Polymorphism of erythrocyte phosphoglucomutase,adenylate kinase and adenosine deaminase in northern Thailand

Summary Phenotypes of the erythrocyte enzymes phosphoglucomutase (PGM) (n-587), adenylate kinase (AK) (n=695), and adenosine deaminase (ADA) (n=616) were determined by horizontal starch gel electrophoresis in Thai subjects from norther Thailand, mainly from the provinces of Chiang Mai and Lamphun. The following gene frequencies were calculated: PGM ₁ ¹ 0.7385 PGM ₁ ² 0.2487 PGM ₁ ⁶ 0.0102 PGM ₁ ⁷ 0.0026, AK ¹ 0.9950 AK ² 0.0050, ADA ¹ 0.9180 ADA ² 0.0820.The regular, apparently autosomal transmission of the PGM ₁ ⁶ and PGM ₁ ⁷ alleles was demonstrated in 7 families revealing sufficient data.

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Zusammenfassung Die Phänotypen der Erythrocytenenzyme Phosphoglucomutase (PGM) (n=587), Adenylatkinase (AK) (n=695), and Adenosindeaminase (ADA) (n=616) wurden mittles horizontaler Stärkegelelektrophorese bei Thailändern aus Nordthailand, hauptsächlich aus den Provinzen Chiang Mai und Lamphun, bestimmt. Auf Grund der Ergebnisse wurden die in der englischen Zusammenfassung angegebenen Genfrequenzen berechnet. Die regelmäßige, anschinend autosomale Vererbung der Allele PGM ₁ ⁶ und PGM ₁ ⁷ wurde in 7 Familien mit ausreichenden Daten nachgewiesen.

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Established and supported by Stiftung Volkswagenwerk.     

Zur formalen Genetik der 6-Phosphogluconatdehydrogenasen (EC: 1.1.1.44); Untersuchung von 220 Familien

Summary The 6-PGD-polymorphism has been investigated in a series of 220 families from Southwestern Germany comprising 546 children. The formal hypothesis two autosomal loci 6-PGD_A and 6-PGD_B, two alleles 6-PGD_B ^b and 6-PGD_B ^b1 at the locus 6-PGD_B covers the results.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

AK,ADA and 6-PGD polymorphisms in Cosenza province (Calabria,Italy)

We report the allele frequencies of polymorphic red cells adenilatechinase (AK), adenosindeaminase (ADA) and 6-phosphogluconate dehydrogenase (6-PGD) enzymes in a population living in the province of Cosenza (Calabria, Southern Italy). AK^*1 (0.957), ADA^*1 (0.942) and 6-PGD^*A (0.965) frequencies are compared with samples obtained from other italian regions.     

Power of assaying inbreeding through sampling of phenotypes and mating types

Summary Several enzyme polymorphisms and hemoglobin variants were typed in a sample of n=219 non-related Greek blood-donors. The following gene frequencies were observed: p^a=0.201, p^b=0.701, p^c=0.098; PGD^A=0.985, PGD^c=0.015; AK¹=0.942, AK²=0.058; Hb^A=0.988, Hb^S=0.012. No polymorphic variation was seen in LDH, s-MDH, PHI, or SOD. The population genetical aspects of these results are discussed.

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Zusammenfassung An einer Stichprobe von n=219 erwachsenen griechischen Blutspendern wurden verschiedene Enzympolymorphismen und Hämoglobinvarianten untersucht. Folgende Genfrequenzen konnten beobachtet werden: p^a=0,201, p^b=0,701, p^c=0,098; PGD^A=0,985, PGD^c=0,015; AK¹=0,0942, AK²=0,058; Hb^A=0,988, Hb^S=0,012. Keine polymorphe Variation wurde bei LDH, s-MDH, PHI und SOD gefunden. Die populationsgenetischen Aspekte dieser Befunde werden diskutiert.

