Sandfly fever virus activity in central/northern Anatolia, Turkey: first report of Toscana virus infections

Sandfly fever viruses (SFVs) cause febrile diseases as well as aseptic meningitis/encephalitis and include serotypes sandfly fever Sicilian virus (SFSV), sandfly fever Naples virus (SFNV) and Toscana virus (TOSV). Infections are endemic in the Mediterranean basin and data on SFV activity in Turkey are limited. In this study, sera from 1533 blood donors from the Ankara, Konya, Eskisehir and Zonguldak provinces of Turkey were evaluated for SFV exposure by indirect immunofluorescence test (IIFT) and confirmed by virus neutralization test (VNT). One hundred and two patients with central nervous system (CNS) infections of unknown aetiology were also tested via IIFT and real-time reverse-transcription PCR for SFV/TOSV. Rate of overall IgG reactivity in IIFT was 32.9% (505/1533) among blood donors. TOSV exposure was confirmed by VNT in all study regions. Exposure to the recently-identified serotype sandfly fever Turkish virus, as evaluated by VNT, was revealed in Konya and Ankara. SFNV exposure was identified in Konya and SFSV was observed to be present in all regions except Zonguldak. TOSV RNA was detected in 15.7% (16/102) and was accompanied by TOSV IgM in 25% (4/16) of the patients. Partial L and S sequences suggested that TOSV circulating in Turkey can be grouped into TOSV genotype A strains. Exposure to TOSV and other SFV serotypes was revealed in blood donors and CNS infections by TOSV were identified for the first time in Turkey. Infections are observed to be endemic in central Anatolia and should be considered as aetiologic agents in cases/outbreaks of fever and meningoencephalitis.

Disseminated histoplasmosis associated with hemophagocytic lymphohistiocytosis in kidney transplant recipients

Transplant patients are susceptible to infectious complications due to chronic immunosuppression. We present two cases of persistent fever, weight loss and pancytopenia in kidney transplant recipients (originally concerning for posttransplant lymphoproliferative disease) that were later diagnosed with disseminated histoplasmosis on bone marrow and lymph node biopsy. In both patients, pancytopenia was due to hemophagocytic lymphohistiocytosis (HLH) which has rarely been described in association with histoplasmosis and not previously reported in kidney transplant recipients with this fungal infection. The diagnosis of histoplasmosis can be complex due to nonspecific symptomatology, delays in isolating histoplasma by fungal culture and false-negative antibody titers in immunocompromised patients. A review of the literature including the clinical features of histoplasmosis in immunosuppressed patients (prevalence, current diagnostic testing and treatment options) as well as the association of HLH in immunocompromised states are discussed.

[Effect of the quality of endodontic treatment on the longevity of fixed prostheses. Apropos of 64 pulpectomies]

64 teeth with well-fitting post-retained crown were examined radio graphically and clinically and a number of failures were assessed including the peri-apical condition and the quality of root filling. All the teeth were vital and prosthetic was the only reason of endodontic treatment. Results have shown that 25% of the teeth had areas of radiolucency around the root apices. Various criteria parameters were reviewed for possible correlation to the failures. 15 out of the 16 cases of failures could be related to the endodontic therapy procedures: lack of using a rubber dam during the treatment and an unsatisfactory condensed and sealed root filling. Only one case was related to the deviation of post from the line of canal suggest a lateral perforation. To perform a post-retained crown the quality of endodontics must be considered: the operative procedures should be more controlled and enough time should be observed after root canal therapy to ensure its successful.

Synthesis of a potent wide-spectrum serotonin-, norepinephrine-, dopamine-reuptake inhibitor (SNDRI) and a species-selective dopamine-reuptake inhibitor based on the gamma-amino alcohol functional group

A series of gamma-amino alcohols were synthesized and screened for reuptake inhibition and noncompetitive NMDA antagonism. Compound (+/-)-3f simultaneously and potently inhibits reuptake of 5-HT, NE, and DA, representing a potential wide-spectrum reuptake inhibitor antidepressant. In addition, comparative rat and human studies uncovered a species-selective DA reuptake inhibitor (+/-)-2e, KD(hDAT)/KD(rDAT) = 97.

