The 'gating' residues Ile199 and Tyr326 in human monoamine oxidase B function in substrate and inhibitor recognition

The major structural difference between human monoamine oxidases A (MAO A) and B (MAO B) is that MAO A has a monopartite substrate cavity of ~550 ?(3) volume and MAO B contains a dipartite cavity structure with volumes of ~290 ?(3) (entrance cavity) and ~400 ?(3) (substrate cavity). Ile199 and Tyr326 side chains separate these two cavities in MAO B. To probe the function of these gating residues, Ile199Ala and Ile199Ala-Tyr326Ala mutant forms of MAO B were investigated. Structural data on the Ile199Ala MAO B mutant show no alterations in active site geometries compared with wild-type enzyme while the Ile199Ala-Tyr326Ala MAO B mutant exhibits alterations in residues 100-103 which are part of the loop gating the entrance to the active site. Both mutant enzymes exhibit catalytic properties with increased amine K(M) but unaltered k(cat) values. The altered K(M) values on mutation are attributed to the influence of the cavity structure in the binding and subsequent deprotonation of the amine substrate. Both mutant enzymes exhibit weaker binding affinities relative to wild-type enzyme for small reversible inhibitors. Ile199Ala MAO B exhibits an increase in binding affinity for reversible MAO B specific inhibitors which bridge both cavities. The Ile199Ala-Tyr326Ala double mutant exhibits inhibitor binding properties more similar to those of MAO A than to MAO B. These results demonstrate that the bipartite cavity structure in MAO B plays an important role in substrate and inhibitor recognition to distinguish its specificities from those of MAO A and provide insights into specific reversible inhibitor design for these membrane-bound enzymes.     

Multiple forms of monoamine oxidase in rabbit platelets.

Rabbit platelets were found to contain both types A and B MAO activities. The specific enzymatic activity of rabbit platelet MAO was higher for the substrate serotonin than for phenylethylamine. The K_m's for rabbit platelet MAO indicated that the MAO-B enzyme was similar to human platelet MAO and that both MAO-A and MAO-B enzymes in the rabbit platelet are similar to the corresponding forms in the rabbit brain. The drugs clorgyline and deprenyl confirmed the existence of types A and B MAO in the platelet and furthermore indicated that the type A form accounted for approximately 90% of the total enzymatic activity. Amitriptyline at low (micromolar) concentrations selectively inhibited MAO-B activity in both rabbit platelets and brain.     

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.     

Human recombinant monoamine oxidase B as reliable and efficient enzyme source for inhibitor screening

Interest in inhibitors of monoamine oxidase type B (MAO B) has grown in recent years, due to their therapeutic potential in aging-related neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. This study is devoted to the use of human recombinant MAO B obtained from a Baculovirus expression system (Supersomes MAO B, BD Gentest, MA, USA) as reliable and efficient enzyme source for MAO B inhibitor screening. Comparison of inhibition potencies (pIC50 values) determined with human cloned and human platelet MAO B for the two series of MAO B inhibitors, coumarin and 5H-indeno[1,2-c]pyridazin-5-one derivatives, showed that the difference between pIC50 values obtained with the two enzyme sources was not significant (P>0.05, Student's t-test). Hence, recombinant enzyme is validated as convenient enzyme source for MAO B inhibitor screening.     

Monoamine Oxidases A and B as Components of a Membrane Complex

Abstract: The activities of monoamine oxidase A (MAO A) and monoamine oxidase B (MAO B) represent two independent types of substrate binding site, as indicated by experiments with selective inhibitors and also by substrate competition. We have tried to determine whether A and B active sites of human brain and liver MAO are located on physically separable enzyme forms or as subunits in large membrane-bound complexes. MAO was extracted from several sources by a procedure that was designed to give solubilized enzyme in high-speed supernatants, with ratios of MAO A/MAO B activities similar to those in initial crude homogenates. This solubilized enzyme gave gel filtration profiles that suggested the presence of large molecular complexes. Affinity binding experiments indicated that both MAO A and B activities may occur on the same complexes in tissues that initially contain both activities. These complexes were broken down to enzymatically active subunits by treatment with either low concentrations of sodium dodecyl sulfate, with phospholipase A₂, or with a combination of both agents. Results of this study support a concept of MAO as part of a membrane unit in which A and B are two distinct enzymes embedded in a phospholipid structure. The enzymatic activity of MAO A is critically dependent on associated phospholipids, whereas that of MAO B is not.     

