Altered sensitivity of CD81-deficient mice to neurobehavioral effects of cocaine

CD81, also known as target of the antiproliferative antibody, is known to be expressed in astrocytes and involved in cell adhesion and, recently, we demonstrated its induction exclusively in the accumbens following cocaine. In the present study, the sensitivity of CD81-deficient mice to behavioral effects of cocaine was evaluated. It was found that CD81-deficient mice exhibited altered sensitivity to cocaine as assessed in the place preference conditioning paradigm and locomotor activity. This deficit in place preference conditioning was not accompanied by a deficit in acquisition or retention of water maze behavior. In addition, CD81 knockout mice exhibited higher levels of nucleus accumbens dopamine as compared to their controls. These observations are discussed in the context of the role of CD81 in cocaine-mediated behaviors.

Potential role of nuclear translocation of glyceraldehyde-3-phosphate dehydrogenase in apoptosis and oxidative stress

Recent studies indicating a role of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in apoptosis or oxidative stress has been reported. Using confocal laser-scanning microscopy, we have investigated the cellular distribution of GAPDH in central nervous system (CNS)-derived cells (neuroblastoma mNB41A3), in non-CNS derived cells (R6 fibroblast) and in an apoptosis-resistant Bcl2 overexpressing cell line (R6-Bcl2). Induction of apoptosis by staurosporine or MG132 and oxidative stress by H(2)O(2) or FeCN enhanced the nuclear translocation of endogenous GAPDH in all cell types, as detected by immunocytochemistry. In apoptotic cells, GAPDH expression is three times higher than in non-apoptotic cells. Consistent with a role for GAPDH in apoptosis, overexpression of a GAPDH-green fluorescent protein (GAPDH-GFP) hybrid increased nuclear import of GAPDH-GFP into transfected cells and the number of apoptotic cells, and made them more sensitive to agents that induce apoptosis. Bcl2 overexpression prevents nuclear translocation of GAPDH and apoptosis in untransfected cells, but not in transfected cells that overexpress GAPDH-GFP. Our observations indicate that nuclear translocation of GAPDH may play a role in apoptosis and oxidative stress, probably related to the activity of GAPDH as a DNA repair enzyme or as a nuclear carrier for pro-apoptotic molecules.

Specification of distinct dopaminergic neural pathways: roles of the Eph family receptor EphB1 and ligand ephrin-B2

Dopaminergic neurons in the substantia nigra and ventral tegmental area project to the caudate putamen and nucleus accumbens/olfactory tubercle, respectively, constituting mesostriatal and mesolimbic pathways. The molecular signals that confer target specificity of different dopaminergic neurons are not known. We now report that EphB1 and ephrin-B2, a receptor and ligand of the Eph family, are candidate guidance molecules for the development of these distinct pathways. EphB1 and ephrin-B2 are expressed in complementary patterns in the midbrain dopaminergic neurons and their targets, and the ligand specifically inhibits the growth of neurites and induces the cell loss of substantia nigra, but not ventral tegmental, dopaminergic neurons. These studies suggest that the ligand-receptor pair may contribute to the establishment of distinct neural pathways by selectively inhibiting the neurite outgrowth and cell survival of mistargeted neurons. In addition, we show that ephrin-B2 expression is upregulated by cocaine and amphetamine in adult mice, suggesting that ephrin-B2/EphB1 interaction may play a role in drug-induced plasticity in adults as well.

Purification of a dichlorophenol-indophenol oxidoreductase from rat and bovine synaptic membranes: tight complex association of a glyceraldehyde-3-phosphate dehydrogenase isoform, TOAD64, enolase-gamma and aldolase C