Phosphorylation of tau protein in tau-transfected 3T3 cells

The tau protein of Alzheimer paired helical filaments (PHFs) is aberrantly phosphorylated, as evidenced by its reactivity with several phosphate-dependent antibodies. We sought to identify whether this unusual phosphorylation state exists in tau expressed by transfected NIH 3T3 fibroblasts. Immunoblot analysis of cell clones transfected with constructs for either the 3-repeat or 4-repeat isoforms of tau revealed two tau bands, with the lower band migrating with unmodified tau in each case. Antibodies T3P and tau-1 were used to probe these bands, as they also react with PHF-tau in a phosphate-dependent manner. The epitopes for both antibodies were phosphorylated in both tau isoforms. Only the upper band was phosphorylated at the T3P site whereas phosphorylation at the tau-1 site was not always associated with a shift of tau mobility on gels. Tau in both bands was soluble, in contrast to PHF-tau, and was competent to bind to exogenously added bovine microtubules. Colchicine treatment of the cells resulted in an inhibition of phosphorylation at both sites, through an unknown mechanism. In conclusion human tau expressed in 3T3 cells was phosphorylated at the T3P and tau-1 sites as is PHF-tau, although no PHFs formed and the phosphorylated tau was competent to bind to microtubules.

Human tau isoforms confer distinct morphological and functional properties to stably transfected fibroblasts

Tau protein is a neuronal microtubule-associated protein that promotes the assembly and stability of microtubules. To evaluate the biological significance of tau isoform diversity, NIH-3T3 cells were stably transfected with cDNAs encoding each of the six isoforms present in human brain. Cells expressing different isoforms developed distinct morphologies. Cell lines expressing 3-repeat tau isoforms developed large flat cell bodies while cells expressing 4-repeat isoforms had small, round cell bodies. All transfected cell lines, except those expressing the shortest tau isoform, displayed very long thin neurite-like processes. Tau colocalized with microtubules in both the cell body and the long processes in all of the tau-transfected cells. Tau also displayed a diffuse amorphous staining pattern that was concentrated around the cell nucleus. Microtubule bundling was not enhanced in any of the transfected cells as compared to untransfected controls. The transfected cells showed increased resistance to colchicine treatment. Thus, different tau isoforms can confer unique cellular morphologies to 3T3 cells and can alter the susceptibility of these cells to a microtubule depolymerizing agent.

Effects of site-specific acetylation on omega-conotoxin GVIA binding and function

Chemical modification of omega-conotoxin GVIA (omega-CgTXGVIA) was performed using nonsaturating concentrations of acetic anhydride to generate seven distinct derivatives. Following separation of these peptides using reverse-phase HPLC (RP-HPLC), their individual molecular weights were determined using fast bombardment mass spectrometry (FAB-MS). Three peptides contained a single acetylated amino group, three possessed two acetylated amino groups, and the last contained three acetylations. For each peptide, the specific site of acetylation was confirmed using a scheme of tryptic digestion, under nonreducing conditions, followed by RP-HPLC and FAB-MS. Biological profiles for each peptide were obtained by analyzing their capacity to displace native 125I-omega-CgTx GVIA binding to rat hippocampal membranes and to block K(+)-stimulated 45Ca2+ influx into chick brain synaptosomes. The data indicate that successive additions of acetyl moieties to omega-CgTx GVIA lead to a loss of both binding affinity and Ca2+ influx inhibitory potency. Within the monoacetylated series, acetylation of the amino terminal of Cys-1, as compared to the epsilon-amino group of either Lys-2 or Lys-24, leads to the greatest shift in potency. In summary, these results indicate that basic (i.e., primary amino) groups, which are brought into close proximity as a result of disulfide bridging, are important in the functional blockade of neuronal Ca2+ channels by omega-CgTx GVIA.

