Substrate and inhibitor-related characteristics of monoamine oxidase in C6 rat glial cells

Expression of A and B types of monoamine oxidase in neuroblastoma hybrid cells

Monoamine oxidase (MAO) activity towards kynuramine as substrate was measured in 6 hybrid cells derived by fusion of neuroblastoma and glioma, liver or brain cells, and was compared with that of parental or non-parental clones. Activities varied from the lowest level of less than 0.15 pmol/min/mg protein in a neuroblastoma clone NB2A to the highest level of 127 pmol/min/mg protein in NCB20 mouse neuroblastoma x Chinese hamster embryo brain hybrid cells. The relative proportions of A and B types of MAO activities were determined in homogenates of each cell line by inhibition curves with clorgyline and deprenyl. Although the A type activity was found in all cell lines measured, MAO A was predominant in 9 clones, except for NCB20 hybrid cells, N4G-B-a neuroblastoma x glioma hybrid cells, and G8-1 myoblast. The ratio of type A/type B activity in NCB20, N4G-B-a and G8-1 cells was 20/80, 75/25 and 95/5, respectively. The results suggest that NCB20 cells are highly enriched in MAO type B, and that the NCB20 cell is an excellent model for studying the type B activity found in the brain in vivo.

Molecular mechanism of the relation of monoamine oxidase B and its inhibitors to Parkinson's disease: possible implications of glial cells

Monoamine oxidases A and B (MAO A and MAO B) are the major enzymes that catalyze the oxidative deamination of monoamine neurotaransmitters such as dopamine (DA), noradrenaline, and serotonin in the central and peripheral nervous systems. MAO B is mainly localized in glial cells. MAO B also oxidizes the xenobiotic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to a parkinsonism-producing neurotoxin, 1-methyl-4-phenyl-pyridinium (MPP+). MAO B may be closely related to the pathogenesis of Parkinson's disease (PD), in which neuromelanin-containing DA neurons in the substantia nigra projecting to the striatum in the brain selectively degenerate. MAO B degrades the neurotransmitter DA that is deficient in the nigro-striatal region in PD, and forms H2O2 and toxic aldehyde metabolites of DA. H2O2 produces highly toxic reactive oxygen species (ROS) by Fenton reaction that is catalyzed by iron and neuromelanin. MAO B inhibitors such as L-(-)-deprenyl (selegiline) and rasagiline are effective for the treatment of PD. Concerning the mechanism of the clinical efficacy of MAO B inhibitors in PD, the inhibition of DA degradation (a symptomatic effect) and also the prevention of the formation of neurotoxic DA metabolites, i.e., ROS and dopamine derived aldehydes have been speculated. As another mechanism of clinical efficacy, MAO B inhibitors such as selegiline are speculated to have neuroprotective effects to prevent progress of PD. The possible mechanism of neuroprotection of MAO B inhibitors may be related not only to MAO B inhibition but also to induction and activation of multiple factors for anti-oxidative stress and anti-apoptosis: i.e., catalase, superoxide dismutase 1 and 2, thioredoxin, Bcl-2, the cellular poly(ADP-ribosyl)ation, and binding to glyceraldehydes-3-phosphate dehydrogenase (GAPDH). Furthermore, it should be noted that selegiline increases production of neurotrophins such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrphic factor (GDNF), possibly from glial cells, to protect neurons from inflammatory process.

Differences in the disposition and toxicity of 1-methyl-4-phenylpyridinium in cultured rat and mouse astrocytes

Species difference in the susceptibility to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was investigated in cultured rat and mouse astrocytes, where 1-methyl-4-phenylpyridinium (MPP+), the toxic mediator of MPTP, is formed. Type A monoamine oxidase (MAO) predominated in both rat and mouse astrocytes, while its activity was slightly higher in mouse cells compared to rat cells; MAO-B activity, on the other hand, was significantly lower in mouse astrocytes than in rat astrocytes. Because both types of MAO have been reported to make similar contributions to MPP+ production in astrocytes, their total activity was examined and results indicated that there was no significant difference between these two species. In addition, MPP+ caused a dose-dependent loss of cell viability as judged by the amount of lactate dehydrogenase released into the incubation medium. The toxicity of MPP+ on astrocytes started to be seen after a 2 day incubation period. Mouse astrocytes were more vulnerable to MPP+ than rat astrocytes. The threshold values for MPP+ toxicity in mouse and rat cultures were 10 microM and 70 microM, respectively. After addition of [3H] MPP+ to the medium, intracellular [3H] MPP+ was found to increase in both cultures. Mouse astrocytes accumulated more MPP+ than rat astrocytes (150 pmol/mg protein vs. 65 pmol/mg protein). When astrocytes were allowed to accumulate [3H] MPP+ and then incubated in fresh medium not containing [3H] MPP+, intracellular levels of [3H] MPP+ in both cells rapidly declined (110 pmol/protein in mouse vs. 40 pmol/mg protein in rat of MPP+ been released).(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of monoamine oxidase activities in synaptic and non-synaptic mitochondria derived from three brain regions: some functional implications