NADH-dichlorophenol-indophenol oxidoreductases (PMOs) were purified from synaptic plasma membranes or synaptic vesicles (small recycling vesicles) from both bovine and rat brains and from a neuroblastoma cell line, NB41A3. Several isoforms could be identified in purified plasma membranes and vesicles. Purification of the enzyme activity involved protein extraction with detergents, (NH4)2SO4 precipitation, chromatography under stringent conditions and native PAGE. PMO activity could be attributed to a very tight complex of several proteins that could not be separated except by SDS/PAGE. SDS/PAGE resolved the purified complex into at least five proteins, which could be micro-sequenced and identified unambiguously as hsc70, TOAD64 and glyceraldehyde-3-phosphate dehydrogenase tightly associated with the brain-specific proteins aldolase C and enolase-gamma. Enzyme activity could be purified from both synaptic plasma membranes and recycling vesicles, yields being much greater from the latter source. Highly purified plasma membranes (prepared from a neuroblastoma cell line NB41A3 by iminobiotinylation of intact cells and affinity purification with avidin and anti-avidin antibodies under very stringent conditions) also displayed PMO activity tightly associated with TOAD64. The association of PMO in a tight complex was confirmed by its immunoprecipitation from cellular and membrane extracts of NB41A3 using antibodies directed against any component protein of the complex followed by immunodetection with antibodies directed against the other members. Antibodies also inhibited the enzyme activity synergistically. In addition, induction of the different components of the complex during dichlorophenol-indophenol stress was demonstrated by the S1 RNase-protection assay in synchronized NB41A3 cells. The role of the complex in membrane fusion and cellular response to extracellular oxidative stress during growth and development is discussed.

The plasma membrane NADH-diaphorase is active during selective phases of the cell cycle in mouse neuroblastoma cell line NB41A3. Its relation to cell growth and differentiation

Plasma membrane oxidoreductases have been described in all cells and use extracellular impermeant electron acceptors (DCIP, Ferricyanide) that are reduced by NADH. They appear to regulate the overall cell activity in response to oxidative stress from the cellular environment. An NADH-DCIP reductase has been described at the plasma membrane of NB41A3, a neuroblastoma cell line (Zurbriggen and Dryer (1993) Biochim. Biophys. Acta 1183, 513-520) whose activation with extracellular impermeant substrates promotes cell growth. Elutriation was performed to separate cells and the various fractions were analysed for enzyme activity on intact cells combined with flow cytometry. These studies showed that the enzyme is mostly induced and activated during the G1 and during the G2/M-phases. These observations were further corroborated with specific inhibitors of the cell cycle. A three-fold increase in enzyme activity was observed in the presence of alpha-amanitin, a specific cell cycle inhibitor of the G1-phase. Taxol, a specific inhibitor of the M-phase, also induces a significant increase in enzyme activity. FACS analysis of taxol -treated and alpha-amanitin-treated cells corroborated these data. The cells have been synchronized and the enzyme activity was measured at different time intervals. An activity increase was observed after ca. 2-3 h, that corresponds to a raise in the M-phase, according to FACS data. Furthermore, NTera-2 cells - a human neuroblastoma cell line that differentiates into fully mature neurones in the presence of retinoic acid - exhibit a 50% decrease in the enzyme activity during the G0-phase upon differentiation, compared to undifferentiated cells. Together the data presented in this paper show that this plasma membrane NADH-diaphorase affects cell growth and differentiation and is strongly modulated at various phases of the cell cycle.

Developmental changes in the localization of the transplasma membrane NADH-dehydrogenases in the rat brain