Tau protein induces bundling of microtubules in vitro: comparison of different tau isoforms and a tau protein fragment

Expression of tau protein in non-neuronal cells can result in a redistribution of the microtubule cytoskeleton into thick bundles of tau-containing microtubules (Lewis et al.: Nature 342:498-505, 1989; Kanai et al.: J Cell Biol 109:1173-1184, 1989). We reconstituted microtubule bundles using purified tubulin and tau in order to study the assembly of these structures. Taxol-stabilized tubulin polymers were incubated with various concentrations of recombinant human tau and examined by electron microscopy. With increasing concentrations of tau 3 (tau isoform containing three microtubule binding domains) or tau 4 (isoform containing four microtubule binding domains) the microtubules changed orientation from a random distribution to loosely and tightly packed parallel arrays and then to thick cables. In contrast, tau 4L, the tau isoform containing four microtubule binding domains plus a 58-amino acid insert near the N-terminus, showed minimal bundling activity. tau 4-induced bundling could be inhibited by the addition of 0.5 M NaCl or 0.4 mM estramustine phosphate, conditions which are known to inhibit tau binding to microtubules. A tau construct that contained only the microtubule binding domains plus 19 amino acids to the C-terminus was fully capable of bundling microtubules. Phosphorylation of tau 3 with cAMP-dependent protein kinase had no effect on its ability to induce microtubule bundling. These results indicate that tau protein is directly capable of bundling microtubules in vitro, and suggests that different tau isoforms differ in their ability to bundle microtubule filaments.

Immunological characterization of the region of tau protein that is bound to Alzheimer paired helical filaments

Tau protein is known to be present in the paired helical filaments (PHFs) of Alzheimer brains. This study investigated the fragments of tau protein that remain bound to pronase-treated PHFs and conditions that lead to the release of these tau fragments from the core structure of the PHF. Antibody 423 reacted with PHFs and with fetal rat tau but not with adult rat tau, pig tau, or recombinant human tau. Three other antibodies that react with the tubulin binding region of tau only reacted with PHFs after they were disrupted with formic acid or guanidine. Other antibodies that recognize tau sequences C terminal to the tubulin binding region also recognized pronase-treated PHFs. Antibodies SMI34 and T3P that recognize phosphorylated epitopes were reactive with pronase-treated PHFs. Tau fragments from the PHF were solubilized by acid or guanidine treatment. These findings suggest that the fragments of tau that are bound to PHFs and protected from pronase digestion include sequences from the tubulin binding region to the C terminus of tau. In addition, some of these sequences appear to be conformationally or post-translationally modified.

Differences in the abilities of human tau isoforms to promote microtubule assembly

Three isoforms of human tau protein were compared for their abilities to induce microtubule assembly. The three isoforms, tau 3 (tau containing three microtubule-binding domains), tau 4 (tau containing four microtubule-binding domains) and tau 4L (tau containing four microtubule binding domains plus a 58-amino-acid insert near the N-terminus) were expressed in E. coli and purified using ammonium sulfate precipitation, ion exchange, and size exclusion chromatography. All three isoforms induced microtubule assembly at micromolar concentrations and showed similar critical concentrations for assembly of 0.4-0.45 microM. However, tau 4 induced microtubule formation at a rate five- to tenfold faster than either tau 3 or tau 4L. The rate of microtubule elongation seen with tau 4 was twofold greater than with tau 3 or tau 4L, suggesting that the faster rate of microtubule assembly seen with tau 4 was due, at least in part, to faster elongation. Tau 4 induced a greater number of microtubules to form at steady state than did tau 3 or tau 4L. The microtubules generated with each tau isoform had similar steady-state length distributions and were equally susceptible to cold-induced disassembly. These results indicate that the additional microtubule-binding domain in tau 4 enhances microtubule assembly, while the 58-amino-acid insert negates the stimulatory effect of the fourth microtubule-binding domain.

