAO-B content, the metabolism of benzylamine by MAO-A can be detected.

Dopamine oxidation and its inhibition by (-)-deprenyl in man

Dopamine is predominantly oxidized by a (-)-deprenyl sensitive form of MAO in the human striatum, and (-)-deprenyl, acting at some suitably low selective inhibitory concentration may, therefore, be of benefit in Parkinson's disease. 10(-6)M was the most effective (-)-deprenyl concentration in vitro for discriminating between the inhibition of MAO A and B. The correlation between the A/B ratio present in different human brain regions and the sensitivity of dopamine oxidation to 10(-6)M deprenyl was 0.84 (p<0.001). This suggests that all dopamine oxidation can be accounted for by the joint contribution of MAO A and B and that it is unnecessary to postulate a special form of the enzyme which metabolizes dopamine. In the brain, the striatum has the highest proportion of MAO B, and in several cortical regions, relatively more dopamine is oxidized by MAO A. In other human tissues also the deprenyl sensitivity of dopamine oxidation correlated with the known A/B ratio, the placenta, lung and jejeunum having the lowest sensitivity and being the richest in MAO A. Km values for dopamine for MAO A and B are similar, 130 and 140 uM respectively, so that the proportion oxidized by the two forms should not vary with substrate concentration.

Amine oxidase histochemistry of the human uterus during the menstrual cycle

The enzymes monoamine oxidase A (MAO A), monoamine oxidase B (MAO B) and benzylamine oxidase (BzAO) have been locaized histochemically in the human uterus during various phases of the menstrual cycle. The results show a large increase in MAO A activity in th endometrial gland cells in the secretory phase of the cycle. MAO B activity was found in both endometrium and myometrium but did not show a cyclical variations in activity. BzAO was localized primarily in the tunica media of the myometrial blood vessels. These observations have been supported by parallel biochemical assay.s

Tele-Methylhistamine is a specific MAO B substrate in man

Tele-methylhistamine, the first metabolite of histamine in tissues which lack diamine oxidase, is shown to be a substrate for human MAO B. Human liver homogenates were incubated with 3H-tele-methylhistamine and the products separated using thin-layer chromatography. The major product was 3-methylimidazoleacetic acid, the oxidatively deaminated metabolite of tele-methylhistamine. The reaction was inhibited by low concentrations of (-)deprenyl, the specific MAO B inhibitor. Tele-methylhistamine was also found to inhibit competitively the oxidation of phenylethlamine, but not that of 5-hydroxytryptamine, providing further evidence that it is oxidized by MAO B itself and not a related enzyme. This finding implies that (-)deprenyl and other MAO inhibitors used clinically may interfere with histamine metabolism.