The acetylenic monoamine oxidase inhibitors clorgyline, deprenyl, pargyline and J-508: their properties and applications

An improved fluorimetric assay for brain monoamine oxidase

A fluorimetric assay for monamine oxidase that uses kynuramine as the substrate is described. This method is more economic, rapid, reproducible, and sensitive than other similar procedures. Its validity has been confirmed by the study of the subcellular distribution, the effects of inhibitors, and the kinetics of enzyme activity. Previous reports of substrate inhibition by kynuramine may have to be reassessed in the light of evidence presented here.

The release of endogenous dopamine from rat striatum during incubation in vitro

In this study the conditions for quantitative estimation of the spontaneous and KCl-stimulated release of endogenous dopamine from rat striatum during in vitro incubation have been investigated. KCl concentrations of 15-35 mM produced increasing stimulation of dopamine release during 20 min of incubation in vitro, while KCl concentrations of 45 and 60 mM produced no further significant increase in release. In the presence of 4.7 or 20 mM KCl, dopamine release was linear throughout a 20 min period of incubation; in the presence of 25 or 60 mM KCl, however, dopamine release was not linear, reaching a maximum within 8 min of incubation. Three structurally unrelated inhibitors of monoamine uptake were shown to enhance the net spontaneous release of endogenous dopamine at low concentrations, but to inhibit release at high concentrations. The ethanol-induced enhancement of striatal dopamine release was used to demonstrate the importance of an appropriate concentration of an amine uptake inhibitor in the incubation media. It is concluded that the incubation of rat striatal tissues in vitro, coupled with amine analysis by high performance liquid chromatography with electrochemical detection, constitutes an effective technique for the quantitative estimation of endogenous dopamine release, providing appropriate concentrations of a reuptake inhibitor and a monoamine oxidase inhibitor are included in the incubation media. Where rates of KCl-stimulated amine release with time are to be determined, however, a low concentration of KCl must be used, or the tissues must be subjected to incubation for short periods of time.

An allylamine derivative (MDL 72145) with potent irreversible inhibitory actions on rat aorta semicarbazide-sensitive amine oxidase

(E)-2-(3,4-dimethoxyphenyl)-3-fluoroallylamine (MDL 72145) was found to be an extremely potent inhibitor of the semicarbazide-sensitive amine oxidase (SSAO) in rat aorta homogenates. Considerable inhibition, which was not reversed by dialysis, could be produced under appropriate in-vitro conditions at drug concentrations around 10 nM. The pseudo first order kinetics for time-dependent inhibition by MDL 72145 (10-100 nM) were found to be consistent with a bimolecular reaction between enzyme and inhibitor with a rate constant for this step of 2 X 10(6) min-1 M-1. A similar rate of inhibition under an oxygen atmosphere to that obtained under nitrogen was produced upon incubation of enzyme with inhibitor, suggesting that oxidation of the inhibitor to an active metabolite was not required for its activity. Incubation of homogenates for very short periods (1 min) with inhibitor (0.05-0.5 microM) and benzylamine (1-10 microM) as substrate indicated non-competitive kinetics for the early interaction of enzyme with the drug. Benzylamine (50 microM), but not pyridoxal phosphate (100 microM), was able to protect SSAO from inhibition by 10 nM MDL 72145. However, pyridoxal phosphate alone appeared to produce some irreversible inhibition of the enzyme. Dialysis against buffer containing 50 microM or 1 mM benzylamine was unable to reactivate SSAO inhibited by 10 nM MDL 72145. It is concluded that MDL 72145 irreversibly inhibits SSAO by acting at, or near, the substrate binding site, but the exact nature of the complex formed remains to be identified.

Age-related changes in monoamine oxidase and semicarbazide-sensitive amine oxidase activities of rat aorta

Aorta MAO-A and SSAO activities were measured on young (3 months) and old (23-26 months) rats. A significant decrease (30-40%) in SSAO activity was found with benzylamine as substrate and the decrease was due to a reduction in Vmax. No significant changes in MAO-A activity were found in the aorta of old rats. beta-PEA is oxidized mainly by SSAO in rat aorta. However, the significance of this is unclear since the physiological role of that enzyme remains unknown.