Infection with murine retrovirus confers resistance to the neurotoxin 1-methyl-4-phenylpyridinium ion in PC 12 cells

High concentrations of the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+) are toxic to the catecholaminergic cell line PC12, derived from rat phenochromocytoma. Prolonged exposure of wild-type PC12 cells to 500 microM MPP+ yields toxin-resistant colonies at a frequency of 2 X 10(-4). These spontaneously arising MPP(+)-resistant cells are morphologically quite distinct from wild-type PC12 cells, and are lacking in most of their characteristic catecholaminergic properties. In contrast, among PC12 cells infected with the murine retrovirus ZIPNEOSV(X), 20% are resistant to the toxin MPP+, a resistance frequency approximately 1,000 times higher than for uninfected cells. The morphology and catecholaminergic phenotype of the virus-infected MPP+ resistant cells are quite similar to those of wild-type PC12 cells. The results presented in this study suggest a unique mechanism of MPP+ resistance in the infected PC12 cells which may be conferred by the presence and/or expression of the retrovirus ZIPNEOSV(X).

Retroviral-mediated gene transfer. Applications in neurobiology

There are now many examples of the successful expression of genes transduced by retroviruses in studies from outside the field of neuroscience. Retroviruses will undoubtedly also prove to be effective tools for neuro-scientists interested in expressing cloned neurotransmitter and receptor genes. There are also other less obvious applications of retroviruses, such as their insertional mutagenic effects, which may be useful in studies of the genetic factors and biochemical mechanisms involved in, for example, neurotoxicity. Strong cellular promoters have been identified by retroviral infection and subsequent rescue of the flanking genomic DNA. Retroviruses can be employed again to reintroduce these regulatory sequences back into cells. In this way the complexities of gene expression in the many subpopulations of neurons may be unraveled. Retroviruses can also serve as very useful genetic markers in studies of development and lineage relationships. Retroviruses may be used to efficiently transfer oncogenes into neuronal cells to create new cell lines. This application exploits one of the natural traits of retroviruses--oncogenesis--which led to their original discovery. Finally, there are neurotropic retroviruses that could serve as important vectors for delivering genes into neurons. Studying these retroviruses may lead to an understanding of how they cause neuropathologic changes in the CNS.

Kinetic differences between type I and type II benzodiazepine receptors

The kinetic properties of [3H]flunitrazepam [( 3H]FNZ) binding to central benzodiazepine receptors of bovine brain membranes at 0 degrees have been studied. Both dissociation and association (under pseudo-first order conditions) kinetics are biphasic, enabling definition of "fast" and "slow" compartments for both processes. In four brain regions examined, the proportion of receptor in the rapidly associating and rapidly dissociating compartments correlates with the proportion of Type I benzodiazepine receptor as determined from equilibrium radioligand binding studies in the four brain regions examined. Preincubation with the Type I-selective drugs CL-218,872 or methyl-beta-carboline-3-carboxylate (Ro 22-7497) reduces the size of the fast but not the slow kinetic compartments. Potencies of the drugs in eliciting these alterations in [3H]FNZ kinetics correlate with their potencies in displacing [3H]FNZ from Type I benzodiazepine receptors. Thus, in membrane preparations the more rapidly associating and dissociating site appears to represent the pharmacologically defined Type I benzodiazepine receptor, whereas the Type II receptors display slower association and dissociation kinetics. Soluble Type I receptors also display more rapid dissociation and association kinetics than soluble Type II receptors.