The function of transplasma membrane oxidoreductases (PMO's) has been further studied by means of investigating the postnatal (PN) developmental changes in the tissue localization of six isoenzymes previously characterized (see accompanying paper). The changes were followed in the midbrain for PMO-I, -II, and -V and in the brainstem for PMO-III, -IV and -VI. PMO-I is not observed before PN5 and develops as long vertical fibers located mainly in the pontine nucleus and in the dorsal raphe nucleus until it merges all over the midbrain except for the aqueduct and the superior colliculus after PN10. At that stage it is highly expressed in the trigeminal nucleus and the dorsal raphe, but its expression then strongly decreases and PMO-I disappears almost completely later on. Similarly PMO-II only develops around PN5, first in the dorsal and caudal linear raphe and later on (at PN7) also in the pontine nucleus and in the median raphe; at PN10 PMO-II gradually had vanished from these areas and strongly developed in the dorsal raphe and in the mesencephalic trigeminal nucleus. Later on PMO-II also decreases from these areas. PMO-III slowly develops within the gigantocellular reticular nucleus from PN1 to PN5 and later on reaches the facial nucleus (after PN5), the density of PMO-III in these regions at PN10 being much higher than in the adult. PMO-IV follows a similar developmental pattern in the midbrain, with an optimal density around PN10 also. PMO-V appears only at about PN5, first within the dorsal raphe in parallel fibers and in multipolar neurons. It disappears from the fibers around PN10 and remains present in neurons up to adulthood. PMO-VI appears at early stages within the gigantocellular reticular nucleus and after PN5 within the central gray in vertical fibers. At later stages PMO-VI is found in the spinal trigeminal nucleus, at first within the neuropil then in multipolar neurons that remain present up to adulthood. These datas suggested that the different isoenzymes are expressed at various stages in specific areas. The role of PMO's in neuronal development is discussed.

Distribution of six transplasma membrane NADH-dehydrogenases in rat brain tissue

Transplasma membrane redox plays a significant role in cellular activation and growth. Six isoenzymes could be prepared from purified rat brain synaptic plasma membrane. Polyclonal antibodies have been prepared against six transplasma membrane oxydoreductases (PMO-I to PMO-VI) and the tissue distribution of the various iso-enzymes have been investigated in adult rat brains by means of immunohistochemistry. PMO-I is densely observed in layers I, IV and V of the parietal cortex, in CA1 of the hippocampus (except for the molecular layer), in the caudate putamen, in the dorsal, granular and ventral parts of the auditory nuclei, in some loci of the vestibular nuclei as well as in the deep cerebellar nucleus and in the granular layer of the cerebellar cortex. PMO-II is mainly located in the polymorphic layer of the dentate gyrus and in the deep cerebellar nucleus and in the granular layer of the cerebellar cortex. PMO-III is abundant in the piriform cortex, in the pyramidal layers of both CA1 and CA2, in the diagonal band of the basal ganglia, in the supraoptic nucleus and in various loci of the magnetocellular paraventricular nucleus of the hippothalamus as well as in the vestibular nuclei from the brain stem. In addition PMO-III is also densely present in motor nuclei (oculomotor, facial, hypoglossal and ambiguus nuclei), in the reticular formation and in the deep cerebellar nucleus as well as in the Purkinje layer of the cerebellar cortex. PMO-IV has a similar location but is less abundant in the vestibular nuclei of the sensory brain stem and in the motor nucleus. PMO-V in contrast is poorly present in most brain areas compared to the other iso-enzymes, apart of the Purkinje layer of the cerebellar cortex. Finally PMO-VI is mainly present in the oriens layer and in the stratum radiatum of the hippocampus formation, in the supraoptic and lateral magnocellular nucleus of the hypothalamus, in the mesencephalic trigeminal nucleus, in the ventral auditory nucleus and in the facial nucleus of the brain stem as well as in red nucleus of the reticular formation and in the Purkinje layer of the cerebellar cortex. These data show that the iso-enzymes are located in specific brain nuclei. The significance of the results in respect to the yet very poorly defined function of PMO's is discussed.

Selective inhibition of adenylate cyclase in bovine cortex by quinones: a novel cellular substrate for quinone cytotoxicity

