The characteristics and distribution of monoamine oxidase (MAO) activity in different tissues of the rainbow trout, Salmo gairdneri

Monoamine oxidase (MAO) activity was determined fluorometrically in brain, intestine, kidney and liver tissues of the rainbow trout, Salmo gairdneri. MAO activity was inhibited by various drugs in a concentration-related manner, with single sigmoid inhibition curves, the inhibitors of type A MAO, harmaline and clorgyline being more effective than deprenyl, an inhibitor of type B MAO. Intestine exhibited greatest MAO activity followed by liver and brain with kidney showing least activity. The Michaelis constants (Km) also showed variability between tissues. Inhibition of MAO by harmaline was non-competitive and dependent on the concentration of substrate present.     

Biochemistry and physiology of monoamine oxidase (MAO) activity in the ring dove (Streptopelia risoria). II. Distribution of MAO in different tissues

Monoamine oxidase (MAO) activity was measured fluorometrically in liver, kidney, intestine and brain of adult male and female ring doves. Liver MAO was inhibited in a concentration-related fashion by clorgyline and harmaline (MAO type A inhibitors) where a plateau in the inhibition curve occurred with about 15% activity remaining, and also by the type B inhibitor deprenyl, which produced a plateau when about 85% activity remained. Kidney, intestine and brain MAO were inhibited in a biphasic manner by harmaline. Results with inhibitors suggest that 85% of liver MAO, 86% of kidney MAO, 88% of intestine and 75% of brain MAO is type A. Using 10(-6) M harmaline to differentiate between MAO-A and MAO-B type activities, the apparent maximal velocities (Vmax) and Michaelis constants (Km) were determined in different tissues. Most activity occurred in the intestine, with proportionally lesser amounts of kidney, liver and brain. The majority of MAO present was in the A form. Except for kidney, Km of MAO-B was higher than that of MAO-A. Both MAO-A and -B activities were higher in the intestines of male birds, although sex differences in content and type of MAO activity were not observed in other tissues of the ring dove.     

Effect of R-(-)-Deprenyl and Harmaline on Dopamine- and Peroxynitrite-Induced Membrane Permeability Transition in Brain Mitochondria

The present study examined the effect of MAO inhibitors, deprenyl and harmaline, on the membrane permeability transition in brain mitochondria. Deprenyl, harmaline, and antioxidant enzymes (SOD and catalase) attenuated alteration of the swelling, membrane potential, cytochrome c release, and Ca²⁺ transport in mitochondria treated with dopamine. In contrast, deprenyl and harmaline did not reduce the peroxynitrite-induced change in membrane permeability. Deprenyl and harmaline inhibited the decrease in thioredoxin reductase activity and the thiol oxidation in mitochondria treated with dopamine but did not decrease the effect of peroxynitrite. Deprenyl and harmaline significantly decreased the formation of melanin from dopamine. The results suggest that deprenyl and harmaline may protect brain mitochondria against the toxic action of dopamine oxidation by the maintenance of thioredoxin reductase activity, inhibition of thiol oxidation, and inhibition of dopamine oxidation product formation. In contrast, MAO inhibitors may not defend brain mitochondria against damaging action of peroxynitrite.     

Monoamine oxidase activity in several tissues of the goldfish. Carassius auratus

1. Monoamine oxidase (MAO) was determined fluorometrically in male goldfish tissues, and the effects of specific inhibitors determined. 2. MAO activity in kidney greater than intestine greater than pancreas greater than brain approximately liver greater than testis. Apparent Michaelis constant (Km) was higher in the first three and lower in the last three tissues. 3. Specific MAO type A inhibitors (harmaline, clorgyline) were much more effective than a type B inhibitor (deprenyl) in reducing MAO activity. 4. Apparently goldfish tissues contain only a single type A-like MAO.     