     

Effects of various metabolites on two phosphoglucomutase allozyme activities from Drosophila melanogaster

The effects of various metabolites on the two most common phosphoglucomutase allozymes (PGM^A and PGM^B) in Drosophila melanogaster have been investigated in vitro. 2,3-Diphosphoglycerate (2,3DPG) inhibited PGM^A and PGM^B to the same degree in the presence of 25 µM glucose-1,6-diphosphate (G1, 6P₂). However a higher concentration of G1,6P₂ partially reversed the inhibition of PGM^A exerted by 2,3DPG, so that in the presence of 150 µM G1,6P₂ the inhibition of PGM^A was half that of PGM^B at pH 6.0. Glycerol-3-phosphate (G3P) had no significant effect at pH 7.4 but exerted an activating effect at pH 6.0 which was more pronounced in the case of PGM^B. ATP, citrate, and fructose-1, 6-diphosphate (F1,6P₂) inhibited both PGM^A and PGM^B. The differences found in vitro between these two allozymes can have a significant impact on in vivo function and, therefore, on the maintenance of PGM polymorphism in experimental populations of D. melanogaster studied in the laboratory.     

Studies on the polymorphism of the red cell acid phosphatase in the population of Northern Bavaria (Germany)

Summary Within a population sample of 2000 blood donors of Northern Bavaria the AP polymorphism has been investigated. The calculated gene frequencies are: P^A=0.3463, P^B=0.5970, P^C=0.0568.

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Zusammenfassung An einer Bevölkerungsstichprobe von 2000 Blutspendern aus Nordbayern wurde der AP-Polymorphismus untersucht. Die errechneten Genfrequenzen sind: P^A=0,3463, P^B=0,5970, P^C=0,0568.

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With technical assistence of A. Melchertsen.     

Mutationsversuche an Kulturpflanzen

Zusammenfassung Die im Jahre 1951 zunächst allgemein gegen die RassenA undD nachgewiesene Mehltauresistenz von 8 röntgeninduzierten Wintergerstenmutanten ist differenziert und erweitert worden. Mut. 501 ist gegen 10 von 14 untersuchten Rassen der RassengruppenA, D, B undC resistent (A ₁,A ₂,A ₃,A ₄,D ₁,D ₃,B ₂,B ₃,C ₂,C ₃) und gegen vier anfällig (B ₅,B ₆,C ₄,C ₅). Die übrigen 7 Mutanten 511, 512, 513, 514, 515, 520 und 525a zeigen übereinstimmendes Resistenzverhalten: Von den 16 untersuchten Rassen der RassengruppenA, D, B, C sind die Mutanten gegen acht resistent (A ₂,A ₅,D ₁,B ₂,B ₃,B ₇,C ₂,C ₄) und für die anderen acht anfällig (A ₁,A ₃,A ₄,D ₃,B ₅,B ₆,C ₃,C ₅). Die Mutanten stellen durch diese ihnen eigentümliche, bisher nicht bekannte Kombination der Resistenz gegen verschiedene Rassen vonErysiphe graminis hordei wertvolle Differentialwirte dar.Die Genetik der Mehltauresistenz ist bei den acht Mutanten für die RasseA ₂ untersucht worden. Zwei unabhängige Gene sind nachgewiesen: eines bedingt die Resistenz von Mut. 501 (Obsistens) und ein zweites diejenige der übrigen sieben Mutanten 511, 512, 513, 514, 515, 520, 525a (Resistens). Diese sieben Mutanten sind also bzgl. der Mehltauresistenz genetisch identisch, während Mut. 501 heterogen ist. Vererbt wird die Mehltauresistenz bei allen acht Mutanten monogen und vollständig bzw. unvollständig dominant. Lediglich Mut. 520 scheint sich labil zu verhalten. alle Mutanten zeigen zusätzliche Merkmalsänderungen.     

