Selective inhibition of monoamine oxidase A by purpurin,an anthraquinone

Monoamine oxidase (MAO) catalyzes the oxidation of monoamines that act as neurotransmitters. During a target-based screening of natural products using two isoforms of recombinant human MAO-A and MAO-B, purpurin (an anthraquinone derivative) was found to potently and selectively inhibit MAO-A, with an IC₅₀ value of 2.50 μM, and not to inhibit MAO-B. Alizarin (also an anthraquinone) inhibited MAO-A less potently with an IC₅₀ value of 30.1 μM. Furthermore, purpurin was a reversible and competitive inhibitor of MAO-A with a K_i value of 0.422 μM. A comparison of their chemical structures suggested the 4-hydroxy group of purpurin might play an important role in its inhibition of MAO-A. Molecular docking simulation showed that the binding affinity of purpurin for MAO-A (?40.0 kcal/mol) was higher than its affinity for MAO-B (?33.9 kcal/mol), and that Ile 207 and Gly 443 of MAO-A were key residues for hydrogen bonding with purpurin. The findings of this study suggest purpurin is a potent, selective, reversible inhibitor of MAO-A, and that it be considered a new potential lead compound for development of novel reversible inhibitors of MAO-A (RIMAs).     

Inhibition of monoamine oxidase by 8-benzyloxycaffeine analogues

Based on recent reports that several (E)-8-styrylcaffeinyl analogues are potent reversible inhibitors of monoamine oxidase B (MAO-B), a series of 8-benzyloxycaffeinyl analogues were synthesized and evaluated as inhibitors of baboon liver MAO-B and recombinant human MAO-A and -B. The 8-benzyloxycaffeinyl analogues were found to inhibit reversibly both MAO isoforms with enzyme–inhibitor dissociation constants (K_i values) ranging from 0.14 to 1.30 μM for the inhibition of human MAO-A, and 0.023–0.59 μM for the inhibition of human MAO-B. The most potent MAO-A inhibitor was 8-(3-methylbenzyloxy)caffeine while 8-(3-bromobenzyloxy)caffeine was the most potent MAO-B inhibitor. The analogues inhibited human and baboon MAO-B with similar potencies. A quantitative structure–activity relationship (QSAR) study indicated that the MAO-B inhibition potencies of the 8-benzyloxycaffeinyl analogues are dependent on the Hansch lipophilicity (π) and Hammett electronic (σ) constants of the substituents at C-3 of the benzyloxy ring. Electron-withdrawing substituents with a high degree of lipophilicity enhance inhibition potency. These results are discussed with reference to possible binding orientations of the inhibitors within the active site cavities of MAO-A and -B.     

α-Tetralone derivatives as inhibitors of monoamine oxidase

In the present study, a series of fifteen α-tetralone (3,4-dihydro-2H-naphthalen-1-one) derivatives were synthesised and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The α-tetralone derivatives examined are structurally related to a series of chromone (1-benzopyran-4-one) derivatives which has previously been shown to act as MAO-B inhibitors. The results document that the α-tetralones are highly potent MAO-B inhibitors with all compounds exhibiting IC₅₀ values in the nanomolar range (<78 nM). Although most compounds are selective inhibitors of MAO-B, the α-tetralones are also potent MAO-A inhibitors with ten compounds exhibiting IC₅₀ values in the nanomolar range (<792 nM). The most potent MAO-B inhibitor, 6-(3-iodobenzyloxy)-3,4-dihydro-2H-naphthalen-1-one, exhibits an IC₅₀ value of 4.5 nM with a 287-fold selectivity for MAO-B over the MAO-A isoform, while the most potent MAO-A inhibitor, 6-(3-cyanobenzyloxy)-3,4-dihydro-2H-naphthalen-1-one, exhibits an IC₅₀ value of 24 nM with a 3.25-fold selectivity for MAO-A. Analyses of the structure–activity relationships for MAO inhibition show that substitution on the C6 position of the α-tetralone moiety is a requirement for MAO-A and MAO-B inhibition, and that a benzyloxy substituent on this position is more favourable for MAO-A inhibition than phenylethoxy and phenylpropoxy substitution. For MAO-B inhibition, alkyl and halogen substituents on the meta and para positions of the benzyloxy ring enhance inhibitory potency. It may be concluded that α-tetralone derivatives are promising leads for design of therapies for Parkinson’s disease and depression.     