Monoclonal antibody production by receptor-mediated electrically induced cell fusion

Fusion of myeloma cells and B lymphocytes to form hybridomas which produce monoclonal antibodies has been a major advance, but the poor efficiency and randomness of viral or polyethylene glycol fusion techniques generally gives poor yields of specific, high affinity antibodies. High voltage electrical fields with dielectrophoresis to ensure cell alignment can fuse a limited number of cells under direct microscopic examination, but it is not possible to identify B-cells destined to secrete relevant antibodies. However, B-cells express, on their surface, antigen receptor immunoglobulins of the same antigenic specificity as the secreted antibodies. Binding of antigen to surface immunoglobulins stimulates proliferation and differentiation of B-cells into plasma cells. Here we report the use of the selective, high affinity interaction of antigen with surface immunoglobulins on B-cells to facilitate a close adherence to myeloma cells. The antigen, covalently conjugated to avidin, binds to the surface immunoglobulins on B-cells. This B-cell-antigen-avidin complex binds to biotin covalently attached to the surface of myeloma cells. An intense electric field across a bulk cell suspension then produces selective fusion of cells in contact, that is, of myeloma cells with B-cells which make the appropriate antibody. We have used this technique with several antigens, and all resultant hybridomas secrete appropriate antibodies with very high affinity.

Autoradiographic visualization of angiotensin-converting enzyme in rat brain with [3H]captopril: localization to a striatonigral pathway

We have visualized angiotensin-converting enzyme (ACE; dipeptidyl carboxypeptidase, peptidylpeptide hydrolase, EC 3.4.15.1) in rat brain by in vitro [3H]captopril autoradiography. [3H]Captopril binding to brain slices displays a high affinity (Kd = 1.8 X 10(-9) M) and a pharmacological profile similar to that of ACE activity. Very high densities of [3H]captopril binding were found in the choroid plexus and the subfornical organ. High densities were present in the caudate putamen and substantia nigra, zona reticulata. Moderate levels were found in the entopeduncular nucleus, globus pallidus, and median eminence of the hypothalamus. Lower levels were detectable in the supraoptic and paraventricular nuclei of the hypothalamus, the medial habenula, the median preoptic area, and the locus coeruleus. Injection of ibotenic acid or colchicine into the caudate putamen decreased [3H]captopril-associated autoradiographic grains by 85% in the ipsilateral caudate putamen and by greater than 50% in the ipsilateral substantia nigra. Thus, ACE in the substantia nigra is located on presynaptic terminals of axons originating from the caudate putamen, and ACE in the caudate putamen is situated in neuronal perikarya or at the terminals of striatal interneurons. The lack of effect of similar injections into the substantia nigra confirmed that the caudate putamen injections did not cause trans-synaptic changes. The presence of [3H]captopril binding is consistent with an ACE-mediated production of angiotensin II in some brain regions. Although [3H]captopril autoradiography reveals ACE in a striatonigral pathway, there is no evidence for angiotensin II involvement in such a neuronal pathway.

Differential localization of type I and type II benzodiazepine binding sites in substantia nigra

A number of studies have suggested the existence of multiple benzodiazepine binding sites in the brain. We have recently reported the physical separation of two apparent benzodiazepine binding site subtypes, the pharmacological properties, and distribution in tissue sections of which correspond to the putative type I and type II sites. Benzodiazepine and gamma-aminobutyric acid (GABA) receptors have been shown to interact, and lesions of the GABAergic striatonigral pathway, which lead to GABA supersensitivity, both increase the numbers of GABA binding sites and enhance GABA-stimulated benzodiazepine binding. We demonstrate here that degeneration of striatonigral fibres increases the density of putative type I benzodiazepine binding sites in the substantia nigra and decreases the density of the putative type II sites. This suggests that type I sites that increase after denervation are postsynaptic, whereas the type II sites reduced by the lesion may be localized to axons or terminals of the striatonigral pathways.