Zur Genetik der 6-Phosphogluconatdehydrogenase (EC: 1.1.1.44): Eine neue Variante F (Freiburg)

Zusammenfassung Unter 2455 Personen fand sich zweimal eine Variante mit einer schneller wandernden Fraktion. Der Phänotypus ist der Friedenship-Variante ähnlich, aber nicht durch NADP im Gel modifizierbar. Die Kinderphänotypen in den Familien der Probanden sprechen nicht gegen die Annahme eines Alleles 6-PGD^f. Die Genhäufigkeit für 6-PGD^f liegt bei 0,0004.

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A new variant F of phosphogluconate dehydrogenase

Summary Within 2455 individuals two have a faster moving variant 6-PGD AF. The zymogram is similar to the Friendship variant but not altered after addition of NADP to the starch gel. The allele is segregating in the two families. The gene frequency of 6-PGD^f is 0.0004.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Factors driving spatial and temporal variation in production and production / biomass ratio of stream‐resident brown trout (Salmo trutta) in Cantabrian streams

1. The objective was to identify the factors driving spatial and temporal variation in annual production (P_A) and turnover (production/biomass) ratio (P/B_A) of resident brown trout Salmo trutta in tributaries of the Rio Esva (Cantabrian Mountains, Asturias, north‐western Spain). We examined annual production (total production of all age‐classes over a year) (P_A) and turnover (P/B_A) ratios, in relation to year‐class production (production over the entire life time of a year‐class) (P_T) and turnover (P/B_T) ratio, over 14 years at a total of 12 sites along the length of four contrasting tributaries. In addition, we explored whether the importance of recruitment and site depth for spatial and temporal variations in year‐class production (P_T), elucidated in previous studies, extends to annual production. 2. Large spatial (among sites) and temporal (among years) variation in annual production (range 1.9–40.3 g m^?2 per year) and P/B_A ratio (range 0.76–2.4 per year) typified these populations, values reported here including all the variation reported globally for salmonids streams inhabited by one or several species. 3. Despite substantial differences among streams and sites in all production attributes, when all data were pooled, annual (P_A) and year‐class production (P_T) and annual (P/B_A) and year‐class P/B_T ratios were tightly linked. Annual (P_A) and year‐class production (P_T) were similar but not identical, i.e. P_T = 0.94 P_A, whereas the P/B_T ratios were 4 + P/B_A ratios. 4. Recruitment (Rc) and mean annual density (N_A) were major density‐dependent drivers of production and their relationships were described by simple mathematical models. While year‐class production (P_T) was determined (R² = 70.1%) by recruitment (Rc), annual production (P_A) was determined (R² = 60.3%) by mean annual density (N_A). In turn, variation in recruitment explained R² = 55.2% of variation in year‐class P/B_T ratios, the latter attaining an asymptote at P/B_T = 6 at progressively higher levels of recruitment. Similarly, variations in mean annual density (N_A) explained R² = 52.1% of variation in annual P/B_A, the latter reaching an asymptote at P/B_A = 2.1. This explained why P/B_T is equal to P/B_A plus the number of year‐classes at high but not at low densities. 5. Site depth was a major determinant of spatial (among sites) variation in production attributes. All these attributes described two‐phase trajectories with site depth, reaching a maximum at sites of intermediate depth and declining at shallower and deeper sites. As a consequence, at sites where recruitment and mean annual density reached minimum or maximum values, annual (P_A) and year‐class production (P_T) and annual (P/B_A) and year‐class P/B_T ratios also reached minimum and maximum values.     

Endogenous Expression of Adenosine A<Subscript>1</Subscript>, A<Subscript>2 </Subscript>and A<Subscript>3</Subscript> Receptors in Rat C6 Glioma Cells