Quinones are widely distributed substances of often potential toxicological significance. On the other hand, cyclic AMP is known to promote a cell-survival response and to retard apoptosis [Berridge, Tan and Hilton (1993) Exp. Hematol. 21, 269-276]. Therefore the effects of quinones on adenylate cyclase were tested. Adenylate cyclase is rapidly inhibited by quinones, with IC50 values of 40-45 microM for p-benzoquinone (BQ) or 200 microM for dichlorophenol-indophenol (DCIP), with 2-substituted quinones being inactive. Membrane solubilization decreases the IC50 values for BQ and DCIP to 18 microM and 40 microM respectively. The inhibition is not affected by GTP, GDP or analogues, or by cholera and pertussis toxins; therefore it is not mediated by a G-protein or the activation of a defined receptor. Further, the inhibition stoichiometrically competes with forskolin activation of adenylate cyclase, equimolar concentrations of quinone and forskolin restoring the enzyme activity to its basal value. Reduction of BQ with sodium dithionite stoichiometrically prevents the inhibition of adenylate cyclase; in turn, oxidation of hydroquinone with ferricyanide fully restores it, indicating that the oxidized state of the quinone is required for inhibition. In addition, BQ is cytotoxic in vivo on HepG2, a human hepatocellular carcinoma cell line, but the effect can be prevented with forskolin. In plasma membranes, BQ tightly binds only one major and two minor proteins; these BQ-binding proteins were purified by means of labelling with [14C]BQ followed by PAGE under native conditions. Together these observations indicate that the action of quinone can be traced to targeting a limited number of proteins at the plasma membrane in a highly selective way and to affecting key enzymes such as adenylate cyclase.

An NADH-diaphorase is located at the cell plasma membrane in a mouse neuroblastoma cell line NB41A3

Plasma membranes from most mammalian cells display significant transplasma membrane oxidoreductase (PMO) activity. The enzymes use an extracellular, impermeant electron acceptor as substrate and intracellular reduced pyridine nucleotide as electron donor. The plasma membrane from a neuroblastoma cell line, NB41A3, has been biotinylated and purified by immunoprecipitation with avidin and antiavidin-antibodies. The protein recovery of an immunopurified membrane preparation was < 0.15% of the protein content in the cell extract. The preparation displays an increase in the specific activity of PMO's of 15- to 20-fold compared to the activity in whole cells. With this approach the presence of a NADH-diaphorase within the cell plasma membrane can be demonstrated. This activity accounts for about one third of the total cellular diaphorase activity. The PMO activity cannot be attributed to an increased permeabilization of the plasma membrane induced upon biotinylation nor to intracellular activity from lysed cells. Activation of basal metabolism (glycolysis) stimulates PMO activity up to approx. 54%, presumably through a raise of the intracellular NADH store. PMO also promotes cell growth at low substrate concentrations (0.1-1 microM). Native gel electrophoresis of iminobiotinylated and affinity purified plasma membrane extracts displays two diaphorase-positive bands, indicating that a homogeneous cell population may express several PMO activities at the plasma membrane.

Imaging of neonatal arterial thrombosis

The case of a neonate who presented with symptoms of upper limb ischemia related to spontaneous multiple arterial and venous thromboses that were demonstrated by colour Doppler sonography and digital subtracted angiography is reported. The presentation of limb ischaemia at birth may be the warning sign of simultaneous cerebral infarction.

Cellobiose dehydrogenases of Sporotrichum (Chrysosporium) thermophile

Both cellobiose dehydrogenases of Sporotrichum (Chrysosporium) thermophile, ATCC 42464, obtained after fractionation with DEAE-Trisacryl chromatography and named cellobiose dehydrogenase I and II have been purified to homogeneity by different chromatographic techniques. Both enzymes are slightly glycosylated flavocytochrome-b proteins with similar catalytic properties but with distinct molecular masses (91 kDa and 192 kDa for enzymes I and II, respectively) and isoelectric point (4.1 versus 3.45). Examination by SDS/PAGE clearly showed that the larger enzyme II is a homodimer, whose subunit is close to, but different from dehydrogenase I which is homogeneous by this technique. After limited digestion of both enzymes with papain, two main fractions with residual activity are formed, one carrying the heme, the other being the flavin component; each fraction is characterized by its particular chromatographic behaviour. The flavin carrying component shows an atypical (for flavoprotein) three-banded spectrum indicative of the presence of a flavin derivative. Both enzymes react very slowly with oxygen clearly forming some superoxide radicals and possibly hydrogen peroxide. Cellobiose and other cellodextrins are oxidized at their reducing glycosyl moiety to the corresponding aldonic acid. With the use of the autooxidable phenazinemethosulphate, cellulose (either in a hydrated form or crystalline) is also oxidized at free reducing ends so that appreciable amounts of cellobionic acid are released upon enzymatic hydrolysis.