Characteristics of the Inhibition of Rat Brain Monoamine Oxidase In Vitro by MD780515

Abstract: The inhibition of type A and B MAO in rat forebrain crude membrane preparation by MD780515. (3-{4-[(3-cyanophenyl)methoxy]phenyl)-5-(methoxymethyl)-2-oxazolidinone Centre de Recherche Delalande, France) has been investigated in vitro with 5-hydroxytryptamine and β-phenylethyl-amine as substrates. The inhibition of the high-affinity binding of [³H]harmaline, a specific marker of type A MAO, was also studied. In the experimental conditions used, MD780515 appeared to be a pure mixed MAO inhibitor (MAOI) of 5-HT deamination, both K_m , and V_max being altered [K₁ (Dixon) = K_i , (slope) = 2 nM; K_i (intercept) = 12 nM]. Phenylethylamine oxidation could be considered to be noncompetitively inhibited by MD780515 (K_i (slope) = 78 nM; K_i , (intercept) = 103 nM). Dixon and intercept replots were hyperbolic, suggesting that, at high concentrations, PEA could be deaminated by both forms of MAO. This hypothesis was confirmed by biphasic inhibition curves of 80 μM-PEA obtained when MD7805 15 , clorgyline, harmaline and deprenyl were used as MAOIs. MD780515 was a potent inhibitor (IC₅₀= 1–2 nM) of [³H]harmaline binding. Comparatively, clorgyline, 'cold' harmaline and Lilly 51641 inhibited ³H ligand binding, with IC₅₀ of 5, 7 and 40 nM respectively. In conclusion, MD780515 is a reversible, specific and potent type A MAOI.     

Possible mechanism of selective inhibition of rat liver mitochondrial monoamine oxidase by chlorgiline and deprenyl]

The inhibition of the deamination of serotonin (the main substrate of monoamine oxidase (MAO) type A) by chlorgiline and deprenyl and of beta-phenylethylamine (the main substrate of the B type MAO) by fragments of rat liver mitochondrial membrane as well as the influence of 4-ethylpyridine on this process were studied. It was shown that the MAO activity of the mitochondrial membrane fragments was highly sensitive to chlorgiline, when serotonin was used as substrate, whereas a high sensitivity toward deprenyl was observed with beta-phenylethylamine as substrate. 4-Ethylpyridine (5.10(-3) M), a competitive and reversible inhibitor of the MAO activity, inhibited deamination of serotonin and beta-phenylethylamine by 34 and 30%, respectively. In experiments with chlorgiline (the specific inhibitor of MAO type A) 4-ethylpyridine (5.10(-3) M) introduced into the samples after preincubation of mitochondria with increasing concentrations of chlorgiline (30 min, 23 degrees C) decreased the inhibition by chlorgiline of the deamination of beta-phenylethylamine, but sharply increased the inhibitory effect of chlorgiline on the oxidation of serotonin. In analogous experiments with deprenyl (the specific inhibitor of MAO type B) 4-ethylpyridine (5.10(-3) M) decreased the inhibitory effect of deprenyl not only on the deamination of serotonin (substrate of MAO A), but also on the oxidation of beta-phenylethylamine (the main substrate of MAO type B). The decrease in the inhibitory effect of deprenyl on the deamination of beta-phenylethylamine after the addition of 4-ethylpyridine may be intensified upon preincubation of deprenyl with mitochondria in the presence of 4-ethylpyridine. The data obtained demonstrate the difference in the type and mechanism of inhibition of the deamination of serotonin by chlorgiline as well as in the type and mechanism of oxidation of beta-phenylethylamine by deprenyl. The possible mechanism of selective blocking of MAO activity by chlorgiline and deprenyl was discussed in terms of our previous data on the existence in the active center of mitochondrial MAO of specific sites for substrate binding, differing in their structure-functional characteristics.     