Age-related changes in the activities of the amine metabolizing enzymes of rat eye

Eye MAO-A, MAO-B, semicarbazide-sensitive amine oxidase (SSAO) and aldehyde reductase (AR) activities were measured in young and old rats. When enzyme activity is expressed as nmol (mg protein)-1 min-1, a significant decrease (18-23%) of SSAO activity in the eye of old rats was found, whereas there was no significant difference in MAO-A and MAO-B activities. A significant increase of AR activity with D-xylose (67%), DL-glyceraldehyde (64%), D-glucuronate (43%) and D-glucose (21%) was found in the eye of old rats. These results suggest that changes in the activities of the amine metabolizing enzymes of rat eye with age might have consequences for their function in senescence; particularly, the increase of AR activity might be involved in cataract formation.

Intra- and extraneuronal monoamineoxidase-A and -B activities after central axotomy (hemisection) on rats

Hemitransection of the left side of rat brain results in a selective increase (40%) in the activity of MAO-B in the left side striatum, as compared to the right, unoperated side. This increase is shown to be the result of an increase in the activity of extraneuronal MAO-B using a "low substrate concentration method" with dopamine as substrate. This result is compatible with the hypothesis, that in certain degenerative processes such as aging, Alzheimer's disease, Huntington's disease and axotomy there is a stimulated growth of extraneuronal cells, which are relatively rich in MAO-B activity.

Membrane lipid environment of carp brain and liver mitochondrial monoamine oxidase

Delipidation of carp liver mitochondria by treatment with methyl ethyl ketone (MEK) or Triton X-100 and then perchlorate greatly reduced MAO activities. Treatment with only Triton X-100 resulted in less reduction in activity. The Km values of the remaining activities were similar regardless of these treatments. The sensitivities towards clorgyline and l-deprenyl of the remaining activity in the Triton X-100-treated residue and the phospholipase C-treated carp brain mitochondria were found to be unchanged, but those of the activity remaining in the MEK-treated residue were similarly decreased. No evidence was obtained suggesting conversion of carp MAO to either MAO-A or MAO-B by the modification of the mitochondrial lipid environment by the treatments employed.

Chronic antidepressants and GABA "B" receptors: a GABA hypothesis of antidepressant drug action

Amitryptyline (10 mg/kg), desipramine (5 mg/kg), citalopram (10 mg/kg) and viloxazine (10 mg/kg) were administered to rats either acutely (decapitation 1 hr after i.p. injection) or subacutely (by subcutaneous minipump implantation for 18 days followed by decapitation 24 hr after removal). After acute administration there was not any consistent alteration in GABA levels, GAD activity, 3H GABA "A" or 3H-GABA "B" receptor binding or 3H-nipecotic acid binding to the recognition site for GABA uptake in the frontal cortex or hippocampus. Upon subacute antidepressant drug infusion, GABA levels, GAD activity and 3H-GABA-"A" binding showed only scattered differences in drug treated animals as compared to saline treated rats. However, 3H-GABA "B" binding in the frontal cortex was consistently elevated after all drug treatments (in % of control: amitryptyline = 155%; desipramine = 151%; citalopram = 173%; viloxazine = 189%). Scatchard analysis showed that this was due to a Bmax increase without an effect in Kd. These findings were reproduced by subacute administration of pargyline, a MAO inhibitor. These data suggest that GABA "B" receptors may be involved in the mechanism of action of antidepressant drugs and provide a link between GABAergic and monoaminergic hypotheses of depression.

Age-related changes in benzylamine oxidase activity in rat tissues

Brain, liver, heart, lung, kidney and duodenum benzylamine oxidase (BZAO) activities were measured from young and old rats. Protein content was found to decrease in liver (-17%), kidney (-20%) and duodenum (-17%) but remained unchanged in brain, heart and lung of old rats compared with that of young rats. A significant decrease (-41%) of BZAO activity was found in lung whereas a significant increase of enzyme activity was found in brain (+49%) and kidney (+25%) and no change was found in heart and duodenum of old rats. BZAO was not detected in either young or old rat liver. Kinetic analysis for lung BZAO activity of old rats showed that Vmax was decreased but Km was unchanged in comparison with that of young rats. Since, as we have shown previously, MAO-A and -B activity in lung of old rats was also found to be decreased, the decrease of lung BZAO activity with increasing age merits further investigation, lung playing an important role in removing amines from the circulation.