Two distinct solubilized benzodiazepine receptors: differential modulation by ions

The modulation of solubilized type 1 and type 2 benzodiazepine receptors from cow brain by gamma-aminobutyric acid (GABA), divalent cations, and anions has been evaluated. GABA stimulates [3H] flunitrazepam binding of both receptor subtypes, whereas divalent cations and anions selectively stimulate solubilized type 2 receptors. Of numerous anions examined, only chloride, bromide, and iodide enhance [3H] flunitrazepam binding. Chloride and bromide increase mainly receptor affinity for [3H]flunitrazepam, whereas iodide largely influences Bmax values. Divalent cations also stimulate soluble type 2 receptors. Calcium, zinc, manganese, barium, and magnesium have similar potencies in enhancing [3H]flunitrazepam binding, whereas copper and nickel are about 4 to 5 times more potent. The 2- to 3-fold increase in type 2 receptor binding by divalent cations involves change in numbers of binding sites. Effects of combinations of GABA, calcium, and chloride suggest that they may exert their modulating effects on type 2 receptors through different mechanisms. GABA, calcium, and chloride also protect [3H]flunitrazepam binding from heat inactivation, indicating a close link in the native state between the GABA, ions, and the benzodiazepine recognition sites. Since physiologic concentrations of calcium and chloride influence type 2 receptors, these ions may be involved in those pharmacologic effects of benzodiazepines mediated by type 2 sites.

Autoradiographic differentiation of multiple benzodiazepine receptors by detergent solubilization and pharmacologic specificity

We have used light microscopic autoradiography to localize differentially Type 1 and 2 benzodiazepine receptors using pharmacologic and detergent treatment techniques. The triazolopyridazine CL218872 has preferential affinity for Type 1 receptors from which it displaces [3H]flunitrazepam selectivity. In biochemical experiments [3H]flunitrazepam binding sites with pharmacologic properties of Type 2 receptors are preferentially solubilized from brain membranes by detergents. We have now treated rat brain slices either with CL218872 or 2% sodium cholate and evaluated the autoradiographic distribution of [3H]flunitrazepam-labeled receptors. Whether calculated from the drug or detergent treated preparations, the relative densities of apparent Type 1 and 2 receptors are the same. Thus differential sensitivity to determine solubilization distinguishes the same two populations of receptors as Type 1 and 2 specific drugs.

Size of acetylcholine receptors in the membrane. An improved version of the radiation inactivation method

The radiation inactivation method was used to study the size of acetylcholine receptors in the intact membrane-bound state. This technique was reinvestigated, and modifications were made which remove substantial difficulties affecting previous applications of it to such proteins. The molecular size was deduced here by reference to a set of protein standards: an inactivation ratio was defined relative to a given internal enzyme molecular weight standard, and a linear calibration plot for the inactivation ratios of the protein standards was constructed and applied. The acetylcholine receptor in Torpedo electric organ, cat denervated muscle, and chick embryonic muscle was found by this method to exist in the membrane as a homogeneous population of the same size in each case. This receptor, when identified thus by the alpha-neurotoxin-binding target structure, has an apparent molecular weight of 300000 or a molecular volume of about 350 nm3. In comparison, the molecular weight of the cat muscle receptor when solubilized, as analyzed by gel electrophoresis after extensive cross-linking, was found to be 270000 +/- 20000. These two values are thought to be equivalent by virtue of the situation and structure of the receptor protein in the cell membrane. If a disulfide-bridge dimeric receptor exists in the membrane (as other evidence has indicated for Torpedo), each monomer acts independently there in binding alpha-neurotoxin, since the monomers can be inactivated independently by irradiation in the Torpedo membrane. In the muscle membrane no evidence for the existence of receptor dimers, of any kind, has been found.

Physical separation and characterization of two types of benzodiazepine receptors

Two distinct benzodiazepine receptors are solubilized differentially by various detergents. The receptor sites that resist solubilization, designated type I, are most highly concentrated in the cerebellum and corpus striatum whereas the more readily solubilized receptors, type II, are most enriched in the hippocampus. The type I receptors display higher affinity for beta-carboline esters and a triazolopyridazine whereas several benzodiazepines do not differentiate the two receptors. The type I receptors can be solubilized with 2% Triton X-100/1 M NaCl; they retain the same drug specificity as in the particulate state.