[3H]HARMALINE AS A SPECIFIC LIGAND OF MAO A—I. PROPERTIES OF THE ACTIVE SITE OF MAO A FROM RAT AND BOVINE BRAINS

A high-affinity (K_d= 5.9 nM) specific binding site for [³H]harmaline was detected in membranes from rat and bovine brains. Studies of the regional and subcellular distributions of this binding indicated its close association with monoamine oxidase type A activity (MAO A) measured with [³H]serotonin ([³H]5-HT) as the substrate. Maximal binding capacity and MAO A activity were found in mitochondrial enriched fractions. Mitochondria of synaptosomal or extra-synaptosomal origin exhibited very similar properties with respect to [³H]harmaline binding characteristics and MAO A activity. Among psychoactive drugs, only monoamine oxidase inhibitors (MAO I) prevented the specific binding of [₃H]harmaline. Logit-log inhibition curves of binding by MAO I gave only one slope which was not significantly different from 1.0, suggesting the existence of only 1 category of specific sites for [³H]harmaline in the membrane preparations from rat and bovine brains. Consistent with the preferential inhibition of MAO A by harmaline, other MAO I of this class, i.e. clorgyline and Lilly 51641, were 10²-2 × 10³ times more efficient than deprenyl and pargyline, two inhibitors of MAO type B, in displacing [³H]harmaline from its specific binding site. K_i and IC₅₀ values for the inhibition of [³H]harmaline binding by MAO I and MAO substrates (tryptamine, 5-HT, norepinephrine) were almost identical with those characterizing their action on MAO A activity with [³H]5-HT as the substrate. In conclusion, the specific binding site for [³H]harmaline exhibited all the expected properties of the active site of MAO A. Like the technique of precipitation with a specific antibody, binding of [³H]harmaline should be of great help for studying the structural characteristics of the active site of MAO A and determining the number of MAO molecules in tissues under various physiological conditions.     

Enhanced selectivity of pargyline as an inhibitor of type B monoamine oxidase in harmaline-treated rats

Pargyline, a slightly selective inhibitor of type B monoamine oxidase (MAO), inhibited phenylethylamine oxidation by 88 ± 1% and 81 ± 1% in rat brain and liver, respectively, at 24 hrs after injection of a 30 mg/kg i.p. dose. Serotonin oxidation was inhibited to a lesser extent, 68 ± 4% and 68 ± 2%, respectively, in brain and liver. In rats treated with harmaline, a short-lasting reversible MAO inhibitor selective for type A MAO, the inhibition of phenylethylamine oxidation after pargyline injection still occurred but the inhibition of serotonin oxidation was prevented. These results illustrate that a selective MAO inhibitor can be used to enhance the selectivity of an irreversible inhibitor, presumably by occupying active sites on a certain form of MAO temporarily and thereby preventing its inactivation. In heart, inhibition of both phenylethylamine and serotonin oxidation by pargyline was prevented by harmaline; this finding supports other evidence that phenylethylamine is metabolized by type A MAO in rat heart.     

Different effects of monoamine oxidase inhibition on MPTP depletion of heart and brain catecholamines in mice

