An evaluation of the role of complement depletion in experimental membranous nephropathy in the rat

Passive Heymann nephritis (PHN), a model of experimental membranous nephropathy produced by the administration of anti-Fx1A antibody, was studied by micropuncture measurement of glomerular hemodynamics and by assessment of immunologic and morphologic findings. The effect of complement depletion on these parameters was evaluated by administering cobra venom factor. Five days after administration of anti-Fx1A Ab to PHN controls, abnormal proteinuria developed and nephron filtration rate decreased due to modest reductions in nephron plasma flow and major reductions (75%) in the glomerular ultrafiltration coefficient. Glomerular capillary hydrostatic pressure gradient was significantly increased and decreased tubular reabsorption was also evident. Complement depletion prevented abnormal proteinuria and normalized tubular reabsorption and some of the glomerular hemodynamic parameters (nephron plasma flow and glomerular capillary hydrostatic pressure gradient). Values for the glomerular ultrafiltration coefficient, a possible index of membrane damage, were significantly improved (100%) after cobra venom factor treatment, although they remained below normal values. Only minimal differences in glomerular and epithelial cell morphology and appearance of electron-dense material were noted between PHN and PHN + cobra venom factor. These data suggest therefore that both complement-dependent and independent mechanisms contribute to explain the changes in nephron filtration and reabsorption that occur in this model of experimental membranous nephropathy.

Circular dichroic spectra of elapid cardiotoxins

Cardiotoxins isolated from elapid snake venoms constitute a chemically homogeneous family of molecules. Within this group several biologically different subclasses exist. We report a comparative analysis of the structure of 20 cardiotoxins using circular dichroism, immunological methods and secondary-structure prediction. It is shown that cardiotoxins fall within two structural subclasses. Toxins of group I are characterized by (a) CD spectra having an intense positive band close to 192.5 nm and a negative trough at 225 nm with no positive band around 230 nm, (b) strong cross-reactivity with a polyclonal antiserum specific for Naja nigricollis toxin gamma and (c) a high tendency to form a reverse turn in the region of position 11. Toxins of group II are characterized by (a) CD spectra displaying a much weaker positive band at 192.5 nm, a negative band around 210 nm and a positive band at 230 nm, (b) little cross-reactivity with the aforementioned antiserum and (c) a high reverse-turn potential at position 31. It is suggested that the observed differences result from differing curvatures in the antiparallel beta sheet which constitutes the main secondary structure of cardiotoxins.

Phospholipase A2 hydrolysis of membrane phospholipids causes structural alteration of the nicotinic acetylcholine receptor

Thermal perturbation techniques have been used to probe structural alteration of the nicotinic acetylcholine receptor as a function of perturbations of its native membrane environment. Differential scanning calorimetry and a technique involving heat inactivation of the alpha-bungarotoxin-binding sites on the receptor protein reveal that there is a profound destabilization of the acetylcholine receptor structure when receptor-containing membranes are exposed to phospholipase A2. The characteristic calorimetric transition assigned to irreversible denaturation of the receptor protein and the heat inactivation profile of alpha-bungarotoxin-binding sites are shifted to lower temperatures by approx. 7 and 5 C degrees, respectively, upon exposure to phospholipase A2 at a phospholipase/neurotoxin binding site molar ratio of about 1:100. The effects of phospholipase A2 on receptor structure can be (i) reversed by using bovine serum albumin as a scavenger of phospholipase hydrolysis products of membrane phospholipids, and (ii) stimulated by incorporation into the membranes of free, polyunsaturated fatty acids. In particular, linolenic acid (18:3(n-3] causes detectable destabilization of the alpha-bungarotoxin binding sites on the receptor at free fatty acid/receptor molar ratios as low as 10:1. Furthermore, alteration of receptor structure by added phospholipase occurs very rapidly, which is consistent with the observation of rapid in situ phospholipase A2 hydrolysis of membrane phospholipids, particularly highly unsaturated phosphatidylethanolamine and phosphatidylserine. Based on previously published data on the inhibition of acetylcholine receptor cation-gating activity caused by the presence of either phospholipase A2 or free fatty acids (Andreasen T.J. and McNamee M.G. (1980) Biochemistry 19, 4719), we interpret our data as indicative of a correlation between structural and functional alterations of the membrane-bound acetylcholine receptor induced by phospholipase A2 hydrolysis products.

Correction of amino acid sequence of phospholipase A2 I from the venom of Laticauda semifasciata (Erabu sea snake)

The amino acid sequence of phospholipase A2 I from the venom of the sea snake Laticauda semifasciata was reinvestigated. The previously reported sequence at positions 70-80 was corrected to Asp-Cys-Ser-Thr-Glu-Glu-Pro-Asn-Cys-Ser-Thr. The positions of half-cystine residues in the corrected sequence agree with most other phospholipases A2.