Coordinate binding studies of the substrate (factor X) with the cofactor (factor VIII) in the assembly of the factor X activating complex on the activated platelet surface

The assembly of the factor X activating complex on the platelet surface requires the occupancy of three receptors: (1) enzyme factor IXa, (2) cofactor factor VIII(a), and (3) substrate factor X. To further evaluate this three-receptor model, simultaneous binding isotherms of (125)I-factor X and (131)I-factor VIII(a) to activated platelets were determined as a function of time and also as a function of the concentrations of both ligands in the presence of active site-inhibited factor IXa (45 nM) and 5 mM CaCl(2). In the presence of active site-inhibited factor IXa and factor VIIIa there are two independent factor X binding sites: (1) low affinity, high capacity (approximately 9000 sites/platelet; K(d) approximately 380 nM) and (2) low capacity, high affinity (1700 sites/platelet; K(d) approximately 30 nM). A single specific and selective factor X binding site was expressed (1200 sites/platelet; K(d) approximately 9 nM) when the shared factor X/factor II site was blocked by excess factor II (4 microM). In the presence of active site-inhibited factor IXa (4 nM) and factor II (4 microM), factor X binds to 3-fold more platelet sites than procofactor VIII with relatively low affinity (K(d) approximately 250 nM). The activation of procofactor VIII to factor VIIIa increases the affinity of binding to platelets of both factor VIIIa ( approximately 4-fold to K(d) approximately 0.8-1.5 nM) and factor X ( approximately 25-50-fold to K(d) approximately 5-9 nM). In the presence of excess zymogen factor IX, which blocks the shared factor IX/factor IXa binding site, the substrate, factor X, and the active cofactor, factor VIIIa, form a 1:1 stoichiometric complex. These coordinate binding studies support the conclusion that factor X initially binds to a high-capacity, low-affinity platelet binding site shared with prothrombin, which then presents factor X to a specific high-affinity site consisting of factor VIIIa bound to a high-affinity, low-capacity receptor on activated platelets.     

MULTIPLE BINDING SITES OF HUMAN BRAIN AND LIVER MONOAMINE OXIDASE: SUBSTRATE SPECIFICITIES, SELECTIVE INHIBITIONS, AND ATTEMPTS TO SEPARATE ENZYME FORMS

Abstract— MAO of human brain and liver mitochondria was solubilized by a procedure that preserved the substrate and inhibitor selectivities of the original mitochondrial preparation. Techniques that are designed to separate proteins on the basis of molecular size or net surface charge did not yield a physical separation of enzymically active A and B forms, even in the presence of ionic detergents or with limited proteolysis. However, sulfhydryl inhibitors, inorganic salts, ionic detergents, heat treatment, and sonication all tended to cause selective inactivation of serotonin-metabolizing activity in solubilized preparations. Experiments with selectively inhibited (membrane-bound or solubilized) MAO supported the concept of at least two independent kinds of substrate binding site, only one of which metabolizes serotonin (A type) and another (B type) which has a very strong affinity for β-phenethylamine. l -Norepinephrine, tryptamine, dopamine, and tyramine could be classified as common substrates. The lowest K_m values were found for tryptamine at A sites and for β-phenethylamine at B sites. Results of this study suggest that the different MAO sites could be part of the same large molecular complex, which may normally be embedded in the outer mitochondrial membrane so that A sites are more dependent on their lipid environment within this membrane.     

脊髓损伤后脊髓神经细胞膜PAF受体特性的变化

采用３ＨＰＡＦ放射配体结合试验方法测定脊髓神经细胞膜上ＰＡＦ受体的特异性位点,观察脊髓损伤后２、６ｈ、１、３周脊髓神经细胞膜ＰＡＦ受体结合特性变化。结果显示,脊髓神经细胞膜上存在ＰＡＦ高、低亲和力结合位点,脊髓损伤后２、６ｈ、１周组ＰＡＦ受体高、低亲和力位点Ｋｄ值和Ｂｍａｘ均有不同程度下降,与对照组比较,有显著性差异（Ｐ＜０．０５）。表明在伤后早期ＰＡＦ受体亲和力增加,结合位点减少。提示ＰＡＦ受体在脊髓损伤后继发性损害病理生理过程中起一定作用。     

Dual functions of transcription factors, transforming growth factor-beta-inducible early gene (TIEG)2 and Sp3, are mediated by CACCC element and Sp1 sites of human monoamine oxidase (MAO) B gene