Inhibition of Ca2+ efflux by pyridine nucleotides

The effects of pyridine nucleotides on the Mg-dependent ATP-stimulated Ca2+ pump and on the ATP-independent Na(+)-Ca2+ exchanger were investigated in rat brain synaptic plasma membranes. Both Ca2+ efflux mechanisms are inhibited by pyridine nucleotides, in the order NADPH greater than NADP greater than NADH greater than NAD with IC50 = ca. 3-4 mM for NADP or NADPH and ca. 5 mM for the other pyridine nucleotides in the case of the ATP-driven Ca(2+)-pump, and with IC50 = 8 to 10 mM for the Na(+)-Ca2+ exchanger. Oxidizing agents such as DCIP or FeCN also affect the Ca(2+)-efflux mechanisms. DCIP and FeCN inhibit the ATP-driven Ca2+ pump but not the Na(+)-Ca2+ exchanger. Inhibition of the ATP-dependent Ca2+ pump is optimal when both a reduced pyridine nucleotide and an oxidizing agent (e.g. DCIP or FeCN) were added together. Under similar experimental conditions the pyridine nucleotide-mediated inhibition of the Na(+)-Ca2+ exchanger is partially removed. Therefore Ca(2+)-efflux mechanisms appear to be controlled in part through the redox environment, probably by means of transplasma membrane dehydrogenases.

Plasma membrane dehydrogenases in rat brain synaptic membranes. Multiplicity and subunit composition

Plasma membrane redox enzymes have been investigated in synaptic membranes from rat brain nerve terminals. UV-Vis spectra of intact synaptic plasma membranes are presented and the presence of a b-type cytochrome, detectable at 77 degrees K and sensitive to NADH or NADPH, is shown. The molecular characterization of rat synaptic NADH-dehydrogenases was further performed on solubilized enzymes using a recently developed nondissociating polyacrylamide gel electrophoresis technique. Synaptic plasma membranes were solubilized with 1% sodium cholate or Triton X-114 and centrifuged. The supernatant retained over 60% of the NADH-dehydrogenase activity, tested with either DCIP2 or ferricyanide as substrates, together with NADH. Both enzyme activities were insensitive toward rotenone. This extraction procedure also solubilized about 50% of the proteins. When submitted to polyacrylamide gel electrophoresis under nondenaturing conditions and stained for NADH-dehydrogenase activity, five bands of different mobilities were detected. The multiple NADH-dehydrogenases of synaptic plasma membranes were investigated by means of band excision and the five excised bands each submitted to amino acid analysis and to 2-D electrophoresis. The subunit composition of each band was then deduced, together with the molecular weight and pI of each respective subunit. NADH-dehydrogenases have also been purified by means of FPLC on Mono-P (chromatofocusing) followed by gel filtration on Superose 12. NADH-Dehydrogenase IV and V could be purified in their active forms by this approach.

Effects of plasma membrane oxidoreductases on Ca2+ mobilization and protein phosphorylation in rat brain synaptosomes

