Botulinum neurotoxin type B. Its purification, radioiodination and interaction with rat-brain synaptosomal membranes

Neurotoxin from Clostridium botulinum type B was purified to homogeneity by by affinity and ion-exchange chromatography; specific neurotoxicity of this protein (Mr of approximately equal to 155 000) following trypsinisation attained a level of 2 X 10(8) mouse LD50 units/mg protein. 125I-iodination of the toxin to high specific radioactivities (19-63 TBq/mmol) yielded typically greater than 65% of its original toxicity; dodecyl sulphate gel electrophoresis under reducing conditions, after trypsinisation, showed that the larger polypeptide (Mr of approximately equal to 101 000) was labelled preferentially. Saturable binding of the 125I-labelled neurotoxin to rat cerebrocortical synaptosomes was observed and Scatchard analysis showed a low content of acceptors with high affinity (Kd = 0.3-0.5 nM;Bmax approximately equal to 30-60 fmol/mg protein, together with a much larger population of weak-affinity sites. No significant differences in binding affinity were seen in competition experiments using native or fully activated (trypsinized) neurotoxin, indicating that chain cleavage is not essential for acceptor-toxin interaction. Type A botulinum neurotoxin showed a limited capacity to inhibit the synaptosomal binding of labelled type B toxin, even at high concentrations (1 muM), and other neurotoxins were without effect, emphasising the acceptor selectivity. Near-complete loss of specific toxin binding was produced by preincubation of synaptosomes with neuraminidase whereas inhibition of the low-affinity sites with wheat-germ agglutinin was less pronounced; such inactivation was prevented by inclusion of selective inhibitors (2,3-dehydro-2-deoxy-N-acetylneuraminic acid and N-acetylglucosamine, respectively). These observations implicate N-acetylneuraminic acid and, possibly, other sugar moieties as constituents of the toxin acceptors. Trypsinisation of synaptosomes gave incomplete inhibition of binding when assayed with 1 nM or 10 nM 125I-iodinated toxin. Detailed analysis of the actions of neuraminidase, trypsin and heat treatment on the concentration dependence of toxin binding suggest the existence of at least two distinguishable populations of sites that contain N-acetylneuraminic acid, with a protein component being associated with the acceptors of lower affinity. These findings are discussed in relation to those previously reported for type A neurotoxin and to the possible physiological significance of such membrane acceptors.     

Biosynthesis of porphyrins and corrins. 1. 1H and 13C NMR spectra of (hydroxymethyl)bilane and uroporphyrinogens I and III

High-field NMR spectroscopic methods have been applied to study the reactions catalyzed by porphobilinogen (PBG) deaminase and uroporphyrinogen III (uro'gen III) cosynthase, which are the enzymes responsible for the formation of the porphyrin macrocycle. The action of these enzymes in the conversion of PBG, [2,11-13C]PBG, and [3,5-13C]PBG to uro'gens I and III has been followed by 1H and 13C NMR, and assignments are presented. The principal intermediate that accumulated was the correspondingly labeled (hydroxymethyl)bilane (HMB), the assignments for which are also presented.     

Critical micelle concentrations of lipoteichoic acids.

Purified lipoteichoic acids (LTAs) from several gram-positive organisms have been shown, by methods involving spectral changes of an added merocyanine dye probe, to have critical micelle concentrations in the range of 1 to 10 micrograms/ml, suggesting that acylated LTAs in their monomer forms may represent the major configuration of extracellular LTAs in bacterial culture fluids. The critical micelle concentrations obtained did not differ markedly with degree of carbohydrate substitution of the polymers. The significance of these findings in relation to the biological properties of LTA is discussed.     

Optical difference spectrum of the electron acceptor Ao in photosystem I

Optical measurements were made during low temperature photoreduction of photosystem I acceptors, A₁ and A₀. In the presence of a significant amount ofA₁ (detected by EPR), no absorbance changes occurred between 750-350 nm, indicating that this species is not a chlorophyll or pheophytin molecule. Spectral changes in this region that may be correlated with the appearance of A⁻₀, suggest that this component is a chlorophyll a anion monomer. The species is present in reaction centres in a ratio of 0.94 A_o/P700.