Monoamine oxidase activities of dissociated cell fractions from rat ventricular muscle

Cell fractions enriched in cardiac muscle cells (myocytes), on the one hand, and in non-myocytes, on the other, were prepared by dissociation of rat ventricular tissue with collagenase. Amine oxidase activities in homogenates of these cell fractions and also in homogenates of the corresponding undissociated ventricular tissue were compared. In addition, the activity of alkaline phosphatase (AP), an enzyme found predominantly associated in the heart with non-myocytes, particularly capillary endothelial cells, was also measured. No significant difference in the activity of MAO-A (assayed with 1 mM 5-hydroxytryptamine) was found between myocyte and non-myocyte fractions. In contrast, the activities of alkaline phosphatase (AP) and also the semicarbazide-sensitive amine oxidase (SSAO), assayed with 1 microM benzylamine (BZ), were both significantly higher in non-myocytes, by several-fold, than in myocyte fractions. Studies of the inhibition by clorgyline of 1 mM BZ metabolism confirmed that both MAO-A and MAO-B can also contribute to BZ oxidation in the rat heart. These experiments indicated different ratios of MAO-A: MAO-B in the various cell fractions. The ratios of the percentage contributions of MAO-A and MAO-B, respectively, to the total metabolism of 1 mM BZ were 78:20 (myocytes), 43:52 (non-myocytes) and 57:32 (undissociated tissue). These results suggest that MAO-B, in addition to AP and SSAO, may be associated preferentially with non-myocyte constituents of the rat heart. Although cardiac myocytes appear to contain predominantly MAO-A, this enzyme form is also localized, with high activity, to the non-myocyte fraction. However, since the non-myocyte fraction is heterogeneous in its cell content, containing vascular components of the coronary microcirculation, as well as other cells of connective tissue origin, the exact cellular localization of the enzyme activities within this fraction has not yet been defined.

Developmental changes in the activity and substrate specificities of mouse brain monoamine oxidase

Developmental changes in monoamine oxidase (MAO) activity in the mouse brain were investigated with the substrates beta-phenylethylamine (PEA), tryptamine, and 5-hydroxytryptamine (5-HT). In the newborn brain, MAO activity towards PEA was found to be much lower than the adult and to be inhibited by clorgyline in a double-sigmoidal fashion. The inhibition curve shifted to a single-sigmoidal pattern with age. MAO activity towards 5-HT as substrate was inhibited by 90% and in a single-sigmoidal manner by clorgyline throughout the postnatal life. Lineweaver-Burk plots with PEA as substrate presented two linear lines (apparent Km: 28.6 and 4.1 microM) for the newborn and one line (apparent Km: 11.4 microM) for the adult, respectively. The plot with high Km value for the newborn brain disappeared in a clorgyline-treated preparation. These findings suggest that age-dependent alterations in the ratio of MAO-A/MAO-B activity affect the substrate specificity of the enzyme.

Monoamine oxidase and semicarbazide-sensitive amine oxidase in spontaneously hypertensive and in normotensive control rats

The aim of the present work was to compare monoamine oxidase (MAO) and semicarbazide sensitive amine oxidase (SSAO) activity in several tissues from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Contribution of MAO-A, -B and SSAO to the metabolism of each substrate in each tissue was defined from experiments where the decrease of oxidative deamination of each substrate at a given concentration was measured as a function of increasing concentrations of a selective MAO-A, -B or SSAO inhibitor. In the heart, aorta and, to a lesser extent, the femoral arteries MAO-A activity was higher in SHR than in WKY. Similarly in the liver the enzyme activity was higher in SHR than in WKY but was due to the -B form of MAO. In all the other tissues studied (duodenum, brain, lungs, adrenals and kidneys) no difference in MAO-A, MAO-B or SSAO activity was found between SHR and WKY, except for the kidneys and brain, if the differences in the weights of these organs in SHR are taken into account.

On the substrate specificities of the two forms of monoamine oxidase

By use of the selective irreversible inhibitors clorgyline and selegiline [(-)-deprenyl], it is possible to determine the Km and Vmax values of the two forms of monoamine oxidase towards a variety of substrates. The validity of this procedure is demonstrated for the deamination of dopamine by rat liver monoamine oxidase -A and -B, and the specificities of the two forms are analysed in terms of their kinetic parameters towards different substrates.