Sulfanylphthalonitrile analogues as selective and potent inhibitors of monoamine oxidase B

It has recently been reported that nitrile containing compounds frequently act as potent monoamine oxidase B (MAO-B) inhibitors. Modelling studies suggest that this high potency inhibition may rely, at least in part, on polar interactions between nitrile functional groups and polar moieties within the MAO-B substrate cavity. In an attempt to identify potent and selective inhibitors of MAO-B and to contribute to the known structure–activity relationships of MAO inhibition by nitrile containing compounds, the present study examined the MAO inhibitory properties of series of novel sulfanylphthalonitriles and sulfanylbenzonitriles. The results document that the evaluated compounds are potent and selective MAO-B inhibitors with most homologues possessing IC₅₀ values in the nanomolar range. In general, the sulfanylphthalonitriles exhibited higher binding affinities for MAO-B than the corresponding sulfanylbenzonitrile homologues. Among the compounds evaluated, 4-[(4-bromobenzyl)sulfanyl]phthalonitrile is a particularly promising inhibitor since it displayed a high degree of selectivity (8720-fold) for MAO-B over MAO-A, and potent MAO-B inhibition (IC₅₀ = 0.025 μM). Based on these observations, this structure may serve as a lead for the development of therapies for neurodegenerative disorders such as Parkinson’s disease.     

Inhibition of monoamine oxidase by 8-[(phenylethyl)sulfanyl]caffeine analogues

In a previous study we have investigated the monoamine oxidase (MAO) inhibitory properties of a series of 8-sulfanylcaffeine analogues. Among the compounds studied, 8-[(phenylethyl)sulfanyl]caffeine (IC₅₀ = 0.223 μM) was found to be a particularly potent inhibitor of the type B MAO isoform. In an attempt to discover potent MAO inhibitors and to further examine the structure–activity relationships (SAR) of MAO inhibition by 8-sulfanylcaffeine analogues, in the present study a series of 8-[(phenylethyl)sulfanyl]caffeine analogues were synthesized and evaluated as inhibitors of human MAO-A and -B. The results document that substitution on C3 and C4 of the phenyl ring with alkyl groups and halogens yields 8-[(phenylethyl)sulfanyl]caffeine analogues which are potent and selective MAO-B inhibitors with IC₅₀ values ranging from 0.017 to 0.125 μM. The MAO inhibitory properties of a series of 8-sulfinylcaffeine analogues were also examined. The results show that, compared to the corresponding 8-sulfanylcaffeine analogues, the 8-sulfinylcaffeins are weaker MAO-B inhibitors. Both the 8-sulfanylcaffeine and 8-sulfinylcaffeine analogues were found to be weak MAO-A inhibitors. This study also reports the MAO inhibition properties of selected 8-[(phenylpropyl)sulfanyl]caffeine analogues.     

Novel sulfanylphthalimide analogues as highly potent inhibitors of monoamine oxidase B

Monoamine oxidase (MAO) plays an essential role in the catabolism of neurotransmitter amines. The two isoforms of this enzyme, MAO-A and -B, are considered to be drug targets for the therapy of depression and neurodegenerative diseases, respectively. Based on a recent report that the phthalimide moiety may be a useful scaffold for the design of potent MAO-B inhibitors, the present study examines a series of 5-sulfanylphthalimide analogues as potential inhibitors of both human MAO isoforms. The results document that 5-sulfanylphthalimides are highly potent and selective MAO-B inhibitors with all of the examined compounds possessing IC₅₀ values in the nanomolar range. The most potent inhibitor, 5-(benzylsulfanyl)phthalimide, exhibits an IC₅₀ value of 0.0045 μM for the inhibition of MAO-B with a 427-fold selectivity for MAO-B compared to MAO-A. We conclude that 5-sulfanylphthalimides represent an interesting class of MAO-B inhibitors and may serve as lead compounds for the design of antiparkinsonian therapy.     