Photosystem I Primary acceptor Optical difference spectrum Chlorophyll a monomer     

Metabolism of icosa-5,11,14-trienoic acid in human platelets and the inhibition of arachidonic acid metabolism in human platelets by icosa-5,8,14-triynoic and icosa-5,11,14-triynoic acids

Two fatty acids differing from arachidonic acid in lacking one of the internal double bonds (20:35,8,14 and 20:35,11,14) and their 1-C¹⁴ and acetylenic analogues were synthesized. 20:35,8,14 was not metabolized by human platelets but 20:35,11,14 yielded a small amount (1.5% conversion) of two hydroxy fatty acids in a three (11-hydroxy-5,12,14-icosatrienoic acid) to one (15-hydroxy-5,11,13-icosatrienoic acid) proportion. Indomethacin inhibited formation of both hydroxy fatty acids indicating that they are produced via cyclooxygenase. Both ethylenic acids were weak inhibitors of cyclooxygenase (substrate 20 μM arachidonic acid) (ID₅₀: 8.8 μM 20:3^{5,8,14; 11.2 μM} 20:3^5,11,14) but were inactive against lipoxygenase (RD₅₀ > 100 μM). Similarly, both acetylenic analogues were poor inhibitors of lipoxygenase (ID₅₀: 23.4 μM 20:3^5,8,14; 47.8 μM 20:3^5,11,14) but although 20:3^5,8,14 was inactive against cyclooxygenase (ID₅₀ > 100 μM) the 20:3^5,11,14 was a potent inhibitor (ID₅₀: 0.35 μM). The results are interpreted on the basis that hydrogen removal by the lipoxygenase is from C10 and by the cyclooxygenase from C13 but only in 20:3^5,11,14 are these hydrogens (C13) located at the center of a 1,4 pentadiene system (ethylenic) or a 1,4 pentadiyne system (acetylenic).     

Effect of platelet-activating factor on airway vascular permeability: possible mechanisms

We studied the effects of the potent inflammatory mediator, platelet-activating factor (PAF), on vascular permeability in airways (and other tissues) of guinea pigs by measuring extravasation of circulating Evans blue dye. PAF caused a dose-dependent increase in vascular permeability. At 1 ng/kg iv, PAF caused an increase in Evans blue extravasation of 220% (P less than 0.05) in the trachea, with the greatest effect at a dose of 100 ng/kg (858%; P less than 0.01). Histamine (150 micrograms/kg iv) caused a 320% increase over base line in the trachea and 200% in main bronchi; this effect was equivalent to that induced by 10 ng/kg PAF in the trachea and 1 ng/kg in main bronchi. The duration of effect of PAF was greatest in main bronchi (less than 10 min). Platelet depletion with a cytotoxic antibody, or the cyclooxygenase inhibitor, indomethacin, or the cyclooxygenase-lipoxygenase inhibitor, BW 7556, did not affect the vascular permeability response to PAF. The PAF-receptor antagonist, BN 52063, inhibited Evans blue extravasation in the airways in a dose-dependent manner, with complete inhibition at 5 mg/kg. Thus PAF-induced airway vascular leakage is mediated by specific receptors but not by products of arachidonic acid metabolism or by platelets. Increased airway microvascular leakage induced by PAF may lead to plasma extravasation and airway edema, factors that may contribute to the airway narrowing and hyperresponsiveness induced by PAF.     

Blocking of human T lymphocyte functions by anti-Leu-2 and anti-Leu-3 antibodies: differential inhibition of proliferation and suppression

We have shown previously that monoclonal antibodies to the Leu-2 and Leu-3 T cell antigens block the response of their respective subsets in allogeneic MLR. The present study was an effort to explore the mechanism of inhibition and to determine if anti-Leu-2 and anti-Leu-3 antibodies affect the responses to stimuli in addition to alloantigens. Our results indicate that antibodies to Leu-2 and Leu-3 have profound inhibitory effects on proliferation by their respective T cell subsets responding to a variety of stimuli, including specific soluble antigens and alloantigen. This effect was characterized by the following features: a) For optimal inhibition of proliferation, antibody must be present at the onset of antigenic stimulation. b) Inhibition is augmented by increasing the concentration of antibody or decreasing the concentration of antigen. c) Fab fragments of both anti-Leu-2a and anti-Leu-3a antibodies also block proliferation. In addition to their effects on T cell proliferation, anti-Leu-3 antibody blocked T cell-dependent lg synthesis induced in MLR, and anti-Leu-2 antibody prevented the induction, in vitro, of Leu-2+3- suppressor cells of lg synthesis. Taken together, these results suggest that antibodies to antigenic determinants on the Leu-2 and Leu-3 molecules competitively block segments of these structures that bind to alloantigen or nominal antigen. On the other hand, anti-Leu-2a antibody failed to block suppression of the MLR by in vivo activated, antigen-specific Leu-2+3- suppressor cells, which suggests that the Leu-2a epitope does not transmit antigen-specific signals from these differentiated suppressor T cells.