Monoamine oxidases (MAO) A and B catalyze the oxidative deamination of many biogenic and dietary amines. Abnormal expression of MAO has been implicated in several psychiatric and neurodegenerative disorders. Human MAO B core promoter (-246 to -99 region) consists of CACCC element flanked by two clusters of overlapping Sp1 sites. Here, we show that cotransfection with transforming growth factor (TGF)-beta-inducible early gene (TIEG)2 increased MAO B gene expression at promoter, mRNA, protein, and catalytic activity levels in both SH-SY5Y and HepG2 cells. Mutation of the CACCC element increased the MAO B promoter activity, and cotransfection with TIEG2 further increased the promoter activity, suggesting that CACCC was a repressor element. This increase was reduced when the proximal Sp1 overlapping sites was mutated. Similar interactions were found with Sp3. These results showed that TIEG2 and Sp3 were repressors at the CACCC element but were activators at proximal Sp1 overlapping sites of MAO B. Gel-shift and chromatin immunoprecipitation assays showed that TIEG2 and Sp3 bound directly to CACCC element and the proximal Sp1 sites in both synthetic oligonucleotides and natural MAO B core promoter. TIEG2 had a higher affinity to Sp1 sites than CACCC element, whereas Sp3 had an equal affinity to both elements. Thus, TIEG2 was an activator, but Sp3 had no effect on MAO B gene expression. This study provides new insights into MAO B gene expression and illustrates the complexity of gene regulation.     

Sigma binding sites in the brain; an emerging concept for multiple sites and their relevance for psychiatric disorders

An increasing amount of evidence suggests the existence of specific binding sites for psychotomimetic drugs from the opiate-benzomorphan and arylcyclohexylamine series. The sigma binding sites have preferential affinity for the dextrorotatory isomers of certain opiate benzomorphans, such as (+)SKF 10047, (+)cyclazocine and (+)pentazocine and also for some neuroleptics (e.g., haloperidol). The PCP receptor has preferential affinity for phencyclidine (PCP) analogs and other non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists. The physiological significance of the PCP receptor is associated with the blockade of the NMDA type of the glutamate receptor, implying a neuroprotective role of the PCP receptor. However, the significance of the sigma binding sites is less conspicuous. It is not only that drugs from distinct pharmacological classes display a certain degree of affinity for the "sigma/haloperidol" binding sites, but also that drugs which do not induce or block psychotomimetic activity, i.e., (+)3-(3-hydroxyphenyl)-N-(1-propyl) piperidine [(+)3-PPP] and 1,3-di-o-tolyl-guanidine (DTG), display relatively high affinity for the sigma binding sites. The diversity of the compounds which are proposed to interact with the sigma receptors and the variety of the responses elicited by these drugs suggest the existence of sigma receptor subtypes. The finding that the type A of monoamine oxidase (MAO) inhibitors, which are used in treatment of affective disorders, display high affinity for the sigma binding sites suggests their involvement in affective or schizoaffective disorders. Revealing the existence of sigma receptor subtypes may help to elucidate their association with various psychiatric disorders.     

Monoamine oxidase: To B or not to B?

That the enzyme, monoamine oxidase (E.C. 1.4.3.4. amine: O₂ oxidoreductase, MAO) exists in multiple forms was first suggested by Johnston (1) who studied the effects of the irreversible inhibitor clorgyline on MAO. It has been proposed that MAO can be classified into two types, A and B, according to their inhibitor sensitivity and substrate specificity. Type A MAO was found to be solely responsible for the deamination of 5-hydroxytryptamine (5-HT) and shows high sensitivity to clorgyline, while type B MAO metabolizes 2-phenethylamine (PEA) and benzylamine (BA) and is less sensitive to clorgyline. Subsequently, it was shown that type B MAO is highly sensitive to the irreversible inhibitor deprenyl (2).Recently, the “multiple forms” concept has been questioned (3–5) mainly because of increasing evidence which is contradictory to some earlier findings. As an alternative, another hypothesis was put forward insinuating that MAO is an enzyme with multiple binding sites but only one molecular entity (3,4,6,7). This account will focus on some experimental findings accumulated mainly since 1978 and which, although equivocal, strongly support the “one molecular entity” hypothesis of MAO.     

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.     