Selected C7-substituted chromone derivatives as monoamine oxidase inhibitors

A series of C7-substituted chromone (1-benzopyran-4-one) derivatives were synthesized and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. The chromones are structurally related to a series of C7-functionalized coumarin (1-benzopyran-2-one) derivatives which has been reported to act as potent MAO inhibitors. The results of the current study document that the chromones are highly potent reversible inhibitors of MAO-B with IC₅₀ values ranging from 0.008 to 0.370 μM. While the chromone derivatives also exhibit affinities for MAO-A, with IC₅₀ values ranging from 0.495 to 8.03 μM, they are selective for the MAO-B isoform. Structure–activity relationships (SAR) show that 7-benzyloxy substitution of chromone is suitable for MAO-B inhibition with tolerance for a variety of substituents and substitution patterns on the benzyloxy ring. It may be concluded that 7-benzyloxychromones are appropriate lead compounds for the design of reversible and selective MAO-B inhibitors. With the aid of modeling studies, potential binding orientations and interactions of selected chromone derivatives in the MAO-A and -B active sites are examined.     

Design of novel nicotinamides as potent and selective monoamine oxidase a inhibitors

A series of N-(2-morpholinoethyl)nicotinamide (1–13) and N-(3-morpholinopropyl)nicotinamide derivatives (14–26) have been designed, synthesized and evaluated in vitro for their monoamine oxidase (MAO) A and B inhibitory activity and selectivity. Most of these synthesized compounds proved to be potent, and selective inhibitors of MAO-A rather than of MAO-B. 5-Chloro-6-hydroxy-N-(2-morpholinoethyl)nicotinamide (13) displayed the highest MAO-A inhibitory potency (IC₅₀ = 0.045 μM) and a good selectivity. 2-Bromo-N-(2-morpholinoethyl)nicotinamide (3) was the most potent MAO-B inhibitor with the IC₅₀ value of 0.32 μM, but it was not selective. Molecular dockings of compound 13 were performed in order to give structural insights regarding the MAO-A selectivity.     

Differential substrate specificity of monoamine oxidase in the rat heart and renal cortex

Although it is known that substrate specificities differ with species and within each species with the tissues, in the rat heart no natural substrate was found for MAO-B. beta-phenylethylamine (beta-PEA) has always been considered the "endogenous" substrate of MAO B. We thought worthwide to evaluate the effect of Ro 41-1049 and lazabemide, both members of a class of highly selective, mechanism-based and reversible inhibitors for MAO-A and MAO B, respectively on the metabolization of beta-PEA by the rat heart. Also the lack of molecular data on rat heart MAOs, prompted us to better characterize rat heart MAOs, both kinetically and using molecular biology techniques. K(m) values for deamination of beta-PEA in the rat heart were 13-fold those in the kidney, by contrast, K(m) values for deamination of 5-HT were quite similar in both tissues. Unexpectedly, the selective MAO-A inhibitor Ro 41-1049 was by far the most potent inhibitor of beta-PEA (20 microM) deamination in the rat heart, while clorgyline, another MAO A inhibitor, and lazabemide, a MAO B inhibitor, had intermediate efficacy; selegiline was found unable to inhibit deamination of beta-PEA. In the rat renal cortex lazabemide and selegiline both inhibited beta-PEA deamination. The reduction of beta-PEA concentration to just 200 nM, the use of heart membranes instead of tissue homogenates or the use of heart membranes pre-treated with 1% digitonine failed to change this pattern of inhibition. Semicarbazide was found not to alter deamination of beta-PEA. Western blot showed the presence of both isoforms (55 kd and 61 kd) in the renal cortex. In the heart there was a predominance of the A form, the B form being undetected. The RT-PCR products for both MAO-A and MAO-B, were found to have the expected sizes. In conclusion, we found mRNA for MAO-B but were unable to detect the protein itself or its activity when using beta-PEA as the substrate.     

2-Heteroarylidene-1-indanone derivatives as inhibitors of monoamine oxidase

In the present study a series of fifteen 2-heteroarylidene-1-indanone derivatives were synthesised and evaluated as inhibitors of recombinant human monoamine oxidase (MAO) A and B. These compounds are structurally related to series of heterocyclic chalcone derivatives which have previously been shown to act as MAO-B specific inhibitors. The results document that the 2-heteroarylidene-1-indanones are in vitro inhibitors of MAO-B, displaying IC₅₀ values of 0.0044–1.53 μM. Although with lower potencies, the derivatives also inhibit the MAO-A isoform with IC₅₀ values as low as 0.061 μM. An analysis of the structure-activity relationships for MAO-B inhibition indicates that substitution with the methoxy group on the A-ring leads to a significant enhancement in MAO-B inhibition compared to the unsubstituted homologues while the effect of the heteroaromatic substituent on activity, in decreasing order is: 5-bromo-2-furan > 5-methyl-2-furan > 2-pyridine ≈ 2-thiophene > cyclohexyl > 3-pyridine ≈ 2-furan. It may therefore be concluded that 2-heteroarylidene-1-indanone derivatives are promising leads for the design of MAO inhibitors for the treatment of Parkinson’s disease and possibly other neurodegenerative disorders.     