Effect of neonatal hypothyroidism on the serotonin system of the rat brain

The effects of neonatal thyroidectomy and thyroid hormone replacement therapy on the development of serotonin-containing neurons in discrete rat brain nuclei were studied. Newborn male rats were rendered hypothyroid by the injection of 125 mu Ci 131I, and, after 45 days, were compared with normal littermate controls and 131I-injected animals subsequently maintained by daily T4 injections. The serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents of discrete brain nuclei removed by punches of frozen brain slices were measured by HPLC with electrochemical detection. 5-HT and 5-HIAA contents were significantly increased in many nuclei of the hypothyroid rat brain. By blocking the biosynthesis of 5-HT with p-chlorophenylalanine we found that the activity of tryptophan hydroxylase is an important step in the stimulatory effect of hypothyroidism on the 5-HT and 5-HIAA contents. Furthermore, we demonstrated after blockage of monoamine oxidase activity with pargyline, a less pronounced decline of 5-HIAA in neonatal hypothyroid animals, thus causing a relative accumulation of this metabolite. These results demonstrate that there are important modifications of the 5-HT system in the brain of neonatal hypothyroid rats. This may have an important role in the development of hypothyroid-induced impairments of central nervous system function.

Serotonin binding in mouse brains. Some methodological aspects

The binding of (3H)5-hydroxytryptamine ([3H]5-HT, serotonin) to crude homogenates of brains from three different strains of mice has been studied. The strains, C57/BL, DBA and BALB did not show significant differences in the binding characteristics, with Kd values around 6-7 nM and Bmax 270-310 fmoles/mg protein. Various methodological aspects were investigated and found to be important for the binding assay. The presence of ascorbic acid (5.7 mM) thus caused a significant increase in Bmax by 30% without any change in the Kd values. This increase seemed to be due to a decrease in the non-specific binding rather than to an increase in the total binding of serotonin. The presence of a specific MAO-A inhibitor, clorgyline (10 microM), during the assay, resulted in a significant reduction of Bmax by 20% without any change in the affinity for serotonin, when tissue which had been frozen was used. Less than 2% of added serotonin was metabolized during the binding procedure in the absence of clorgyline. Thus, the decrease in binding capacity caused by clorgyline, may be due to a non-competitive blocking effect of the serotonin binding structure. These results indicate that the use of MAO inhibitors in (3H)5-HT binding assays on frozen tissue, rather than being necessary, might negatively affect the result. Neither storage of the brains at -70 degrees nor postmortem storage of the animals for 60 hours at 4 degrees resulted in obvious changes in the (3H)5-HT binding characteristics.

Monoamine oxidase in pancreatic islets, exocrine pancreas, and liver from rats. Characterization with clorgyline, deprenyl, pargyline, tranylcypromine, and amezinium

Monoamine oxidase (MAO) was characterized in tissue homogenates from pancreatic islets, exocrine pancreas, and liver from rats. Phenylethylamine was preferentially deaminated by pancreatic islet MAO while 5-hydroxytryptamine was preferentially deaminated by MAO from exocrine pancreas, and tyramine was a good substrate for both tissues. All three substrates were well deaminated by liver tissue. Clorgyline, a selective inhibitor of MAO-A, preferentially inhibited deamination of 5-hydroxytryptamine by all three tissue homogenates, while deprenyl, a selective inhibitor of MAO-B, preferentially inhibited deamination of phenylethylamine. In the case of pargyline, a less selective MAO-B inhibitor, the preference in favour of phenylethylamine was less pronounced. According to these results, MAO in pancreatic islets can be classified as predominantly type B enzyme species and MAO in exocrine pancreas as predominantly type A enzyme species while both types of the enzyme are present in the liver. Using the same three MAO substrates and compared with the effects of the selective enzyme inhibitors, clorgyline and deprenyl, tranylcypromine can be classified as a potent nonselective inhibitor of MAO in homogenates of all three tissues investigated with a slight preference in favour of the inhibition of the B-form of the enzyme, while in contrast amezinium can be classified as a weak nonselective inhibitor of MAO with a slight preference in favour of the inhibition of the A-form of the enzyme. All MAO inhibitors tested also inhibited insulin secretion by isolated incubated rat pancreatic islets, however only at IC50 which were two to three decimal powers higher than those necessary for the inhibition of the MAO activity, thus indicating that inhibition of MAO activity and inhibition of insulin secretion are apparently not closely related.