The heterogeneity of monoamine oxidase (MAO; EC 1.4.3.4) activities was studied in two fractions of synaptic mitochondria (SM & SM2) and one fraction of non-synaptic ("free") mitochondria (M) isolated from three rat brain regions (cerebral cortex, striatum, and pons & medulla) by the Lai and Clark (1979, 1989) method in order to elucidate the heterogeneity of MAO at the subcellular and brain regional levels. The activities toward serotonin (MAO-A), benzylamine (MAO-B), and dopamine (MAO-DA) in SM2 from all three regions were different from the corresponding values in SM. In addition, the various MAO activities in SM and SM2 showed heterogeneous distribution with respect to the three brain regions investigated. Both the distribution of MAO-A and MAO-B in non-synaptic mitochondria (M) did not show marked regional differences although MAO-DA in M varied depending on the region. These results clearly demonstrate that the distribution of MAO activities toward different substrates is heterogeneous both at the subcellular and the brain regional levels. The MAO-A:MAO-B ratios in the various mitochondrial fractions also showed trends that are consistent with this hypothesis. Furthermore, in fraction SM of synaptic mitochondria, this ratio was consistently higher than values in the other two mitochondrial fractions (SM2 & M) irrespective of the region from which they were isolated. In view of the functional importance of MAO in the regulation and compartmentation of amine metabolism, the heterogeneity of MAO at subcellular and regional levels may assume pathophysiological importance in neurological diseases (e.g., Parkinsonism) with which altered amine metabolism is associated.

Neurochemical characterization of the alterations in the noradrenergic afferents to the cerebellum of adult rats exposed to X-irradiation at birth

A single dose of x-irradiation was applied on the cephalic end of newborn rats, and the alterations in the noradrenergic afferents to the cerebellum were studied 180 days later. A net increase in the noradrenaline content of cerebellum was found (122% of nonirradiated controls). The response of noradrenaline content to reserpine injection (0.9 mg/kg, i.p.) was similar in exposed and control rats. Likewise, the 3H release induced by Ro 4-1284 from cerebellar cortex slices labeled with [3H]noradrenaline was unmodified by x-rays, although a mild increase in the spontaneous efflux of 3H was found. The retention of 3H by the slices was reduced in exposed animals (58% of controls). Both the in vitro activity of tyrosine hydroxylase and the accumulation of L-3,4-dihydroxyphenylalanine (L-DOPA) were not significantly different between x-treated rats and controls. In contrast, monoamine oxidase activity was markedly reduced in x-irradiated cerebellum (38% of controls). The x-ray-induced decrease in cerebellar weight (-60%) resulted in marked increases in noradrenaline concentration (223%), tyrosine hydroxylase activity per milligram of protein (206%), and 3H retention (50%). The accumulation of L-DOPA per gram of tissue was also increased at every time considered. These data indicate that x-irradiation at birth produces a cerebellar loss not completely shared by the noradrenergic afferents, and a permanent imbalance between the noradrenergic afferent input and its target cells might eventually result.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma amine oxidase activities in Norrie disease patients with an X-chromosomal deletion affecting monoamine oxidase

