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

Nicotinic acetylcholine receptor inhibitors derived from snake and snail venoms

The nicotinic acetylcholine receptor (nAChR) represents the prototype of ligand-gated ion channels. It is vital for neuromuscular transmission and an important regulator of neurotransmission. A variety of toxic compounds derived from diverse species target this receptor and have been of elemental importance in basic and applied research. They enabled milestone discoveries in pharmacology and biochemistry ranging from the original formulation of the receptor concept, the first isolation and structural analysis of a receptor protein (the nAChR) to the identification, localization, and differentiation of its diverse subtypes and their validation as a target for therapeutic intervention. Among the venom-derived compounds, α-neurotoxins and α-conotoxins provide the largest families and still represent indispensable pharmacological tools. Application of modified α-neurotoxins provided substantial structural and functional details of the nAChR long before high resolution structures were available. α-bungarotoxin represents not only a standard pharmacological tool and label in nAChR research but also for unrelated proteins tagged with a minimal α-bungarotoxin binding motif. A major advantage of α-conotoxins is their smaller size, as well as superior selectivity for diverse nAChR subtypes that allows their development into ligands with optimized pharmacological and chemical properties and potentially novel drugs. In the following, these two groups of nAChR antagonists will be described focusing on their respective roles in the structural and functional characterization of nAChRs and their development into research tools. In addition, we provide a comparative overview of the diverse α-conotoxin selectivities that can serve as a practical guide for both structure activity studies and subtype classification. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Saposin B-dependent reconstitution of arylsulfatase A activity in vitro and in cell culture models of metachromatic leukodystrophy

Arylsulfatase A (ASA) catalyzes the intralysosomal desulfation of 3-O-sulfogalactosylceramide (sulfatide) to galactosylceramide. The reaction requires saposin B (Sap B), a non-enzymatic proteinaceous cofactor which presents sulfatide to the catalytic site of ASA. The lack of either ASA or Sap B results in a block of sulfatide degradation, progressive intralysosomal accumulation of sulfatide, and the fatal lysosomal storage disease metachromatic leukodystrophy. We studied the coupled Sap B-ASA reaction in vitro using detergent-free micellar and liposomal assay systems and in vivo using cell culture models of metachromatic leukodystrophy. Under in vitro conditions, the reaction had a narrow pH optimum around pH 4.3 and was inhibited by mono- and divalent cations, phosphate and sulfite. Bis(monoacylglycero) phosphate and phosphatidic acid were activators of the reaction, underscoring a significant role of acidic phosphoglycerolipids in sphingolipid degradation. Desulfation was negligible when Sap B was substituted by Sap A, C, or D. Up to a molar ratio between Sap B and sulfatide of 1:5, an elevation of Sap B concentrations caused a sharp increase of sulfatide hydrolysis, indicating the requirement of unexpected high Sap B levels for maximum turnover. Feeding of ASA-deficient, sulfatide-storing primary mouse kidney cells with ASA caused partial clearance of sulfatide. Co-feeding of Sap B or its precursor prosaposin resulted in the lysosomal uptake of the cofactor but did not promote ASA-catalyzed sulfatide hydrolysis. This suggests that Sap B is not a limiting factor of the coupled Sap B-ASA reaction in mouse kidney cells even if sulfatide has accumulated to unphysiologically high levels.

Uptake and accumulation of tritiated uteroglobin by day-6 rabbit blastocysts

Uptake of uteroglobin (UGL) by day-6 rabbit blastocysts and the intracellular fate of this protein were studied by light- and electron-microscopic autoradiography, immunocytochemistry and acid-phosphatase cytochemistry. UGL, labelled with N-succinimidyl-(2-3-3H)-propionate, was administered to embryos in vitro for 25 min to 4 h. The kinetics, determined from light-microscopic autoradiographs, showed a continuous uptake of the labeled protein over a 4-h period of incubation. At the ultrastructural level, increasing numbers of silver grains and an intense UGL immunoreaction in protein vacuoles and crystalloid bodies of trophoblast cells indicated that 3H-UGL had accumulated in these organelles. The presence of crystalloid inclusions in protein vacuoles suggests their origin by a condensation of the protein content, including UGL. Lysosomes containing radioactivity were rarely found, suggesting a very low degradation rate of the 3H-UGL. Protein vacuoles and crystalloid bodies exhibited no acid phosphatase reaction. The enzyme was mainly found outside the basal and lateral cell membranes of trophoblast cells, and on the rough endoplasmic reticulum of endoderm cells.