Inhibitory and stimulatory adenosine receptors have been identified and characterized in both membranes and intact rat C6 glioma cells. In membranes, saturation experiment performed with [³H]DPCPX, selective A₁R antagonist, revealed a single binding site with a K _D = 9.4 ± 1.4 nM and B _max = 62.7 ± 8.6 fmol/mg protein. Binding of [³H]DPCPX in intact cell revealed a K _D = 17.7 ± 1.3 nM and B _max = 567.1 ± 26.5 fmol/mg protein. On the other hand, [³H]ZM241385 binding experiments revealed a single binding site population of receptors with K _D = 16.5 ± 1.3 nM and B _max = 358.9 ± 52.4 fmol/mg protein in intact cells, and K _D = 4.7 ± 0.6 nM and B _max = 74.3 ± 7.9 fmol/mg protein in plasma membranes, suggesting the presence of A_2A receptor in C6 cells. A₁, A_2A, A_2B and A₃ adenosine receptors were detected by Western-blotting and immunocytochemistry, and their mRNAs quantified by real time PCR assays. Giα and Gsα proteins were also detected by Western-blotting and RT-PCR assays. Furthermore, selective A₁R agonists inhibited forskolin- and GTP-stimulated adenylyl cyclase activity and CGS 21680 and NECA stimulated this enzymatic activity in C6 cells. These results suggest that C6 glioma cells endogenously express A₁ and A₂ receptors functionally coupled to adenylyl cyclase inhibition and stimulation, respectively, and suggest these cells as a model to study the role of adenosine receptors in tumoral cells.     

Red cell enzyme polymorphisms in Punjabis in North India

Seven red cell isoenzyme systems were investigated on a sample of 140 Punjabis from Hoshiarpur and Chandigarh, shown to be representative by comparison of their blood group frequencies with other samples from the area. Phenotype and gene frequencies are given for adenosine deaminase, adenylate kinase, acid phosphatase, 6-phosphogluconate dehydrogenase, phosphoglucomutase locus 1 and 2, lactate dehydrogenase and phosphohexose isomerase. The high frequencies of the ADA² and AK² genes in Indian samples and the presence of the rare variant 3-1 of phosphohexose isomerase are confirmed.     

Polymorphismus der Adenosindesaminase

Zusammenfassung Es wurden die Erythrocyten-ADA-Isoenzymmuster bei 500 unausgelesenen Personen der West-Berliner Bevölkerung untersucht. Die gefundenen Genhäufigkeiten betragen: ADA¹=0,942 und ADA²=0,058.

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Adenosine deaminase polymorphismStudy on a population sample from West-Berlin

Summary The ADA isoenzyme patterns in the blood of 500 unselected individuals living in West-Berlin have been determined. The found gene frequencies are ADA¹=0.942 and ADA²=0.058.

     

Cytogenetic studies in Papaver L.; the x= 6 and x= 7 diploids

Although hybridization between species in Papaver is difficult, a combination of karyotypic and genomic analysis has allowed the definition of up to six, apparently independent genomes, in the 2n= 12 and 1n= 14 diploids. In the 2n= 14 group there is considerable karyotypic differentiation and karyotypes off. atlanticum and P. hybridum are sufficiently dissimilar from each other and from the rest to allow the recognition of the two genomes as unique although no hybrids with other x= 7 diploids were produced. The genome of P. atlanticum is defined as C₇C₇ and that of P. hybridum H₇H_7.P. alpinum and P. rhaeticum only hybridized successfully with each other and the near perfect bivalent formation in their hybrid, together with the extreme similarity of their karyotypes, suggests that they are very closely related. They are designated J₇J₇. All the other x=7 diploids are karyotypically similar and the analysis of meiosis in their hybrids demonstrates thay they all share the same genome to some extent. There is however some differentiation among them. P. commutatum, P. glaucum, P. macrostemum and P. rhoeas are genomically very similar and all can be regarded as A₇A₇. P.fugax and P. tauricola appear to share an identical genome, partially differentiated from A₇A₇ and are defined as A²₇A²₇ while P. postii, although showing some little homology with A₇A₇ is sufficiently differentiated from it to be regarded as more distant than A²₇A²₇ and is described as A³₇A³₇. Although no hybrids between the two 2n= 12 diploids P. apulum and P.pavoninum were produced their karyotypes are sufficiently different to be individually recognized. The only hybrid produced between the x= 6 and x= 7 groups (P. apulum×P. hybridum) showed no homology between the chromosomes of the two genomes and, although this may not be true for any other x=6/x=7 combinations it is best to recognize the two x= 6 genomes as independent of the x= 7. The genome of P. apulum is thus regarded as I₆I₆ and that of P. pavoninum as P₆P₆.     