We have investigated the possible role of plasma membrane oxidoreductases in the Ca2+ export mechanisms in rat brain synaptic membranes. Ca2+ efflux in nerve terminals is controlled both by a high-affinity/low capacity Mg-dependent ATP-stimulated Ca2+ pump and by a low affinity/high capacity ATP-independent Na(+)-Ca2+ exchanger. Both Ca2+ efflux mechanisms were strongly inhibited by pyridine nucleotides, in the order NADP greater than NAD greater than NADPH greater than NADH with IC50 values of ca. 10 mM for NADP and ca. 3 mM for the other agents in the case of the ATP-driven Ca2+ pump and with IC50 values between 8 and 10 mM for the Na(+)-Ca2+ exchanger. Oxidizing agents such as DCIP3 and ferricyanide inhibited the ATP-driven Ca2+ efflux mechanism but not the Na(+)-Ca2+ exchanger. In addition, full activation of plasma membrane oxidoreductases requires both an acceptor and an electron donor; therefore the combined effects of both substrates added together were also studied. When plasma membrane oxidoreductases of the synaptic plasma membrane were activated in the presence of both NADH (or NADPH) and DCIP or ferricyanide, the inhibition of the ATP-driven Ca2+ pump was optimal; by contrast, the pyridine nucleotide-mediated inhibition of the Na(+)-Ca2+ exchanger was partially released when both substrates of the plasma membrane oxidoreductases were present together. Furthermore, the activation of plasma membrane oxidoreductases also strongly inhibited intracellular protein phosphorylation in intact synaptosomes, mediated by either cAMP-dependent protein kinase, Ca2+ calmodulin-dependent protein kinases, or protein kinase C.

The role of iron in the activation of mannonic and altronic acid hydratases, two Fe-requiring hydro-lyases

D-Altronate hydratase and D-mannonate hydratase belong to a class of Fe2+-requiring enzymes, but the function of iron in these enzymes is largely unknown. Methods are described for the convenient preparation of both these hydratases from Escherichia coli and studies related to metal activation are presented. The enzymes are inactive in the absence of a bivalent metal and a reducing agent such as dithiothreitol. Fe2+ at low concentrations activates the enzymes efficiently, but inhibits them over 2 mM. Furthermore Mn2+ is also capable of activating aldonic acid hydratases and appears to be a constituent of the enzyme active center. A marked synergistic activation is observed in the presence of both ions, raising the possibility that the enzyme has two binding sites for ions. Upon activation, the two aldonic acid hydratases incorporate a single Fe atom and contain no Fe-S core, in contrast to other characterized Fe-hydratases, such as aconitase or maleic acid hydratase. The incorporated iron is losely bound (with Kd about 4.5 mM and 20 mM for mannonate and altronate hydratase, respectively) and can be readily removed with EDTA. The enzymes exhibit no requirement for sulphide ions and are insensitive to thiol reagents. A first-order inhibition is observed with iron chelators and can be removed by competition with excess metal ions. No change in the absorption spectra is observed upon oxidation-reduction or activation with metals. The activated enzymes exhibit no electron paramagnetic (EPR) spectrum under anaerobic conditions; in the presence of oxygen, an intense EPR spectrum develops in Fe2+-activated samples with signal at g = 1.98, which upon reaction of the enzyme with the substrate moves into a species with signals at g = 4.15 and g = 9.07, with EPR parameters very similar to those of oxidized rubredoxins.

Isolation and biochemical characterization of maleic-acid hydratase, an iron-requiring hydro-lyase

A procedure for the isolation of maleic acid hydratase (D-malate hydro-lyase, EC 4.2.1.31) of about 95% purity from rabbit kidneys is described. The enzyme consists of a single polypeptide chain of 582 amino-acid residues with an approximate molecular mass of 68 kDa. The enzyme is very unstable and has an absolute requirement for chloride ions. Addition of sodium sulphide during the purification process was essential to maintain the enzyme in an activatable state. The pure preparation has low activity but responds to activation with Fe2+ ions, Na2S and a thiol. The sequence of adding the activating reagents is critical to achieve optimal activity. Ni2+ and to a lesser extent Co2+ can replace iron in the activation process. The enzyme incorporates 4-5 mol iron/mol and 4.5-6 mol sulphide/mol during activation. In this process an [Fe-S] cluster appears to be built up, as indicated by optical and electron paramagnetic resonance (EPR) spectroscopy. In activated samples exposed to air the [Fe-S] cluster is EPR-detectable through an axial signal with g = 2.01 and g = 2.029 whose temperature and power saturation characteristics were similar to those of other [3Fe-xS] clusters. The activated enzyme, however, is readily inactivated even upon minor manipulation with destruction of the iron-sulfur core.