Two individuals with an X-chromosomal deletion were recently found to lack the genes encoding monoamine oxidase type A (MAO-A) and MAO-B. This abnormality was associated with almost total (90%) reductions in the oxidatively deaminated urinary metabolites of the MAO-A substrate, norepinephrine, and with marked (100-fold) increases in an MAO-B substrate, phenylethylamine, confirming systemic functional consequences of the genetic enzyme deficiency. However, urinary concentrations of the deaminated metabolites of dopamine and serotonin (5-HT) were essentially normal. To investigate other deaminating systems besides MAO-A and MAO-B that might produce these metabolites of dopamine and 5-HT, we examined plasma amine oxidase (AO) activity in these two patients and two additional patients with the same X-chromosomal deletion. Normal plasma AO activity was found in all four Norrie disease-deletion patients, in four patients with classic Norrie disease without a chromosomal deletion, and in family members of patients from both groups. Marked plasma amine metabolite abnormalities and essentially absent platelet MAO-B activity were found in all four Norrie disease-deletion patients, but in none of the other subjects in the two comparison groups. These results indicate that plasma AO is encoded by gene(s) independent of those for MAO-A and MAO-B, and raise the possibility that plasma AO, and perhaps the closely related tissue AO, benzylamine oxidase, as well as other atypical AOs or MAOs encoded independently from MAO-A and MAO-B may contribute to the oxidative deamination of dopamine and 5-HT in humans.

Isoelectric focusing of isoenzymes of monkey platelet monoamine oxidase

Monkey platelet monoamine oxidase (MAO) was preferentially found as the B-form of the enzyme as observed from differences in substrate specificities, as well as liver MAO. The isoelectric points and molecular weights of platelet MAO subunits were compared with those of monkey liver using sodium dodecyl sulfate-disc polyacrylamide gel electrophoresis and isoelectric focusing-disc gel electrophoresis. The pI value of monkey liver was a single peak at 6.5, but the pI values of monkey platelets were triple peaks at 5.5, 6.5 and 7.0. The molecular weight of MAO subunits in monkey platelets was similar to that of liver, and was found to be about 60,000. These results indicate that MAO-B of monkey platelets differs from MAO-B of the liver, and that it has different electrophoretic properties.

Biochemistry and genetics of monoamine oxidase

This chapter reviews the two mitochondrial flavin containing isozymes of monoamine oxidase. Section 1, "Biochemistry" discusses assays, substrates and inhibitors, phylogenic and tissue distribution, interactions with lipids, nutritional studies, protein structure, kinetic and chemical mechanistic proposals, and biosynthesis. Section 2, "Inheritance" discusses possible genes involved in expression, genetic studies of platelet MAO-B and fibroblast MAO-A, and chromosomal location. Section 3, "Molecular Genetics" reviews the cloning of their cDNAs, their intra- and interspecies homology and structural inferences made from deduced amino acid sequences. Section 4, "Regulation" gives an overview of levels in development and aging, and effect of drugs. The final section 5, "Role in Human Disease" discusses physiological function and effects of altered levels in humans and animal models including complete absence due to a submicroscopic chromosomal deletion in several human patients.

Histochemical localization of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine oxidation in the mouse brain

The sites in the mouse brain where 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine can be oxidized to the toxic metabolite 1-methyl-4-phenylpyridine were determined using a histochemical technique. The method involved the demonstration of monoamine oxidase activity using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine as the substrate by means of a sensitive coupled peroxidase technique. The distribution of neurons displaying the ability to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine via a monoamine oxidase catalysed reaction was compared to that of various amine systems identified with immunohistochemistry. Dopamine neurons, and in particular the nigrostriatal dopamine cells, did not display the capacity to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Noradrenergic neurons showed intense monoamine oxidase activity when 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was used as substrate, and this activity was blocked by the monoamine oxidase-A inhibitor clorgyline. Serotonin neurons and histamine neurons were also able to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The reaction in these neurons was blocked by deprenyl, an inhibitor of monoamine oxidase-B. Pretreatment with inhibitors of monoamine oxidase-B has been previously shown to prevent the neurotoxic action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on dopaminergic neurons. Therefore, since serotonin and histamine neurons are able to oxidize 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by means of monoamine oxidase-B, these neurons may be involved in the production of the toxic metabolites of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in vivo.