High affinity binding of thrombin to platelets. Inhibition by tetranitromethane and heparin

[¹²⁵I] iodo-α-thrombin has been modified at the macromolecular substrate binding site in order to study the importance of this region in the platelet-thrombin interaction. Modification was effected by the nitration of tyrosine residues with tetranitromethane. This chemical modification abolished the ability of the enzyme to bind with a high affinity to the platelet surface but did not significantly alter low affinity binding. The presence of heparin was also found to inhibit high affinity binding. These results indicate that the high affinity binding site interacts with the fibrinogen binding region of the thrombin molecule and suggests that there are two distinct classes of binding sites for thrombin on the platelet membrane.     

[3H]Harman Binding Experiments. II: Regional and Subcellular Distribution of Specific [3H]Harman Binding and Monoamine Oxidase Subtypes A and B Activity in Marmoset and Rat

[3H]Harman (1-[3H]methyl-beta-carboline) was used in a novel radioligand binding assay to label selectively and with high affinity monoamine oxidase (MAO) type A. The concentration of the enzyme was determined in six CNS regions of the primate species marmoset (Callithrix jacchus) and of the rat: hypothalamus, hippocampus, cerebellum, cerebral cortex, striatum, and spinal cord. The specific [3H]harman binding in the CNS of the marmoset reveals the same pharmacological profile and other characteristics (affinity, saturability, and reversibility) as in the CNS of the rat. The regional distribution of the [3H]harman binding density (Bmax) in the CNS exhibits a distinct pattern in the marmoset and the rat and a 35 (hypothalamus) to 75% (hippocampus) lower Bmax in the marmoset than in the rat. The Bmax values of [3H]harman binding in the CNS of the marmoset and the rat combined as well as those from visceral organs of the rat (liver, heart, lung, thymus, spleen, and kidney) correlated positively and highly significantly with the respective Vmax values of specific MAO activity of the A type but not of the B type, determined with kynuramine as the substrate. In subcellular fractionation experiments with rat cerebral cortex, the highest [3H]harman binding density (Bmax) and MAO-A activity (Vmax) were detected in mitochondrial fractions and severalfold lower values in the synaptosomal membrane fraction. In conclusion, we suggest that [3H]harman binding is a biochemical tool as a selective marker to quantify MAO-A in the CNS of different mammalian species as well as in extraneuronal tissues.     

[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.     

Platelet factor 4 selectively inhibits binding of TGF-beta 1 to the type I TGF-beta 1 receptor.

A low molecular weight inhibitor of TGF-beta 1 binding was detected in partially purified human platelet extracts by using Hep 3B hepatoma cells in the binding assays. The inhibitory protein was purified to homogeneity and was identified as platelet factor 4 on the basis of its amino acid sequence. TGF-beta 1 binding to Hep 3B cells was almost completely inhibited by 100 nM concentrations of platelet factor 4, but TGF-beta 1 binding to NRK 49F fibroblasts was inhibited only slightly. Affinity cross-linking experiments revealed that these differences in the inhibition of TGF-beta 1 binding by platelet factor 4 were due to differences in the complements of TGF-beta 1 binding proteins present on these two cell types. In Hep 3B cells the majority of bound TGF-beta 1 was cross-linked to a complex which had an apparent molecular weight of 70 kDa. TGF-beta 1 binding to this protein was the most sensitive to inhibition by platelet factor 4. Based on its size and TGF-beta 1 binding properties, we believe this protein is the type I TGF-beta 1 receptor. Hep 3B cells also had a high-affinity TGF-beta 1 binding protein which appeared as an 80 kDa complex, and which we believe to be the type II TGF-beta 1 receptor. TGF-beta 1 binding to this protein was not inhibited by platelet factor 4. TGF-beta 1 was also cross-linked to complexes of higher molecular weights in Hep 3B cells, but it was not clear whether any of them represented the type III TGF-beta 1 receptor. In NRK 49F cells, the majority of bound TGF-beta 1 was cross-linked to a high molecular weight complex which probably represented the type III TGF-beta 1 receptor. NRK 49F cells also had type I TGF-beta 1 receptors and platelet factor 4 inhibited binding to these receptors in the NRK cells. Since the type I receptor contributed only a small percentage of total TGF-beta 1 binding, however, the overall effects of platelet factor 4 on TGF-beta 1 binding to NRK 49F cells were negligible. We were unable to demonstrate specific or saturable binding of platelet factor 4 to Hep 3B cells using either direct binding or affinity cross-linking assays. Thus, it is not clear whether platelet factor 4 inhibits TGF-beta 1 binding by competition for binding to the type I receptor. Modest concentrations of TGF-beta 1 reduced the adherence of Hep 3B cells to tissue culture dishes.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