Confidence Intervals on Ratios of Variance Components for the Unbalanced Two Factor Nested Model

The model considered in this article is the two-factor nested unbalanced variance component model: for p = 1, 2, …, P; q = 1, 2, …, Q_p; and r = 1, 2, …, R_pq. The random variables Y_pqr are observable. The constant μ is an unknown parameter, and A_p, B_pq and C_pqr are (unobservable) normal and independently distributed random variables with zero means and finite variances σ²_A, σ²_B, and σ²_C, respectively. Approximate confidence intervals on ?_A and ?_B using unweighted means are derived, where The performance of these approximate confidence intervals are evaluated using computer simulation. The simulated results indicate that these proposed confidence intervals perform satisfactorily and can be used in applied problems.     

Studies on blood groups and other genetic markers in forest Nentzi: Variation among the subpopulations

Summary A Samoyed-speaking group of fishers, hunters, and deer breeders numbering 1500 in total has been investigated. Seven territorial subgroups were examined with respect to 15 genetic systems. The presence of A₂, cde, C^wDe, Kp^a, AK², p^c, Gm^fb, and Gm^fnb genes or haplotypes in low or moderate frequencies was observed. An unexpected finding was a deleted Gm(-;n;gb) phenotype in three siblings.Significant local genetic heterogeneity was observed with regard to AB0, Rh, Tf, PGM₁, 6-PGD, and Gm systems. The summed genic heterogeneity was found to be highly significant ( ₁₃₂ ² =663.70, P< 0.001). Mean F_st was equal to 0.0228, reflecting variation among subpopulations dispersed in the forest-tundra area and living under conditions of partial isolation.     

Recent improvements in the development of A<Subscript>2B</Subscript> adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A₁, A_2A, A_2B and A₃ (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A_2B AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A_2B AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A_2B AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N ⁶-, C²-positions of the purine heterocycle and/or at the 5′-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N′-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-β-D-ribofuranuronamide (19, hA₁ K _i = 1050 nM, hA_2A K _i = 1550 nM, hA_2B EC₅₀ = 82 nM, hA₃ K _i > 5 μM) and its 2-chloro analogue 23 (hA₁ K _i = 3500 nM, hA_2A K _i = 4950 nM, hA_2B EC₅₀ = 210 nM, hA₃ K _i > 5 μM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA_2B AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60–6583, hA₁, hA_2A, hA₃ EC₅₀ > 10 μM; hA_2B EC₅₀ = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.     

Cationic state distribution over the P700 chlorophyll pair in photosystem I

The primary electron donor P700 in photosystem I is composed of two chlorophylls, P_A and P_B. P700 forms the cationic [P_A/P_B]^•+ state as a result of light-induced electron transfer. We obtained a P_A^•+/P_B^•+ ratio of 28:72 and a spin distribution of 22:78 for the entire PSI protein-pigment complex. By considering the influence of the protein components on the redox potential for one-electron oxidation of P_A/P_B monomers, we found that the following three factors significantly contributed to a large P_B^•+ population relative to P_A^•+: 1), Thr-A743 forming a H-bond with P_A; 2), P_A as a chlorophyll a epimer; and 3), a conserved PsaA/PsaB pair, the Arg-A750/Ser-B734 residue. In addition, 4), the methyl-ester groups of the accessory chlorophylls A_−1A/A_−1B significantly stabilized the cationic [P_A/P_B]^•+ state and 5), the methyl-ester group orientations were completely different in A_−1A and A_−1B as seen in the crystal structure. When the methyl-ester group was rotated, the spin-density distribution over P_A/P_B ranged from 22:78 to 15:85.     