Evaluation of the toxicity of the dopaminergic neurotoxins MPTP and MPP+ in PC12 pheochromocytoma cells: binding and biological studies

This study was designed to investigate the toxicity of both MPTP and MPP+ using some simple cell systems, such as PC12 and C6 cultures, as models. Exposure of PC12 cells to 0.5 mM MPTP for 72 h resulted in a 50% cell loss with respect to the control cells, and clorgyline, a MAO-A inhibitor, antagonized this toxic effect. Higher concentrations of MPTP demonstrated only a weak cytostatic effect on C6 cells. Moreover, MPP+ showed a toxic effect which was 100 times more evident than MPTP toxicity in the PC12. We found a single, saturable class of [3H]MPP+ binding sites with a relatively high affinity both in PC12 and C6 cell lines. Moreover, the most susceptible cell line towards the toxic effects of both MPTP and MPP+, i.e. PC12, has the higher number of MPP+ binding sites. Our results suggest that MPTP can be toxic not only via MAO-B, but also via MAO-A activity and we propose PC12 as a model to study the intracellular mechanisms of MPTP and MPP+ toxicity.

Evidence for existence of A and B form monoamine oxidase in mitochondria from dog platelets

It is known that platelet MAO appears to behave more like the B-form enzyme than the A-form enzyme based on inhibitor sensitivity and substrate specificity. However, dog platelets showed a different substrate specificity such as high activity with 5-HT and beta-PEA as substrates. Moreover, dog platelet MAO was sensitive to the drugs clorgyline and harmaline with 5-HT as the substrate, while it was sensitive to the drug deprenyl with beta-PEA as the substrate. These results also indicate the existence of two forms of MAO in dog platelets unlike in other platelets such as those from humans. A-form MAO from dog platelets was more stable against heat treatment at 55 degrees C than A-form MAO from dog liver and brain. On the other hand, there was no difference in the heat resistance of the three enzymes with beta-PEA as the substrate. After digestion with trypsin at 37 degrees C for 30 min, it was found that MAO from dog platelets, brain and liver were mostly inhibited with 5-HT as the substrate. In contrast, MAO in brain and liver were inhibited about 10%, but platelet MAO was inhibited about 50% with beta-PEA as the substrate. From these results, it is considered that dog platelet MAO exists as the two forms of the enzyme and has different enzymic properties in comparison with those of MAO from dog liver and brain mitochondria.

[3H]5-hydroxytryptamine binding to brain astroglial cells: differences between intact and homogenized preparations and mature and immature cultures

[3H]5-hydroxytryptamine ([3H]serotonin) binds with high affinity (KD 2-12 nM) to a finite number of sites on brain astroglial cells. The number of binding sites in the C6 glioma line is decreased significantly (Bmax = 315 versus 30 fmol/mg) by homogenization. In intact primary cultures, derived from newborn rat brain, the number of binding sites is far greater in cultures of immature astrocytes than in cultures treated with dibutyryl cyclic AMP (Bmax = 1,520 versus 580 fmol/mg). A role for these receptors in development is suggested.

Assignment of A and B types of monoamine oxidase in NCB20 hybrid cells to those of the parental cells by peptide mapping

The structures of [3H]pargyline-labeled, flavin-containing polypeptides of monoamine oxidase (MAO) from hybrid NCB20 cells, and their parental cells, A/J mouse brain cells and Chinese hamster brain cells, were analyzed and compared by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and limited proteolysis and one-dimensional peptide mapping in SDS gels. After preincubation of mitochondrial preparations with deprenyl or clorgyline, the flavin-containing polypeptide of type A or type B MAO was selectively labeled with [3H]pargyline. SDS-PAGE of [3H]pargyline-labeled mitochondrial samples revealed that the polypeptide with apparent Mr of 62,000 was associated with type A activity in the three types of cells, and that the polypeptide with apparent Mr of 61,000 or 58,000 was associated with type B activity in Chinese hamster brain cells and NCB20 cells or A/J mouse brain cells, respectively. Chymotrypsin digestion of the [3H]pargyline-labeled polypeptides and the peptide mapping in SDS gels from A/J mouse and Chinese hamster brain cells produced identical map patterns between the two type A MAOs, almost the same map patterns (with the exception of one additional peptide fragment) between the two type B MAOs, and different map patterns between type A and type B MAOs. The results of identical treatments of the [3H]pargyline-labeled polypeptides of MAOs in NCB20 cells showed that type A and type B MAO in NCB20 cells were similar to type A MAO of A/J mouse and Chinese hamster brain cells and to type B MAO of Chinese hamster brain cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular localization of MAO A and B in brain: evidence from kainic acid lesions in striatum