Uptake of tritiated uteroglobin by the endometrium of the rabbit during peri-implantation

Uteroglobin, labelled with N-succinimidyl-(2-3-3H)-propionate, was applied in vivo for 3 h to pregnant rabbit uteri 7 and 9 days after mating. Light- and electron-microscopic autoradiographs showed that the endometrial epithelium, both ciliated and non-ciliated cells, is able to take up 3H-uteroglobin, however, with differing intensity. Large areas of labelling were found in the luminal epithelium, whereas the glandular epithelium contained fewer silver grains. Moreover, intensively labelled single cells or symplasms occurred in both luminal and glandular epithelium. They were identified as degenerating or dead cells. After internalization by pinocytosis or phagocytosis, the tritiated uteroglobin was observed in multivesicular bodies or in lysosomes with floccular content. Later, radioactivity was either found within residual bodies or distributed throughout the entire epithelium and the subepithelial stroma, i.e., the silver grains could no longer be assigned to specific cell organelles.

Purification of uteroglobin using monospecific antibodies coupled to divinylsulphone-activated agarose

As a model for the isolation of a labile or trace protein, the purification of uteroglobin (UGL) by immunoaffinity chromatography is described. Antibody was isolated from sheep antiserum by immunoprecipitation, and coupled to divinylsulphone-activated agarose (Mini Leak). For the immunoabsorption stage rabbit uterine mucosal scrapings were defatted and incubated directly with the immunosorbent. After washing and desorption, the UGL preparation contained relatively few high molecular weight impurities and these were removed by gel chromatography. Purification was monitored at each step by two-dimensional SDS polyacrylamide gel electrophoresis and immunoelectrophoresis. Furthermore, affinity-purified UGL was tritiated with N-succinimidyl[2,3-3H]propionate and assayed by fluorography. In order to determine absolute UGL concentrations a competitive ELISA was developed.

alpha-Bungarotoxin immobilized and oriented on a lipid bilayer vesicle surface

We have developed a new method to assess the binding site on alpha-bungarotoxin (alpha-BGT) for the acetylcholine receptor. It involves the covalent attachment of a palmitic acid chain to the toxin molecule, generating monopalmitoyl-alpha-bungarotoxin (PBGT) which is then immobilized on the surface of a lipid vesicle by a process of spontaneous insertion via the acyl chain into preformed unilamellar vesicles (approximately 800 A in diameter). PBGT itself is able to bind specifically to Triton X-100 solubilized acetylcholine receptors with an association constant, KA, of 5.56 X 10(6) M-1 which is approximately 20-fold lower in affinity than native alpha-BGT. Vesicle-associated PBGT binds to acetylcholine receptor enriched microsac membrane vesicles in aqueous buffer with a KA for both lipid and protein of 4.26 X 10(7) M-1. The putative site of acylation on the PBGT molecule is determined by extensive cleavage of a reduced and carboxymethylated PBGT with thermolysin. An acylated fragment is purified by hydrophobic column chromatography and identified by high-pressure liquid chromatography methods from the known primary sequence of the native toxin as a decapeptide including residues Thr47-Glu56 [C. Y. Lee convention used; see Mebs, D., Narita, K., Iwanaga, S., Samejuma, Y., & Lee, C. Y. (1971) Biochem. Biophys. Res. Commun. 44, 711-716]. Sequential hydrolysis of the fragment from the carboxy terminus with carboxypeptidase Y indicates that Lys51 is the sole site of acylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Tetanus toxin-[2,3-3H]propionamid. Synthesis, specific activity, immunoreactivity, toxicity and ascent into the spinal cord

Tetanus toxin was tritiated with various amounts of N-succinimidyl-[2,3-3H]propionate. A tenfold molar excess of reagent over toxin was used for the introduction of about 5 nmol of [2,3-3H]propionate into 1 nmol of toxin, corresponding to a specific activity of about 48 MBq/mg protein. The labelling procedure did not change significantly the immunoreactivity of tetanus toxin, but reduced the toxicity in mice from 12 ng/kg to 16 ng/kg. Tetanus toxin-[2,3-3H]propionamid was carried through the peripheral nerves into the spinal cord. It could be used instead of 125I-labelled toxin for the detection of anti-tetanus toxin antibodies with a detection limit of 0.25 mU/ml of serum.