Pargyline, an inhibitor of monoamine oxidase type B (MAO-B), did not prevent the depletion of heart norepinephrine 24 hr after a single dose of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice. In mice killed 24 hr after the last of 4 daily doses of MPTP, the depletion of dopamine in the striatum and of norepinephrine in the frontal cortex was completely prevented by pargyline, but the depletion of heart norepinephrine was not prevented. These results with pargyline are the same as results obtained earlier with deprenyl, another selective inhibitor of MAO-B. The doses of pargyline and of deprenyl that were used resulted in almost complete inhibition of MAO-B activity (phenylethylamine as substrate) in brain, heart and liver of mice. Deprenyl did not inhibit MAO-A activity (serotonin as substrate) in brain, but pargyline caused some inhibition of MAO-A in brain. In heart and liver, serotonin was oxidized only at about 1/10 the rate of phenylethylamine oxidation, suggesting that MAO-B predominates in these tissues. Both pargyline and deprenyl caused some inhibition of serotonin deamination in heart and liver, suggesting that the oxidation may have been due partly to MAO-B. Experiments with selective MAO inhibitors in vitro showed that only about 20% of the oxidation of serotonin was occurring via MAO-B in heart and liver. The in vitro oxidation of MPTP by MAO in mouse brain, heart and liver was almost completely inhibited by pretreatment with either pargyline or deprenyl. Neither pargyline nor deprenyl had any significant effect on the concentrations of MPTP in brain or heart one-half hr after injection of MPTP into mice. The concentrations of the metabolite, MPP+ (1-methyl-4-phenyl-pyridinium), were markedly reduced in brain and in heart by pretreatment with either pargyline or deprenyl. The data suggest that MPP+ formation, which is necessary for the depletion of brain catecholamines after MPTP injection, may not be necessary for depletion of norepinephrine in heart. Since the oxidation of MPTP in vitro was inhibited more by pargyline or deprenyl pretreatment than was the appearance of MPP+ in vivo, the possibility exists that some MPP+ formation might occur by an enzyme other than MAO.     

Substrate Selectivity of Type A and Type B Monoamine Oxidase in Rat Brain

Abstract: Use of the irreversible inhibitors clorgyline and deprenyl showed that rat brain mitochondria contain type A and type B monoamine oxidase (MAO). Tyramine is a substrate for both types of MAO, whereas serotonin is a preferential substrate for type A MAO. In contrast to MAO in other tissues, type A MAO in brain tissue oxidizes β-phenylethylamine (PEA) at high concentrations (0.5 and 1.0 mM). The proportions of type A and type B MAO activities in the mitochondria estimated from the double-sigmoidal inhibition curves of tyramine oxidation were about 70:30 irrespective of the concentration of tyramine. With PEA as substrate, the ratios of type A to type B activities were found to increase from low values at low concentrations to about 1 at 0.5-1.0 mM-PEA, and even higher at further increased concentrations of PEA. At very low (0.01 mM) and high (10.0 mM) concentrations of PEA, single-sigmoidal curves were obtained; with the high PEA concentration the activity was highly sensitive to clorgyline, whereas with the low concentration it was highly sensitive to deprenyl. In deprenyl-pretreated mitochondrial preparations, all the remaining activity towards 0.5-1.0 mM-PEA was shown to be highly sensitive to clorgyline, demonstrating that this activity was indeed due to oxidation by type A MAO. The opposite result was obtained with deprenyl as inhibitor of clorgyline-pretreated preparations, demonstrating that PEA at this concentration was also oxidized by type B MAO in rat brain mitochondria. The K₃ values of type A and type B MAO for PEA were significantly different. On Lineweaver-Burk analysis, plots with PEA as substrate for type A MAO in a deprenyl-treated preparation were linear over a wide concentration range, whereas those for type B MAO in a clorgyline-treated preparation were not linear, but showed substrate inhibition at higher concentrations of the substrate. It is concluded from the present findings that the effect of the substrate concentration must be considered in studies on the characteristics of multiple forms of MAO in various organs and species.     