The role of iron in the activation-inactivation of aconitase

Methods are described for the convenient preparation of aconitase from beef heart mitochondria in its inactive [3Fe-4S] form and largely in its active [4Fe-4S] form. Inactive aconitase can be activated anaerobically by various reducing agents without addition of iron. Under these conditions, maximally 70-80% of the activity attainable in the presence of added iron can be reached. It is concluded that during activation without added iron, [4Fe-4S] clusters are built from [3Fe-4S] clusters at the expense of a fraction of the 3Fe clusters present. This explains the approximately 75% maximal activation observed and concomitant loss of approximately 25% of total clusters as quantitated by EPR. Time course plots of aconitase activation appear to be second order but are not amenable to simple kinetic analysis because of the requirements of both reduction and Fe2+ for activation. Activation of aconitase with 59Fe leads to rapid (minutes) incorporation of 1 iron atom/cluster, which on subsequent inactivation is readily lost again. With longer incubation times (hour), 59Fe is found in more than a single site/cluster. It is concluded that, in analogy to cluster loss during activation in absence of added iron, the appearance of 59Fe in more than one cluster site can be due to complete breakdown and rebuilding of clusters. However, exchange into intact clusters cannot be ruled out. Ferric iron can be bound nonspecifically to active and inactive aconitase but can be readily removed by chelating agents. Sulfide is not required for activation of aconitase in keeping with the proposal that inactive aconitase, as isolated, contains a [3Fe-4S] cluster. It is demonstrated that oxidation initiates the inactivation of aconitase with concomitant release of iron and formation of 3Fe clusters as determined by EPR.

A spin-label study of the disposition of the Fe-S cluster with respect to the active center of aconitase

It has been reported by Johnson et al. ((1977) Biochem. Biophys. Res. Commun. 74, 384-389) that phenacyl bromide reacts with a single reactive sulfhydryl group of aconitase, abolishing enzyme activity. Substrate or analogs have a protective effect. This group is therefore at the catalytic site of the enzyme. Aconitase is also known to be an Fe-S protein, paramagnetic as obtained on purification (Ruzicka and Beinert (1978) J. Biol. Chem. 253, 2514-2517). We have attempted to obtain information on the location of the Fe-S cluster of aconitase with respect to the catalytically active site by attaching nitroxide-labelled sulfhydryl reagents of the bromoacyl and maleimide type to the sensitive sulfhydryl group. The EPR signals of those spin-labelled sulfhydryl reagents that abolish enzyme activity disappear during reaction with aconitase. EPR spectra at 13 K of the product obtained by reaction of three spin labels (two maleimides and one bromoacyl) with aconitase included a half-field transition at g approximately equal to 4.0 which is characteristic of spin-spin interaction. On the basis of calculations of the dependence of the intensity of the half-field transition on the distance between two interacting unpaired electrons (Eaton and Eaton, (1982) J. Am. Chem. Soc. 104, 5002-5003) the distances between the nitroxide N-O bond and the center of the Fe-S cluster for the three spin labels were calculated to be 10.5, 11 and 13 A. Combined distance and orientation data for the three spin labels indicate that the reactive sulfhydryl group is about 12 A from the center of the Fe-S cluster.

Iron-sulfur stoichiometry and structure of iron-sulfur clusters in three-iron proteins: evidence for [3Fe-4S] clusters