Alkynyl–coumarinyl ethers as MAO-B inhibitors

In this study, alkynyl–coumarinyl ethers were developed as inhibitors of human monoamine oxidase B (MAO-B). A series of 31 new, ether-connected coumarin derivatives was synthesized via hydroxycoumarins, whose phenolic group at position 6, 7 or 8 was converted by means of the Mitsunobu reaction. The majority of the final products were produced from primary alcohols with a terminal alkyne group. The inhibitors were optimized with respect to the structure of the alkynyloxy chain and its position at the fused benzene ring as well as the residue at position 3 of the pyran-2H-one part. A hex-5-ynyloxy chain at position 7 was found to be particular advantageous. Among the 7-hex-5-ynyloxy-coumarins, the 3-methoxycarbonyl derivative 36 was characterized as a dual-acting inhibitor with IC₅₀ values of less than 10 nM towards MAO-A and MAO-B, and the 3-(4-methoxy)phenyl derivative 44 was shown to combine strong anti-MAO-B potency (IC₅₀ = 3.0 nM) and selectivity for MAO-B over MAO-A (selectivity >3400-fold).     

Antidepressant-like profile and MAO-A inhibitory activity of 4-propyl-2H-benzo[h]-chromen-2-one

AimsTo evaluate the central nervous pharmacological profile of 4-propyl-2H-benzo[h]-chromen-2-one (FCS-304) in ICR mice and its monoamine oxidase inhibitor activity.Main methodsFCS-304 was evaluated in screening test of central nervous system in ICR mice and against MAO activity.Key findingsFCS-304 (25–200 mg/Kg, p.o.) significantly reduced immobility time during the forced swimming test (FST) and tail suspension test (TST), but did not show effects in the rota-rod tests, maximal electroshock seizures (MES), pentylenetetrazole seizures, light–dark box, barbiturate sleeping time and cold plate tests. Furthermore, FCS-304 inhibited monoamine oxidase A (MAO-A) with IC₅₀: 2.28 ± 0.15 µM, but not MAO-B.SignificanceThese results suggest that FCS-304 could elicit antidepressant effects related to MAO-A inhibitory activity.     

Molecular cloning and characterization of monoamine oxidase A and B genes in pig.

Monoamine oxidase (MAO) is a key enzyme in the degradation of monoamine neurotransmitters, which are physiologically and pathologically important for animals and humans. MAO exists in two forms, coded by separate genes. Here we cloned and sequenced full length cDNAs of MAO-A and MAO-B in pig. The putative proteins include 527 and 520 amino acids, respectively. The sequences alignment and homology analysis showed that MAO-A and MAO-B are conserved among mammals. The amino acid residues and domains, which have been demonstrated to be important for MAO-A and MAO-B binding of ligands, substrates and inhibitors, were conserved in the pig.     

Modification of substrate-inhibitor affinities of human platelet monoamine oxidase B in vitro

The rate of benzylamine utilization by monoamine oxidase (MAO)-B from human blood platelets was 2-4 times higher than that for octopamine. Both activities were inhibited 100% by 10(-7) M deprenyl (a specific MAO-B inhibitor) and were not affected by clorgyline (a specific MAO-A inhibitor) or by polyclonal antibodies to MAO-A. The preincubation of platelet MAO-B with purified MAO-A from mitochondrial membranes of human placenta resulted in appearance of excess octopamine activity. This additional activity was not precipitated by antibodies to MAO-A or inhibited by deprenyl but was inhibited by clorgyline. Incubation of the MAO-A preparation from placenta at 45 degrees C for 15 min before its preincubation with MAO-B caused 50% loss of both activities. Protease inhibitors had no effect on the modification of MAO. These data indicate that MAO-A or a factor tightly bound to it can modify MAO-B yielding a form of the enzyme with both MAO-A and MAO-B substrate and inhibitor affinities and MAO-B immunospecificity.     