The cellular localization of the two forms of monoamine oxidase (MAO A and MAO B) was studied by measuring their activities in rat striatum following unilateral stereotaxic injection of kainic acid to produce selective degeneration of striatal neurons and subsequent proliferation of astrocytes. The results demonstrated a persistent loss of 15-20% in MAO A activity, whereas MAO B activity decreased initially by 25% and then increased to more than twice the control value by 54 days after lesions. The changes in activity were compared to parallel estimates of the postsynaptic neuronal enzyme markers glutamic acid decarboxylase (GAD) and neuron-specific enolase (NSE), astroglial enzyme markers glutamine synthetase (GS) and non-neuronal enolase (NNE), and the presynaptic enzyme marker DOPA decarboxylase (DDC). The results suggest that a small amount of striatal MAO A is present in kainic acid-sensitive postsynaptic striatal neurons and that MAO B is probably localized in both neurons and astrocytes.

Cellular composition of a cerebral hemisphere primary culture

In this overview attention is given to available markers and methods for characterizing cell elements in a culture system. Primary cultures from newborn rat cerebral hemispheres were grown for 14 days. The population of cells was dominated by astrocytic glial cells (60-70%), but cells with properties of macrophages, endothelial-like cells, mesenchymal-like cells, ependymal-like cells, and oligoblasts were also found. Neither mature neurons nor oligodendroglial cells were observed. The enrichment in astroglial-like cells makes the cultures a satisfactory astroglial-cell model, at least for some purposes.

Specificity of antisera prepared against pure bovine MAO-B

Antisera have been prepared against purified bovine MAO-B that appear to react selectively with MAO-B and not MAO-A, Rabbit and mouse antisera indirectly immune precipitated [125I]bovine MAO-B using inactivated Staphylococcus aureus cells, and binding of antibodies to bovine and rat MAO-B did not inhibit enzyme activity. Two continuous rat cell lines, hepatoma line MH1C1 and glioma line C6, were used to elucidate the specificity of the antisera. MH1C1 cells, which express both MAO-A and MAO-B, showed immune-specific staining with rabbit antiserum, and staining was blocked with pure MAO-B. Further, MAO-B activity and [3H]pargyline-labeled MAO molecules could be immune precipitated from solubilized mitochondrial preparations of MH1C1 cells; and immune fixation of mitochondrial proteins following SDS polyacrylamide gel electrophoresis (SDS-PAGE) revealed staining of the MAO-B, but not of the MAO-A, flavin-containing subunit. In contrast, no immune-specific immunocytochemical staining was observed in C6 cells, which have only MAO-A activity; no MAO-A activity or [3H]pargyline-labeled MAO could be immune precipitated from solubilized mitochondrial preparations of these cells, and no stained bands were observed for mitochondrial proteins resolved by SDS-PAGE and processed for immune fixation. Further support for the selectivity of this antiserum for MAO-B comes from immunocytochemical staining of rat tissues which express varying amounts of MAO-A and MAO-B activities. Hypothalamus and liver, with high levels of MAO-A and MAO-B activities showed a large number of immunoreactive cells, whereas spleen, heart and superior cervical ganglia, with high MAO-A and low MAO-B activities showed only a few or no stained cells. Catecholamine neurons in the substantia nigra, thought to contain MAO-A, did not show immune-specific staining. Skeletal muscle cells with low MAO-A and MAO-B activities did not stain. These studies provide additional evidence that MAO-A and MAO-B are distinct molecules, differentially expressed in different cell types.

Localization of rat striatal monoamine oxidase activities towards dopamine, serotonin and kynuramine by gradient centrifugation and nigro-striatal lesions

Subfractionation of the crude synaptosomal-mitochondrial fraction of rat striatum in a continuous sucrose gradient in a zonal rotor led to the following results. The distribution pattern of monoamine oxidase (MAO) activity towards dopamine (DA) was very similar to the pattern of MAO activity towards serotonin (5HT), but differed from the pattern of MAO activity towards kynuramine (KYN). As 5HT is specifically deaminated by MAO-A while KYN is a common MAO substrate, this supports earlier suggestions that in rat striatal preparations DA is deaminated preferentially by MAO-A. The patterns of the MAO activities towards DA and 5HT were clearly dissimilar, despite considerable overlap, to the patterns of tyrosine hydroxylase (TH) and DOPA decarboxylase (DD) activity, both marking the presence of striatal dopaminergic synaptosomes. The peak activities were separated and all patterns were symmetrical without showing a shoulder. This indicates that rat striatal MAO activity towards DA and 5HT is not specifically or for the greater part localized in dopaminergic terminals. We also investigated the effects of electrolytic and 6-hydroxydopamine lesions of the substantia nigra, both causing extensive degeneration of striatal dopaminergic terminals as appeared from the large decrease of striatal TH and DD activity. However, neither type of lesion induced a reduction of the MAO activity towards any of the substrates used. It is concluded that the amount of MAO activity towards DA and 5HT (probably MAO-A activity) present in dopaminergic terminals is very low compared with the total activity of this enzyme in rat striatal tissue.