The Deduced Amino Acid Sequences of Human Platelet and Frontal Cortex Monoamine Oxidase B Are Identical

Abstract: Monoamine oxidases (MAOs) A and B play important roles in the metabolism of neuroactive, vasoactive amines. Human platelets contain only MAO B, often used as an indicator of brain MAO B. The validity of this model remained to be evaluated. This report describes the molecular cloning of human MAO B from frontal cortex and platelets. Two overlapping PCR-amplified clones of human platelet MAO B and four PCR-amplified clones of human frontal cortex MAO B covering the entire coding region were sequenced using five internal oligomers and M13 reverse and forward primers. The nucleotide sequences of human MAO B cDNA from platelet and frontal cortex were identical to that of human liver MAO B except for three nucleotides that differed in frontal cortex: nucleotides 440 A → G, 794 C → T, and 825 C → T. Whether or not these differences are artifactual, all three represent silent mutations, which would not alter the amino acid of the encoded polypeptides. Thus, the deduced amino acid sequences of MAO B from frontal cortex, platelet, and liver are identical. These findings indicate the validity of using platelet MAO B mRNA as a marker for brain MAO B and provide a new approach to study the role of brain MAO B in humans.     

Estradiol membrane binding sites on human breast cancer cell lines. Use of a fluorescent estradiol conjugate to demonstrate plasma membrane binding systems

A fluorescent estradiol macromolecular complex was used to study and to characterize steroid binding to membranes of living target cells. Ligand binding to plasma membranes was quantitated with a sensitivity of 0.1 nM. In this way, we found two types of estradiol-binding sites on hormone sensitive MCF-7 cells. Type A sites (8000-16000 sites per cell) were rapidly saturated at low concentrations of the estradiol-bovine serum albumin-fluorescein isothiocyanate macromolecular complex (E2-BSA-FITC). They had a greater affinity for the complex than did the type B sites for which a phenomenon of cooperative fixation was shown. The complex binding was displaced by estrogenic molecules, but not by non-estrogenic compounds, such as cortisol or progesterone. We also studied complex binding on another breast cancer cell line, MDA-MB-231 (MDA), without intracellular estrogen receptors. These cells showed a specific plasma membrane binding system for estrogen, but lacked the high affinity type A binding site. Then, we report the effects of enzyme treatments (trypsin, phospholipase A2 and neuraminidase) on E2-BSA-FITC binding to MCF-7 cell membranes. The quantity of complex bound to membranes decreased after phospholipase and neuraminidase treatments and increased after trypsin. But, in the three cases, the binding was no longer specific because it could not be displaced by E2-BSA or by estradiol. The enzymatic effects were reversible and specific binding was totally restored within 24 h. However, in the presence of the protein synthesis inhibitor, cycloheximide, no restoration of specific binding occurred on trypsin-treated cells. Estrogen binding to MCF-7 and MDA cell plasma membranes thus possesses the three characteristics of all mediated transport processes across biological membranes: saturability, substrate specificity, and specific inhibition. However, the high affinity type A binding site was found only on the estrogen-sensitive cell line, MCF-7.     

Studies on the pargyline-binding site of different types of monoamine oxidase

[3H]Pargyline has been covalently linked to active sites of both type A and type B monoamine oxidase (MAO) obtained from various tissues. Rat heart and human placenta were chosen to represent predominantly type A MAO, pig and bovine livers to represent type B MAO, and rat liver and brain to represent mixed type A and type B MAO's. The [3H]pargyline-MAO adducts were isolated and hydrolyzed by proteolytic enzymes, and the labelled peptides (pargyline-binding sites) separated and compared by paper chromatography and by paper electrophoresis at various pH values. Only one common pargyline peptide was obtained from all the different MAO's. The alternative A and B sites were assessed after preincubation of rat liver MAO with the selective inhibitors deprenyl (to block the B site) and clorgyline (to block the A site). Following proteolysis of the [3H]pargyline of both type A and type B MAO from this pretreated rat liver, MAO has been purified by a series of chromatographic and electrophoretic procedures. Micro-Edman degradation, followed by dansylation, revealed the amino acid sequence to be Ser-Gly-Gly-Cys(X)-Tyr. It is concluded that the primary structures immediately surrounding the pargyline-binding sites are identical for both type A and type B MAO in these tissues.