Brain region differences and some characteristics of monoamine oxidase type a and b activities in the vervet monkey

Monoamine oxidase in the vervet monkey showed greater variations in activity in six brain regions when tyramine or phenylethylamine was used as the substrate (3.8- to 4.1-fold differences) than when serotonin was the substrate (1.8-fold differences). With phenylethylamine and tyramine as substrates, the highest MAO specific activities were found in the hypothalamus and the lowest in the cerebellum and cortex. With serotonin as the substrate, the highest specific activities were in the mesencephalon and cortex. The inhibition of tyramine deamination by clorgyline and deprenyl yielded biphasic plots indicative of the presence of MAO-A and MAO-B enzyme forms in the vervet brain. On the basis of these inhibitor curves, the vervet brain could be estimated to contain approximately 85% MAO-B and 15% MAO-A, in contrast to rat brain which contains 45% MAO-B and 55% MAO-A. The inhibition of serotonin deamination by deprenyl in vervet brain yielded a biphasic plot, suggesting that some serotonin deamination in the vervet is accomplished by the MAO-B enzyme form. Estimations of the relative amounts of MAO-A and MAO-B based on inhibitor curves or based on substrate ratios yielded proportionate results which were in close agreement across the different brain regions, supporting the validity of these approaches to estimating MAO-A and MAO-B activities.     

4-(O-Benzylphenoxy)-N-Methylbutylamine (Bifemelane) and Other 4-(O-Benzylphenoxy)-N-Methylalkylamines as New Inhibitors of Type A and B Monoamine Oxidase

4-(O-Benzylphenoxy)-N-methylbutylamine (Bifemelane, BP-N-methylbutylamine), a new psychotropic drug, was found to inhibit monoamine oxidase (MAO) in human brain synaptosomes. It inhibited type A MAO (MAO-A) competitively and type B (MAO-B) noncompetitively. BP-N-methylbutylamine had a much higher affinity to MAO-A than an amine substrate, kynuramine, and it was a more potent inhibitor of MAO-A than of MAO-B. The Ki values of MAO-A and -B were determined to be 4.20 and 46.0 microM, respectively, while the Km values of MAO-A and -B with kynuramine were 44.1 and 90.0 microM, respectively. The inhibition of MAO-A and -B by BP-N-methylbutylamine was found to be reversible by dialysis of the incubation mixture. MAO-A in human placental and liver mitochondria and in a rat clonal pheochromocytoma cell line, PC12h, was inhibited competitively by BP-N-methylbutylamine, while MAO-B in human liver mitochondria was inhibited noncompetitively, as in human brain synaptosomes. BP-N-methylbutylamine was not oxidized by MAO-A and -B. The effects of other BP-N-methylalkylamines, such as BP-N-methylethylamine, -propylamine, and -pentanylamine, on MAO activity were examined. BP-N-methylbutylamine was the most potent inhibitor of MAO-A, and BP-N-methylethylamine and -propylamine inhibited MAO-B competitively, whereas BP-N-methylbutylamine and -pentanylamine inhibited it noncompetitively. Inhibition of these BP-N-methylalkylamines on MAO-A and -B is discussed in relation to their chemical structure.     

New halogenated 3-phenylcoumarins as potent and selective MAO-B inhibitors

With the aim to find out the structural features for the MAO inhibitory activity and selectivity, in the present communication we report the synthesis and pharmacological evaluation of a new series of bromo-6-methyl-3-phenylcoumarin derivatives (with bromo atom in both different benzene rings of the skeleton) with and without different number of methoxy substituent at the 3-phenyl ring. The methoxy substituents were introduced, in this new scaffold, in the meta and/or para positions of the 3-phenyl ring. The synthesized compounds 3–7 were evaluated as MAO-A and B inhibitors using R-(?)-deprenyl (selegiline) and iproniazide as reference inhibitors, showing, most of them, MAO-B inhibitory activities in the low nanomolar range. Compounds 4 (IC₅₀ = 11.05 nM), 5 (IC₅₀ = 3.23 nM) and 6 (IC₅₀ = 7.12 nM) show higher activity than selegiline (IC₅₀ = 19.60 nM) and higher MAO-B selectivity, with more than 9050-fold, 30,960-fold and 14,045-fold inhibition levels, with respect to the MAO-A isoform.     