Recent improvements in the development of A2B adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A₁, A_2A, A_2B and A₃ (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A_2B AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A_2B AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A_2B AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N ⁶-, C²-positions of the purine heterocycle and/or at the 5′-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N′-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-β-D-ribofuranuronamide (19, hA₁ K _i = 1050 nM, hA_2A K _i = 1550 nM, hA_2B EC₅₀ = 82 nM, hA₃ K _i > 5 μM) and its 2-chloro analogue 23 (hA₁ K _i = 3500 nM, hA_2A K _i = 4950 nM, hA_2B EC₅₀ = 210 nM, hA₃ K _i > 5 μM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA_2B AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60–6583, hA₁, hA_2A, hA₃ EC₅₀ > 10 μM; hA_2B EC₅₀ = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis. This article has previously been published in issue 4/4, under doi:.     

Studies of the morphology of chromosomes of some selected species

Karyotypes of twelve species from twenty-four localities in southern Moravia and one locality in Slovakia were investigated. Their counts or karyotypic formulae are as follows:Chenopodium foliosum (Moench) Ascherson: K (2n)=18=16 A^m+2 B^sm;Astragalus austriacus Jacq.: K (2n)=16=8 A^m+8 B^sm;Astragalus exscapus L.: K (2n)=16=10 A^m+4 B^sm+2 C^st;Astragalus cicer L.: K (2n)=64;Astragalus onobrychis L.: K (2n=64 and K (2n)=64+1;Vicia dumetorum L.: K (2n=14=10 A^m+4 B^sm;Vicia sylvatica L.: K (2n)=14=2 A^m+10 B^sm+2 C^st;Vicia pisiformis L.: K (2n)=12=8 A^m+4 B^sm;Vicia cassubica L.: K (2n)=12=4 A^m+6 B^sm+2 C^st;Vicia cracca L. (from five localities in southern Moravia): K (2n)=28=4 A^m+12 B^sm+12 C^st and K (2n)=28+1=5 A^m+12 B^sm+12 C^st;Vicia cracca L. (from one locality in Slovakia): K (2n)=14=2 A^m+6 B^sm+6 C^st;Vicia tenuifolia Roth: K (2n)=24=4 A^m+16 B^sm+4 C^st;Serratula lycopifolia (Vill.) Kern.: K (2n)=60.     

α2-Adrenoceptor Subtypes Identified by [3H]RX821002 Binding in the Human Brain: The Agonist Guanoxabenz Does Not Discriminate Different Forms of the Predominant α2A Subtype

Abstract: Competition [³H]RX821002 ([³H]2-methoxyidazoxan) binding experiments with α₂-adrenoceptor subtype-specific antagonists—BRL 44408 (α_2A selective), ARC 239 (α_2B selective), and others—were performed to delineate through rigorous computer modeling receptor subtypes in the postmortem human brain. In the hippocampus, hypothalamus, cerebellum, and brainstem the whole population of α₂-adrenoceptors appears to belong to the α_2A subtype (100%; B_max = 34–90 fmol/mg of protein). In the frontal cortex, the predominant receptor was the α_2A subtype (87%; B_max = 53 fmol/mg of protein), although a small population of the α_2B/C subtype (13%; B_max = 8 fmol/mg of protein) was also detected. In the caudate nucleus, a mixed population of α_2A (64%; B_max = 9 fmol/mg of protein) and α_2B/C (36%; B_max = 5 fmol/mg of protein) subtypes was detected. In the cortex and caudate and in the presence of ARC 239 (to mask the α_2B/C-adrenoceptors), competition experiments with the agonist guanoxabenz clearly modeled the high- and low-affinity states of the α_2A subtype. In the presence of ARC 239 and the GTP analogue guanylyl-5′-imidodiphosphate together with NaCl and EDTA (to eliminate the high-affinity α_2A-adrenoceptor) guanoxabenz only recognized the low-affinity α_2A-adrenoceptor. The results indicate that in the human brain the predominant α₂-adrenoceptor is of the α_2A subtype and that this functionally relevant receptor subtype is not heterogeneous in nature.