Beef heart aconitase contains 3Fe clusters in its inactive and 4Fe clusters in its active form. The fully active form can be restored from the inactive one by insertion of Fe(2+), whereas S(2-) is not required. Chemical analyses for iron and labile sulfide yield Fe/S(2-) ratios of 0.66-0.74 for the inactive and 0.90-1.03 for the active form. Sulfane sulfur (S(0)) was not detected. We propose on the basis of these data that the inactive form may arise from the active one by loss of one iron only per cluster with the sulfur remaining as S(2-) in a [3Fe-4S] structure. Measurements by extended x-ray absorption fine structure (EXAFS) spectroscopy on the 3Fe form of aconitase yield a Fe..S distance of 2.24 A and a Fe..Fe distance of 2.71 A. This Fe..Fe distance is in agreement with that obtained by EXAFS on ferredoxin II of Desulfovibrio gigas, another 3Fe protein, but disagrees with Fe..Fe distances observed for the 3Fe cluster of Azotobacter vinelandii ferredoxin I by x-ray diffraction-namely, 4.1 A. We suggest that this difference may be due to the presence of a [3Fe-3S] structure in the Azotobacter ferredoxin I crystals vs. a [3Fe-4S] structure in liquid or frozen solutions of aconitase. The [3Fe-3S] cluster has been shown to have a relatively flat twist-boat structure, whereas a [3Fe-4S] cluster could be expected to essentially maintain the compact structure of the [4Fe-4S] cluster. This would explain the differences in Fe..Fe distances. Two possible structural models for a [3Fe-4S] cluster are discussed.

Mössbauer studies of beef heart aconitase: evidence for facile interconversions of iron-sulfur clusters

Beef heart aconitase, isolated under aerobic conditions, has been studied with Mössbauer and EPR spectroscopy. In the oxidized state, the enzyme exhibits an EPR signal at g = 2.01. The Mössbauer data show that this signal is associated with a 3Fe cluster. In dithionite-reduced aconitase, the 3Fe cluster, probably of the [3Fe-3S] type, is in a paramagnetic state of interger electronic spin (S = 2); the Mössbauer spectra exhibit al the unique features reported for proteins with 3Fe clusters. On activation of aconitase with ferrous ion, the paramagnetic 3Fe cluster of dithionite-reduced enzyme is converted into a diamagnetic (S = 0) form. Activation studies with iron enriched in either 27 Fe or 56 Fe suggest that activation transforms the 3Fe cluster into a center that has a [4Fe-4S] core. This conclusion is supported by the observation that EPR signals characteristic of reduced [4Fe-4S] clusters can be elicited under appropriate conditions. It has frequently been assumed that the activation of aconitase with Fe2+ produces an active site containing a single ferrous ion. The data reported here suggest that a ferrous ion is used to rebuild a [4Fe-4S] cluster.

Relationship of the oxidation state of the iron-sulfur cluster of aconitase to activity and substrate binding

It is known that aconitase from mammalian mitochondria is only partially active as isolated but may be activated by incubation with iron, ascorbate, and a thiol, or with dithionite. It has been suggested that the added Fe in the activation mixture is essential for activation and that it is incorporated in the enzyme [Villafranca, J. J., & Mildvan, A. S. (1971) J. Biol. Chem. 246, 772-779; Gawron, O., Waheed, A., Glaid, A. J., & Jaklitsch, A. (1974) Biochem. J. 139, 709-714]. However, it is shown in this paper that, when the enzyme has a full complement of 3Fe and 3S, full activation is reached coulometrically, without iron or other chemical reducing agents. It is clear, therefore, that the role of activators is to reduce the iron--sulfur cluster of the enzyme. The appearance of catalytic activity on reduction of the cluster shows a pronounced lag, as does the decay of activity after reoxidizing the cluster. This suggests that catalytic activity requires a conformational change in the protein which is initiated by reduction of the cluster and that, following reoxidation, activity disappears only after the inactive conformation is assumed. Citrate and the competitive inhibitor trans-aconitate are bound to a comparable extent to the active and inactive forms, but only the active form can bind 1-hydroxy-2-nitro-1,3-propanedicarboxylic acid, a transition-state analogue. This is interpreted to show that in the inactive state aconitase cannot enter the conformation it assumes in the transition state during catalysis.

A rapid and simple method for the isolation of pure eosinophilic leukocytes from horse blood

An improved and short method is described for the isolation of intact eosinophilic leukocytes from horse blood with high yield (1--1.5 g/20 l). Viability and purity of the preparations were verified by light and electron microscopy and by the trypan blue exclusion test. Isolated eosinophils were 98--100% pure, intact and viable, and they could be shown to phagocytise immune-complexes.