Differences in the structures of monoamine oxidases A and B in rat clonal cell lines

[3H]Pargyline-labeled polypeptides associated with the A and B types of monoamine oxidase (MAO) activity in two rat cell lines were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). [3H]Pargyline was bound to MAO A and B in a crude mitochondrial fraction from rat hepatoma cell line MH1C1 and to MAO A in a mitochondrial fraction from rat glioma line C6. Specific radiolabeling of proteins associated with type A or B activity in the hepatoma samples was achieved by incubation with selective B or A inhibitors, respectively, prior to [3H]pargyline binding. Following [3H]pargyline binding, the samples were solubilized by heating in sodium dodecyl sulfate (SDS) in the presence of a reducing agent. SDS-PAGE of [3H]pargyline bound samples revealed a radiolabeled protein band of apparent molecular weight (mol. wt) 63,000 daltons associated exclusively with MAO A activity and a band of apparent mol. wt 60,000 associated exclusively with MAO B activity. Furthermore, when SDS-solubilized, [3H]pargyline-labeled MAO A and B proteins from these cell lines were subjected to limited proteolysis and one-dimensional peptide mapping in SDS gels, different patterns of [3H]pargyline-labeled peptides were obtained. These findings indicate that the A and B forms of MAO activity are associated with enzyme molecules of different primary covalent structures determined by different gene loci.

MAO COMT, and GABA-T activities in primary astroglial cultures

Cultures from cerebral hemispheres of newborn rats contain the enzymes monoamine oxidase (MAO), catechol-O-methyltransferase (COMT), and gamma-aminobutyric acid alpha-ketoglutarate transaminase (GABA-T). The COMT activity was higher in the cultures than in adult rat cerebral hemispheres. The MAO activity was comparable in the cultures and in the rat cerebral hemispheres. The activities of both these enzymes increased with age in the cultures and in the rat brain hemispheres. In the culture the activities were further potentiated by removal of fetal calf serum and addition of 0.1 mM dibutyryl-cyclic AMP (dB-cAMP). GABA-T activity was, however, lower in the cultures than in the adult rat brain hemispheres. The activity increased in brain during postnatal maturation. No changes in the enzyme activity were observed in the cultures, either during growth or after removal of fetal calf serum and addition of dB-cAMP.

Type A and B monoamine oxidase in glial cells in long-term culture

Monoamine oxidase (MAO) in astroglial primary cultures increased during cell development. The enzyme activity towards type B substrate was further enhanced in cultures treated with 0.25 mM dibutyl cyclic AMP (dBcAMP), although dBcAMP (1 mM) does not effect MAO activity in vitro. It has been estimated that the dBcAMP treated cultures contain 70% type A and 30% type B MAO, while in the untreated one, there was virtually only type A MAO. Following the application of Percoll density gradient centrifugation, MAO associated with different subcellular fractions was obtained. The dBcAMP treated fractions were slightly heavier than the untreated ones.

Localization of monoamine oxidases A and B in primate brains relative to neuron-specific and non-neuronal enolases

Using serotonin and phenylethylamine deamination as measures of MAO A and MAO B activity respectively, positive correlations were observed between the activities of MAO A and MAO B in different areas of rhesus monkey and human brains. When the activities of MAO A and MAO B were compared with those of neuron-specific enolase and nonneuronal enolase (isozymes which are markers for neurons and glia), a slight but non-significant correlation was observed, suggesting that a simple distribution of MAO A in neurons and MAO B in glia is unlikely. This conclusion is supported by studies using synaptosomes, but contrasts with that from investigations of MAO from peripheral tissues, where experiments indicate that MAO A is predominantly localized in neurones.