Inhibitory actions of hydralazine upon monoamine oxidizing enzymes in the rat

The inhibition by hydralazine of the clorgyline-resistant amine oxidase (CRAO) and monoamine oxidase (MAO) activities in various rat tissues has been studied. Hydralazine was a potent, time-dependent inhibitor of rat heart CRAO activity in vitro. The inhibition was not reversed by dialysis for 18 hr at 4 degrees, and only partially reversed by dialysis at 37 degrees. Dialysis at 4 degrees in the presence of pyridoxal phosphate (10(-4) M) also did not reverse the inhibition. Ex vivo inhibition of CRAO was found in heart and aorta homogenates in a dose-dependent manner after administration of hydralazine (1-40 mg/kg i.p.) to rats. In contrast, MAO-A activity was unaffected or, in some cases, significantly increased in these tissue homogenates from drug-treated animals. However, in vitro inhibition by hydralazine of both MAO-A and B activities of rat liver mitochondrial fractions was found, and these effects were fully reversible by dialysis for 18 hr at 4 degrees. Inhibition of MAO-A was competitive (Ki of 2.5 X 10(-6) M), while inhibition of MAO-B showed complex mixed non-competitive kinetics. These results indicate that hydralazine possesses different inhibitory properties towards the various amine oxidases in rat tissues, and these actions are discussed in relation to the clinical use of the drug as an anti-hypertensive agent.

The deamination of n-pentylamine by monoamine oxidase and a semicarbazide-sensitive amine oxidase of rat heart

n-Pentylamine is deaminated by homogenates of rat heart. Clorgyline inhibition curves at 10 and 100 microM n-pentylamine indicated that this substrate was deaminated by MAO-A, -B and a clorgyline-resistant amine oxidase sensitive to inhibition by semicarbazide. These results have been compared with two other commonly used monoamine substrates, beta-phenethylamine and benzylamine.

The effect of deprenyl (selegiline) on intra- and extraneuronal dopamine oxidation

It was found that in the rat striatum DA was oxidized extrasynaptosomally to 11% by MAO-A and to 3% by MAO-B. The corresponding intrasynaptosomal oxidations were 84% and 2%, respectively. Those figures were virtually unchanged even if the rat brain MAO-B was selectively inhibited to 87% by deprenyl. In the human brain extrasynaptosomal oxidation was 16% and 66%, respectively, by MAO-A and -B. Intrasynaptosomally the corresponding figures were 12% and 6%, respectively. Selective inhibition of human caudate MAO-B was calculated to give a total reduction of DA oxidation of 63%. The differences between man and rat are due to the proportionately greater oxidation of DA by MAO-B in man, which is a consequence of a higher ratio of concentration of MAO-A/-B in the rat.

N-[2-(o-iodophenoxy)ethyl]cyclopropylamine hydrochloride (LY121768), a potent and selective irreversible inhibitor of type A monoamine oxidase

The effects of N-[2-(o-iodophenoxy)ethyl]cyclopropylamine hydrochloride (LY121768) on types A and B monoamine oxidase (MAO) assayed with radiocarbon-labeled serotonin and phenylethylamine, respectively, were studied in vitro and in vivo. Type A MAO from rat brain was inhibited in vitro by LY121768 with an IC50 of 4 x 10(-10) M, whereas 2500 times higher concentrations of LY121768 (IC50 = 1 x 10(-6) M) were required to inhibit type B MAO. The inhibition of type A MAO increased with time of incubation of LY121768 with enzyme prior to substrate addition and persisted after dialysis, indicative of irreversible inhibition. The irreversible inactivation was prevented by harmaline, a reversible, competitive inhibitor of type A MAO, indicating a requirement for catalytic activity of MAO in the time-dependent inactivation by LY121768. In rats, LY121768 selectively inhibited type A MAO in brain and in liver at low doses. The inhibition of type A MAO persisted for several days after a single 10 mg/kg i.p. dose of LY121768 and was associated with a significant increase in brain dopamine, norepinephrine and epinephrine concentrations and a significant decrease in the concentration of the dopamine metabolites, homovanillic acid and 3,4-dihydroxyphenylacetic acid. The inactivation of type A MAO by LY121768 in vivo was prevented by co-administration of harmaline, indicating a similar mechanism for the in vivo inactivation as for the in vitro inactivation of MAO by LY121768. A reasonable inference from these data and from previous literature is that LY121768 acts as a "suicide substrate" for MAO and inactivates the enzyme by formation of a reactive intermediate which binds covalently to the enzyme. The presence of iodine in the LY121768 molecule not only confers high selectivity for type A MAO but offers a site for radionuclide introduction that might be a useful means of labeling type A MAO in vitro or in vivo for various purposes.