Preparation of neurotoxic 3H-beta-bungarotoxin: demonstration of saturable binding to brain synapses and its inhibition by toxin I

1. Homogeneous beta-bungarotoxin, isolated from the venom of Bungarus multicinctus was radiolabelled with N-succinimidyl-[2.3-(3) H]propionate. Stable, di-propionylated material was obtained which was tritiated on both subunits and had a specific radioactivity of 102 Ci/mmol. 2. After separation from unlabelled toxin by isoelectric focussing, it was shown to exhibit significant biological activity in both the peripheral and central nervous systems but had negligible phospholipase A2 activity towards lecithin or cerebrocortical synaptosomes. 3. The labeled neurotoxin binds specifically to a single class of non-interacting sites of high affinity (Kd = 0.6 nM) on rat cerebral cortex synaptosomes; the content of sites is about 150 fmol/mg protein. This binding was inhibited by unlabelled beta-bungarotoxin with a potency which indicates that tritiation does not alter the affinity significantly. 4. The association of toxin with its binding component and its dissociation were monophasic; rate constants observed were 7.8 x 10(5) M-1 s-1 and 5.6 x 10(-4) s-1 at 37 C, respectively. 5. beta-Bungarotoxin whose phospholipase activity had been inactivated with p-bromophenacyl bromide inhibited to some extent the binding of tritiated toxin but with low efficacy. Taipoxin and phospholipase A2 from bee venom, but not Naja melanoleuca, inhibited the synaptosomal binding of toxin with low potencies in the presence, but not the absence, of Ca2+. 6. Toxin I, a single-chain protein from Dendroaspis polylepis known to potentiate transmitter release at chick neuromuscular junction, completely inhibited the binding of 3H-beta-bungarotoxin with a Ki of 0.07 nM; this explains its ability to antagonise the neuroparalytic action of beta-bungarotoxin. Other pure presynaptic neurotoxins, alpha-latrotoxin and botulinum neurotoxin failed to antagonise the observed binding; likewise tityustoxin, which is known to affect sodium channels, had no effect on 3H-beta-bungarotoxin binding. 7. Trypsinization of synaptosomes completely destroyed the binding activity, suggesting that the binding component is a protein; the functional role of the latter is discussed in relation to the specificity of toxin binding.

Similarity of acetylcholine receptors of denervated, innervated and embryonic chicken muscles. 2. Subunit compositions

The native (9-S) form of the acetylcholine receptor, purified from chick embryonic or adult innervated or denervated chicken muscles, was shown in each case to contain three subunits of Mr about 40000 (predominant), 50000 and 54000. The 40000-Mr subunit (termed alpha) appears to exist in two forms; the minor variant form of it, with Mr about 41000, may have a different carbohydrate content. Bromo[3H]acetylcholine, an affinity alkylating reagent for this receptor, labels the 40000-Mr subunit and the accompanying 41000-Mr variant. The apparent ratio of the subunits varies with the several methods of detection used (protein staining, radio-iodination or tritiation) but the alpha subunit always predominates. The alpha subunit was separated in urea gel isoelectric focusing; its isoelectric point was identical there for the three receptors from innervated, denervated and embryonic muscles. Their respective peptide maps for alpha subunit were identical, and likewise for the 54000-Mr subunit. All of the evidence obtained shows that the subunit structures of the junctional, extra-junctional and embryonic chick muscle receptors are identical. Although the alpha subunit in the receptor from Torpedo electric organ, cat muscle and chicken muscles is distinguishable in all three cases in dodecylsulphate gel electrophoresis, it is affinity-labelled in each case. The peptide map of this subunit after limited proteolysis is also very similar in all three cases. It is concluded that the alpha subunits from these three diverse sources are closely related structurally.

Similarity of acetylcholine receptors of denervated, innervated and embryonic chicken muscles. 1. Molecular species and their purification

Acetylcholine receptors were purified to homogeneity from chicken embryonic, adult innervated and denervated muscles, by bio-specific chromatographies using immobilised alpha-neurotoxin and lentil lectin. A minimum specific activity for the pure receptor was estimated to be 6000 nmol alpha-toxin binding sites/g protein. For analysis, the receptors were radio-iodinated or tritiated to high specific radioactivity with succinimidyl-[2,3-3H]propionate. All of the iodinated protein present in the purified receptor preparation reacted with antibody against the pure acetylcholine receptor from Torpedo marmorata electric organ. In the case of all three muscle types used the same oligomeric forms were obtained. The principal form has a sedimentation coefficient of about 9 S, while a minor species (approximately 5S) was also appreciable in crude preparations of embryonic and denervated muscles. Immunization of rabbits with the homogenous receptor from chicken denervated muscle produced muscle weakness characteristic of experimental autoimmune myasthenia gravis. These antisera were equally reactive towards the receptor --125I-alpha-bungarotoxin complexes from chick innervated and denervated muscles. Likewise, the electrophoretic mobilities of the receptors (9-S form) from all three muscle types were identical, as were the isoelectric points of their complexes with 125I-alpha-bungarotoxin. Collectively, these findings and associated ones on subunit structure denote that the 9-S receptor molecules from junctional and extra-junctional area and embryonic stage of chicken muscle are indistinguishable by all criteria yet applied to them.

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.