Uptake of 6-fluoro-5-hydroxytryptamine and 4,6-difluoro-5-hydroxytryptamine into releasable and non-releasable compartments of human platelets

1 The ring-fluorinated compounds, 6-fluoro-5-hydroxytryptamine and 4,6-difluoro-5-hydroxytryptamine, are sterically similar to 5-hydroxytryptamine (5-HT) but have pKaS for their 5-hydroxyl groups (9.7 and 8.0 respectively) which are lower than that of 5-HT. 2 The rates at which [3H]-5-HT, [3H]-6-fluoro-5-HT, and [3H]-4,6-difluoro-5-HT entered the releasable (vesicular) and non-releasable compartments of washed human platelets during a 1 min incubation period at 37 degrees C were similar. These portions of the uptake process therefore appears to be relatively independent of the pKa of the 5-hydroxyl group. 3 The [3H]-4,6-difluoro-5-HT was unique since material accumulated in the non-releasable compartment during uptake did not migrate into the vesicular compartment under appropriate incubation conditions. 4 The data suggest that the zwitterionic form of 5-HT is not the transported species, but that the electronic configuration about the 5-hydroxyl group may be critical for translocation from a non-releasable to a releasable compartment.

Anomalous compartmentation of 5-hydroxytryptamine in intact human platelets

1 The movement of radiolabelled 5-hydroxytryptamine (5-HT) between the extracellular medium, the thrombin-releasable (vesicular) compartment, and a non-thrombin-releasable compartment has been investigated in washed human platelets. 2 Appreciable amounts of extracellular 5-HT can accumulate in a non-releasable compartment. Depending on the incubation conditions and the amount of 5-HT already present inside the cell, non-releasable 5-HT can either remain non-releasable or rapidly migrate into the vesicular compartment. 3 Measurable amounts of vesicular 5-HT can also enter a non-releasable compartment. However, vesicular 5-HT which becomes non-releasable does not appear to mix with 5-HT becoming non-releasable following uptake from the extracellular medium. 4 Extracellular 5-HT can be added to vesicles and to the non-releasable compartment in the presence of appreciable quantities of 5-HT already in one or both compartments. 5 The vesicular and non-releasable compartments appear to accumulate 5-HT independently of one another. Furthermore, most vesicular accumulation of 5-HT occurs from the extracellular medium, rather than by translocation of non-releasable 5-HT into the vesicular compartment.

Monoamine oxidase in developing chick retina

The developmental changes in monoamine oxidase (MAO) activities towards 5-hydroxytryptamine (5-HT) and beta-phenylethylamine (PEA) in chick retina were investigated. The whole pattern of the developmental changes in MAO activity towards 5-HT was similar to that towards PEA, even though the activity towards 5-HT was much lower than that towards PEA during the period from the 12th day of incubation to the 14th day after hatching. The MAO activities on the 12th day of incubation were low and increased remarkably until the 18th day of incubation. Then they remained unchanged until the 7th day after hatching; therefore, they decreased. The activities measured on the 14th day after hatching are on the same level as those of an adult. The multiplicity of MAO in the retinae of a chick on the 12th day of incubation and of an adult was also studied with specific inhibitors. The result showed that the major part of MAO in chick retina is type B enzyme; that type A MAO in the retina of an adult is, however, relatively higher than that in the retina of the embryo on the 12th day of incubation. It was found that an appreciable amount of MAO activity towards 5-HT in chick retina is due to type B MAO.

Monoamine oxidase activity in cultured human skin fibroblasts

Skin fibroblast cultures were prepared from 21 men, and found to contain types A and B activity of monoamine oxidase, with a possible slight predominance of type A, as evaluated by substrate preferences and differential inhibition by clorgyline and deprenyl. Three women had similar activities. There was a close correlation of activities with different substrates, but there was no quantitative correlation between fibroblast and blood platelet enzyme (type B) activities. The fibroblasts also contained catechol-O-methyltransferase activity exceeding, but poorly correlated with, that in erythrocytes. Fibroblasts may be advantageous in studies of monoamine oxidase in man by providing both types of enzyme as found, for example, in the central nervous system, and by providing a means of removing many in vivo chemical influences from the cells in culture. Nevertheless, great caution must be exercised in generalizing results of this "model" to other tissues, since activities of both enzymes correlated poorly with those in blood cells of the same individuals.