A new type of mitochondrial monoamine oxidase distinct from type-A and type-B

The characteristics of mitochondrial monoamine oxidase (MAO) in carp liver were studied with MAO inhibitors and substrates. This enzyme was thermolabile, but was stabilized in the presence of bovine serum albumin. With clorgyline and deprenyl, single-sigmoidal curves for inhibition of the activity towards tyramine or 5-hydroxytryptamine were obtained; the sensitivities to the two inhibitors were identical. The activity towards beta-phenylethylamine was not completely inhibited by clorgyline or deprenyl, but the remaining activity was inhibited by semicarbazide and the inhibition curves by either clorgyline or deprenyl and semicarbazide were also identical to the curves with the other two substrates. These results suggest that carp liver mitochondria contain "classical" MAO and a clorgyline- and deprenyl-resistant amine oxidase and that the classical MAO does not seem to be MAO-A or MAO-B, which are present in mitochondria of most mammalian tissues.

Effects of clorgyline and deprenil on corticosterone levels in rats

Adrenal and plasma corticosterone levels were measured in adult female rats after i.p. administration of specific MAO inhibitors: clorgyline (IMAO-A) and deprenil (IMAO-B). Brain and adrenal MAO activity was also determined. The IMAO effect was found to depend on 3 interrelated factors: (a) dose of drug, (b) time after injection, (c) specificity in MAO inhibition. One hour after injection, clorgyline (1 and 2.5 mg/kg) but not deprenil (20 mg/kg), appeared to inhibit the adrenocortical stress response to i.p. injection. At higher doses, MAO-B inhibition by deprenil (40 mg/kg) induced a moderate but sustained increase in corticosterone levels, while MAO-A inhibition by clorgyline (5, 10, 20 mg/kg) resulted in a large and sharp rise. This effect of clorgyline was potentiated with alpha-methyl-p-tyrosine and blocked with p-chlorophenylalanine. On this basis, the involvement of serotonergic mechanisms could be considered to explain the stimulatory effect of clorgyline on adrenal cortical function.

Platelet monoamine oxidase: specific activity and turnover number in headache

Monoamine oxidase turnover numbers (molecules of substrate converted to product per minute per active site) have been calculated for the human platelet enzyme using [3H]pargyline. Headache patients with high and low monoamine oxidase specific activities relative to controls were found to have turnover numbers very close to those for controls. This finding suggests that their specific activities vary because of differences in the concentration of active monoamine oxidase molecules, rather than differences in the ability of those enzyme molecules to catalyse the deamination reaction.

A simple method for screening monoamine oxidase (MAO) inhibitory drugs for type preference

A simple method is described for screening monoamine oxidase (MAO) inhibitors for preferential action on either type A or type B MAO. Kynuramine, a substrate for both types of MAO, was used and the sources of enzyme were rat heart (type A) and mouse heart (type B). The method clearly showed that clorgyline (a preferential type A inhibitor) preferentially inhibited rat heart MAO; whereas pargyline (a preferential type B inhibitor) preferentially inhibited mouse heart MAO. Pheniprazine, an inhibitor without type preference, was equally effective in the two tissues. The method was used to substantiate a report by others that amphetamine has preferential inhibitory action against type A MAO.

Deamination of 5-hydroxytryptamine by both forms of monoamine oxidase in the rat brain

Km and Vmax values of monoamine oxidase (MAO) A and B towards 5-hydroxytryptamine were determined for rat brain homogenates after the in vitro inhibition of one of the two forms by the selective inhibitors clorgyline and l-deprenyl. Km values of 178 and 1170 microM, and Vmax values of 0.73 and 0.09 nmol . mg protein-1 . min-1 towards 5-hydroxytryptamine were found for MAO-A and -B, respectively. The Ki for 5-hydroxytryptamine as a competitive inhibitor of beta-phenethylamine oxidation by MAO-B was found to be 1400 microM. The significance of these findings is discussed.