NONEXISTENCE OF A TYPE C MONOAMINE OXIDASE IN RAT BRAIN

Abstract— The possible existence of type C MAO, distinct from type A and type B, in circumventricular structures of rat brain was examined by histological studies on the inhibitory effects of clorgyline. a preferential type A MAO inhibitor and deprenyl, a preferential type B inhibitor, on enzyme. Brain slices were preincubated with the inhibitors and then incubated with 5-HT, the substrate for type A MAO, and stained for MAO activity. Deposits of the product formazan were detected in circumventricular structures of slices of brain preincubated with clorgyline and deprenyl at concentrations of 10^-7–10^-4m at room temperature for 5 min. When the slices were preincubated with either of these inhibitors at room temperature for 60 min, strong activity was observed in this region, whereas when they were preincubated with either 10^-5m -clorgyline or 10^-5m -deprenyl for 20 and 30 min at 37°C, no MAO activity was seen in any region of the brain. Thus, at the higher preincubation temperature, lower concentrations of each inhibitor and a shorter preincubation period were required for inhibition of the enzyme. Preincubation for 60 min at 37°C with a combination of 10^-7m -clorgyline and 10^-8m -deprenyl did not inhibit the enzyme in the circumventricular region completely, but at the same temperature, concentrations of 10^-7m of both inhibitors inhibited the enzyme completely in 10min, Thus the effects of the inhibitors are synergistic. These results indicate that the inhibitory effects of the two inhibitors on the enzyme in circumventricular structures of the brain is time- and temperature-dependent. Moreover, the activity seems to be sensitive to deprenyl even when 5-HT is used as substrate. The results do not support the idea of the existence of type C MAO, distinct from type A and type B MAO.     

Alteration of dopamine synthesis in rat striatum subsequent to selective type A monoamine oxidase inhibition

Abstract: Pretreatment of rat striatal slices with the selective type A monoamine oxidase (MAO) inhibitor clorgyline was found to produce significant inhibition of dopamine (DA) synthesis. DA synthesis was reduced by nearly 50%, but not until more than 90% of the type A enzyme was inhibited. In contrast, complete inhibition of the type B MAO following deprenyl treatment had no effect. It is suggested that interneuronal accumulation of DA following inhibition of type A MAO leads to feedback inhibition at the rate-limiting step in DA biosynthesis, tyrosine hydroxylation. These results are also consistent with the presence of a type A MAO within DA-containing neurons and provide evidence of a regulatory role for type A MAO in the synthesis of brain DA.     

Specificity of endogenous substrates for types A and B monoamine oxidase in rat striatum

Abstract: Studies were designed to evaluate specificity of the transmitter amines serotonin (5-hydroxytryptamine, 5-HT) and dopamine (DA), as well as the trace amines p -tyramine ( p -TA) and β -phenylethylamine (PEA) for types A and B monoamine oxidase (MAO) in rat striatum. 5-HT was found to be a specific substrate for the type A enzyme. However, the specificity of PEA for the type B enzyme was found to be concentration-dependent. When low concentrations of PEA and 5-HT were used to measure type B and type A activities, respectively, both clorgyline and deprenyl were highly selective for the sensitive form of MAO in vivo. However, as the concentration of PEA was increased, the type B inhibitor deprenyl became less effective in preventing deamination of PEA. Conversely, the type A inhibitor clorgyline became more effective in this regard. Kinetic analysis following selective in vivo inhibition showed PEA deamination by both forms of MAO with a 13-fold greater affinity for the type B enzyme. In vivo dose-response curves obtained with the common substrates DA and p -TA showed approximately 20% deamination by the B enzyme. Kinetic values for DA and p -TA deamination in in vivo -treated tissue possessing only type A or type B MAO activity, revealed a 2.5-fold greater affinity for the type A enzyme. These studies show the importance of concentration on substrate specificity in striatal tissue. The results obtained characterize the common substrate properties of DA and p -TA as well as of PEA in rat striatum. In addition, the presence of regional specificity for 5-HT deamination by only type A MAO is demonstrated.     

Inversion of selectivity of N-substituted propargylamine monoamine oxidase inhibitors following structural modifications to quaternary salts

A number of N-substituted-propargylamines are well known mechanism-based MAO inhibitors. Clorgyline and deprenyl in fact represent archetypal MAO-A and MAO-B inhibitors respectively. In the present study several ring-substituted deprenyl structural analogues were synthesized and alterations of selectivity and potency towards MAO-A and MAO-B activities were found. When deprenyl and its structural analogues were further modified to their corresponding quaternary ammonium salts, i.e. by attaching either an extra propargyl or a methyl group to the nitrogen atom, the potency of inhibition of MAO-B activity was drastically reduced and inhibition of MAO-A activity substantially increased. Such a complete inversion of selectivity may be related to a hydrophilic and electrophilic region seemingly present only in the MAO-A but not in the MAO-B molecule. The results also suggest that at least three sites are required for the selectivity and mechanism-based action of an inhibitor towards MAO.     