Acetylcholine receptor from mammalian skeletal muscle. Oligomeric forms and their subunit structures

1. The acetylcholine receptor of cat denervated skeletal muscle was solubilised with Triton X-100 in the presence of protease inhibitors and was shown to have a sedimentation coefficient of about 9 S. This oligomer can be converted to a smaller, active 4-S species. 2. This 9-S glycoprotein was purified to homogeneity (showing pI = 5.0) by improved biospecific chromatography on alpha-neurotoxin and lectin affinity gels, and shown to bind specifically 10--11.5 mumoles [2,3-3H]propionyl-alpha-bungarotoxin/g protein. The association rate constant (3 x 10(5) M-1 s-1 at 25 degrees C) for this reaction was similar to that observed with membrane-bound or unpurified receptor; affinity constants for nicotinic ligands were also similar in all these cases. 3. By a variety of techniques, a major polypeptide of Mr about 43,000 was detected in the pure protein. Likewise, both 9-S and 4-S oligomers isolated in a pure state at high yield (approximately equal to 80%) by a novel technique using anti-toxin immunoglobulin, contained the same size of subunit. 4. Sub-synaptic and extra-synaptic forms of the receptor were alkylated specifically in the membrane-bound state with the affinity reagent bromo[3H]acetylcholine. As in the case of the pure receptor from denervated muscle, the same size polypeptide (Mr 43,000) was labelled. This was true, also, for both the 9-S and the 4-S oligomer of the denervated muscle receptor. 5. Proposed oligomeric structures of acetylcholine receptors containing single and multiple-size subunits are discussed.

Molecular forms of the acetylcholine receptor from vertebrate muscles and Torpedo electric organ. Interactions with specific ligands

Multiple forms of the acetylcholine receptor solubilised from cat denervated muscle were separated by velocity sedimentation centrifugation. The kinetic properties of the two main forms (with sedimentation coefficients of 9 S and 4 S) were investigated using a pure preparation of a suitable probe, [3H]propionyl-alpha-bungaro-toxin. The binding of this toxin to each of these forms of the muscle receptor was consistent with a simple bimolecular reaction with a homogeneous class of binding sites. Negligible dissociation of the receptor-toxin complex was observed. This behaviour was also found for the different forms of the acetylcholine receptor of chick embryo muscle and of Torpedo marmorata electric organ. Association rate constants for binding of the 3H-labelled alpha-toxin to receptor from chick embryo muscle and the 9-S and 4-S forms from cat denervated muscle were 0.54 X 10(5), 1.76 X 10(5) and 2.69 X 10(5) M--1 S--1 respectively, at 25 degrees C. The values obtained for the 9-S and 13-S forms of receptor from T. marmorata were 4.51 X 10(5) and 9.93 X 10(5) M--1 S--1 respectively. The reaction of the 3H-labelled alpha-toxin with the receptor was second-order and linear in the presence of an antagonist, as in its absence, for the 4-S and 9-S forms of the cat denervated muscle receptor. This reaction of the receptor was inhibited by cholinergic ligands, with Ki values for two antagonists tested being greater with the 4-S form than with the 9-S form. Apparent negative interaction is observed for antagonists with this receptor, with Hill coefficients of about 0.78 and 0.64 for the 4-S and 9-S forms respectively. A ligand-induced affinity increase, produced by the agonists but not by the antagonists, was observed in this reaction for both forms of the muscle receptor. Two agonists tested showed no difference between these forms in their high-affinity states in either their binding affinities or Hill coefficients.

Tritiation of alpha-bungarotoxin with N-succinimidyl [2,3-3H]propionate. A useful reagent for labelling proteins

Mono[3H]propionyl-alpha-bungarotoxin, prepared with N-succinimidyl [2,3-3H]propionate (sp. radioactivity 50 Ci/mmol) and purified to homogeneity by electrofocusing, retains its biological activity and stability. Rate constants for its binding to acetylcholine receptor were 4.4-fold lower than for unlabelled toxin; no dissociation was detectable. Analysis of enzymic digests of toxin showed 3H is located mainly of entirely in epsilon-propionyl-lysine.