Type A monoamine oxidase is the target of an endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, leading to apoptosis in SH-SY5Y cells

Mitochondrial monoamine oxidase (MAO) has been considered to be involved in neuronal degeneration either by increased oxidative stress or protection with the inhibitors of type B MAO (MAO-B). In this paper, the role of type A MAO (MAO-A) in apoptosis was studied using human neuroblastoma SH-SY5Y cells, where only MAO-A is expressed. An endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, an MAO-A inhibitor, reduced membrane potential, DeltaPsim, in isolated mitochondria, and induced apoptosis in the cells, which 5-hydroxytryptamine, an MAO-A substrate, prevented. In contrast, beta-phenylethylamine, an MAO-B substrate, did not suppress the DeltaPsim decline by N-methyl(R)salsolinol. The binding of N-methyl(R)salsolinol to mitochondria was inhibited by clorgyline, a MOA-A inhibitor, but not by (-)deprenyl, an MAO-B inhibitor. RNA interference targeting MAO-A significantly reduced the binding of N-methyl(R)salsolinol with simultaneous reduction in the MAO activity. To examine the intervention of MAO-B in the apoptotic process, human MAO-B was transfected to SH-SY5Y cells, but the sensitivity to N-methyl(R)salsolinol was not affected, even although the activity and protein of MAO increased markedly. These results demonstrate a novel function of MAO-A in the binding of neurotoxins and the induction of apoptosis, which may account for neuronal cell death in neurodegenerative disorders, including Parkinson's disease.     

Absence of tissue-bound semicarbazide-sensitive amine oxidase activity in carp tissues

We have previously reported that carp (Cyprinus carpio) tissue mitochondria contain a novel form of monoamine oxidase (MAO), which belongs neither to MAO-A nor to MAO-B of the mammalian enzyme. This conclusion results from the findings that the carp MAO was equally sensitive to a selective MAO-A inhibitor clorgyline and to the MAO-B selective inhibitor l-deprenyl, when tyramine, a substrate for both forms, serotonin or beta-phenylethylamine, a substrate for either A or B-form of mammalian MAO, was used. In the present study, we tried to detect another amine oxidase, termed tissue-bound semicarbazide-sensitive amine oxidase (SSAO), activity in carp tissues. As definition of SSAO was used, such as insensitivity to inhibition of the kynuramine oxidizing activity by an MAO inhibitor pargyline and high sensitivity to the SSAO inhibitor semicarbazide. The results indicated that the oxidizing activity was selectively and almost completely inhibited by 0.1 mM pargyline alone or a combination of 0.1 mM pargyline plus 0.1 mM semicarbazide, but not by 0.1 mM semicarbazide alone. We also tried to detect any SSAO activity by changing experimental conditions, such as lower incubation temperature, higher enzyme protein concentration, a lower substrate concentration and different pH's in the reaction, as the enzyme source. However, still no SSAO activity could be detected in the tissues. These results conclusively indicate that carp tissues so far examined do not contain SSAO activity.     

Inhibition of Monoamine Oxidase by 7-Chloro-4-Nitrobenzofurazan

7-Chloro-4-nitrobenzofurazan (NBD-Cl) is a potent inhibitor of both types of monoamine oxidase (MAO). NBD-Cl competitively inhibited the oxidative deamination of kynuramine catalyzed by human placenta MAO-A, the oxidative deamination of benzylamine catalyzed by bovine liver MAO-B, the oxidative deamination of serotonin catalyzed by rat brain MAO-A, and the oxidative deamination of phenylethylamine catalyzed by rat brain MAO-B. In addition, a time-dependent inactivation of MAOs by NBD-Cl has been demonstrated upon incubation of the enzyme preparations with NBD-Cl at pH 9, but not at pH 7.5. The time-dependent inhibition of MAO by NBD-Cl could be prevented by the addition of 4-nitrophenyl azide, an active site-directed label of MAO, during incubation of the enzyme with NBD-Cl. On the basis of these findings, it is suggested that at pH 9, NBD-Cl modifies one (or more) essential lysine residue(s) in the active sites of the two types of MAO.