Mechanism of inhibition by chlorgyline and deprenyl of tyramine deamination by mitochondrial monoamine oxidase of rat liver

The inhibition by chlorgyline and deprenyl of deamination of tyramine, i. e. substrate of two forms of monoamine oxidase (MAO) A and B, by fragments of rat liver mitochondrial membrane and the effects of competitive reversible inhibitors of the MAO activity, e. g. 4-ethylpyridine, benzyl alcohol, O-benzyl-hydroxylamine and 2-oxyquinoline, on this process were studied. It was shown that all the inhibitors used sharply increase the inhibiting effect of chlorgyline on tyramine deamination, the degree of the stimulating effect being the same irrespective of whether the inhibitors are added to the samples before or after a 30-min preincubation of chlorgyline with the enzyme at 23 degrees, i. e. after the onset of irreversible inhibition. The stimulating effect is due to the independent action of two inhibitors on the two different sites of the MAO active center: chlorgyline--on the isoalloxazine ring of FAD, that of 4-ethylpyridine, benzyl alcohol, O-benzylhydroxylamine, 2-oxyquinoline, respectively, on the hydrophobic region involved in tyramine binding. In similar experiments with deprenyl all the competitive inhibitors used, when added to the samples after a 30-min incubation of the inhibitor with the enzyme at 23 degrees, remove the inhibiting effect of deprenyl on tyramine deamination. The decrease of the inhibiting effect of deprenyl is indicative of an existence of competitive interactions between deprenyl and the above-mentioned compounds and of the reversible inhibition by deprenyl of tyramine deamination under the given experimental conditions. The data obtained revealed the differences in the type and mechanism of action of chlorgyline and deprenyl on tyramine deamination and showed that these inhibitors act on different sites of the MAO active center, responsible for tyramine oxidation. Chlorgyline blocks primarily the "flavin moiety" of the MAO molecule, essential for the catalytic act, while the effect of deprenyl is directed to the hydrophobic part of the enzyme active center essential for the enzyme binding to tyramine. In this case the irreversible inhibiting effect is achieved at a slower rate and the reversibility of tyramine oxidation by deprenyl is maintained for a longer period of time than the chlorgyline inhibition of deamination of this amine.     

Monoamine oxidase in amphistomes and its role in worm motility

The quantitative assay of mitochondrial monoamine oxidase (MAO) activity revealed a higher enzyme level in Explanatum explanatum than Gastrothylax crumenifer. The specific MAO inhibitors, chlorgyline, pargyline, deprenyl and nialamide produced different degrees of interspecific inhibition. The differential effects on enzyme activity of chlorgyline and deprenyl suggests the possible existence of polymorphic forms of the enzyme, MAO-A and MAO-B, in amphistomes. These specific inhibitors also had a differential influence on the in vitro motility of amphistomes, further indicating the involvement of different forms of MAO in the oxidative deamination of biogenic monoamines which might be partly responsible for neuromuscular coordination in amphistomes. The experimental procedures used in this study could be conveniently used for quick screening and evaluation of some of the qualitative effects of anthelmintic drugs under in vitro conditions.     

Modulation of brain mitochondrial function by deprenyl

The present study shows that deprenyl, a known inhibitor of monoamine oxidase B (MAO B), may generate changes in mitochondrial function. Brain submitochondrial membranes (SMP), synaptosomes and cytosolic fractions were incubated with different deprenyl concentrations and nitric oxide synthase (NOS) activity was measured. The effect of deprenyl on oxygen consumption, calcium-induced permeability transition and hydrogen peroxide (H(2)O(2)) production rates was studied in intact mitochondria. Respiratory complexes and monoamine oxidase activities were also measured in submitochondrial membranes. Incubation of brain submitochondrial membranes with deprenyl 10, 25 and 50 microM inhibited nitric oxide synthase activity in a concentration-dependent manner. The same effect was observed in cytosolic fractions and synaptosomes. Monoamine oxidase activity was inhibited at lower deprenyl concentrations (from 0.5 microM). Cytochrome oxidase (complex IV) activity was found 42% increased in the presence of 25 microM deprenyl in a condition of maximal nitric oxide synthase activity. Incubation of brain mitochondria with deprenyl 25 microM produced a 60% increase in oxygen uptake in state 3, but no significant changes were observed in state 4. Pre-incubation of brain mitochondria with deprenyl 0.5 and 1 microM inhibited calcium-induced mitochondrial permeability transition and decreased hydrogen peroxide production rates. Our results suggest that in vitro effects of deprenyl on mitochondrial function can occur through two different mechanisms, involving nitric oxide synthase inhibition and decreased hydrogen peroxide production.     

Phenylethanolamine is a specific substrate for type B monoamine oxidase

The characteristics of phenylethanolamine as both a competitive inhibitor and as a substrate for monoamine oxidase (MAO) were studied using rat brain and liver homogenates. Although phenylethanolamine, even at high concentrations (1 mM), produced minimal inhibition of MAO when serotonin (a substrate for type A MAO) was used as the substrate, it was a potent competitive inhibitor (K_i=11 μM) of the deamination of phenylethylamine (a substrate for type B MAO). When phenylethanolamine was used as a substrate, deprenyl, a selective inhibitor of type B MAO, was found to produce a single sigmoid inhibition curve at low concentrations of the inhibitor (pI₅₀=7.5). These results indicate that phenylethanolamine is a specific substrate for type B MAO. Identification of the products formed under the assay conditions show that phenylethanolamine is converted to both mandelic acid and phenylethylene glycol by liver homogenates but only to the latter, neutral metabolite by brain homogenates.     

Guinea Pig Striatum as a Model of Human Dopamine Deamination: The Role of Monoamine Oxidase Isozyme Ratio, Localization, and Affinity for Substrate in Synaptic Dopamine Metabolism

The kinetic properties of type A and type B monoamine oxidase (MAO) were examined in guinea pig striatum, rat striatum, and autopsied human caudate nucleus using 3,4-dihydroxyphenylethylamine (dopamine, DA) as the substrate. MAO isozyme ratio in guinea pig striatum (28% type A/72% type B) was similar to that in human caudate nucleus (25% type A/75% type B) but different from that in rat striatum (76% type A/24% type B). Additional similarities between guinea pig striatum and human caudate nucleus were demonstrated for the affinity constants (Km) of each MAO) isozyme toward DA. Endogenous concentrations of DA, 3-methoxytyramine, 3,4-dihydroxyphenylacetic acid, and homovanillic acid were also measured in guinea pig and rat striatum following selective type A (clorgyline-treated) and type B (deprenyl-treated) MAO inhibition. In guinea pig, DA metabolism was equally but only partially affected by clorgyline or deprenyl alone. Combined treatment with clorgyline and deprenyl was required for maximal alterations in DA metabolism. By contrast, DA metabolism in rat striatum was extensively altered by clorgyline but unaffected by deprenyl alone. Finally, the deamination of DA in synaptosomes from guinea pig striatum was examined following selective MAO isozyme inhibition. Neither clorgyline nor deprenyl alone reduced synaptosomal DA deamination. However, clorgyline and deprenyl together reduced DA deamination by 94%. These results suggest that the isozyme localization and/or isozyme affinity for DA, rather than the absolute isozyme content, determines the relative importance of type A and type B MAO in synaptic DA deamination. Moreover, based on the enzyme kinetic properties of each MAO isozyme, guinea pig striatum may serve as a suitable model of human DA deamination.     

A new type of mitochondrial monamine 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 β-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.