Antibody conjugates with cobra venom factor. Synthesis and biochemical characterization

Immunoconjugates are semi-synthetic hybrid proteins which bear great promise to become a new generation of anti-tumor agents. While many immunoconjugates have been shown to be selectively cytotoxic in in vitro model systems, dramatic in vivo anti-tumor effects have not been reported. To improve the activity of immunoconjugates, careful structure-function analyses have to be performed. We report here such an analysis for immunoconjugates consisting of a monoclonal anti-tumor antibody (MoAb) and cobra venom factor (CVF), the complement-activating glycoprotein from cobra venom, synthesized with the heterobifunctional crosslinking reagent N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP). It is shown that a reaction mixture after protein coupling contains free MoAb and CVF as well as hybrid proteins of different compositions (dimers (MoAb-CVF), trimers (MoAb2-CVF, MoAb-CVF2), tetramers (MoAb-CVF3, MoAb2-CVF2, MoAb3-CVF), and some higher oligomers). While free MoAb and CVF can be removed by size exclusion chromatography, separation of different oligomeric hybrid proteins is not possible by this method. From the biochemical characterization of the hybrid proteins, which included the determination of sedimentation coefficients, recording of circular dichroism spectra with subsequent determination of secondary structure, and ultrastructural analysis by transmission electron microscopy, it was concluded that the two proteins do not undergo major structural changes upon coupling, and that the coupling of the two proteins is random with no preferential relative orientation. The functional inactivation of CVF was substantial (approximately 70%) due to both derivatization with SPDP and subsequent conjugation to the MoAb, with conjugation being relatively more inactivating than derivatization. In contrast, the binding activity of the antibody was far less susceptible to inactivation. In conclusion, our data indicate that immunoconjugate synthesis with heterobifunctional crosslinking reagents results in a mixture of heterogeneous hybrid proteins and causes substantial functional inactivation. For successful in vivo anti-tumor activity of future immunoconjugates with CVF and other protein ligands better methods for immunoconjugate synthesis will have to be developed.

Effects of phospholipase of Enhydrina schistosa venom on nerve, motor end-plate and muscle of the mouse

Phospholipase A2 from Enhydrina schistosa (sea snake) venom was administered intravenously or intramuscularly to mice. The effects on the contractile responses and neuromuscular transmission were studied in soleus (a slow-twitch muscle) and extensor digitorum longus (e.d.l.--a fast-twitch muscle). These muscles were also studied by morphological methods which included light and electron microscopy and enzyme histochemistry. There was a rapid and irreversible reduction of resting membrane potentials of muscle fibres of soleus which then failed to contract in response to direct or indirect stimulation. The e.d.l. was less severely affected. Spontaneous and evoked release of transmitter remained detectable at almost all end-plates tested and there was no apparent reduction in the sensitivity of the post-synaptic membrane. Most muscle fibres of soleus but only about 50% of those in e.d.l. became necrotic, the histochemical fibre types 1 and 2A being affected. Electron microscopy showed that in the acute phase (up to 24 h) there were lesions of the plasmalemma and severe disorganization of the internal structure of muscle fibres. In contrast, motor nerve terminals remained intact as did myelinated axons in the intramuscular nerves. There was a transient increase in the number of omega-shaped profiles in the axolemma and of coated vesicles in nerve terminals at 1-3 h, but by 24 h motor nerve terminals were normal in appearance. In regenerated muscle, post-synaptic folds at motor end-plates were reduced in depth and number.

The influence of cytotoxins from Central Asian cobra venom and melittin from bee venom on the thermodynamic properties of phospholipid bilayer

The interaction of cytotoxin Vc1 and Vc5 from Central Asian cobra and melittin from the bee venom with multilayer liposomes prepared from dimiristoylphosphatidylcholine with an addition of phosphatidic acid have been studied by the method of differential scanning calorimetry. Incorporation of Vc1, Vc5 and melittin into the lipid resulted in pronounced changes in the thermodynamic properties of the lipid. Polypeptides studied induced lateral phase separation in the lipid. Interaction between molecules of the toxins and the lipid resulted in the formation of a new lipid phase characterized by a higher melting temperature and lower phase transition enthalpy.

Black mamba bites. A report of 2 cases

The clinical features of confirmed cases of black mamba snakebites in a 14-month-old child and a 34-year-old man are presented. The steps taken in management are described and reviewed. The importance of early aggressive treatment and general principles of mamba bite management are discussed. To date there has been no reported case of confirmed and medically treated black mamba bite in a child so young.

Pseudonajatoxin b: unusual amino acid sequence of a lethal neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis

The complete amino acid sequence of pseudonajatoxin b, a basic neurotoxin from the venom of the Australian common brown snake, Pseudonaja textilis, was determined by automated Edman analysis of the reduced carboxymethylated polypeptide and of peptides derived by digestion of it with Staphylococcus aureus V8 proteinase. Pseudonajatoxin b consists of a single polypeptide chain of 71 amino acids with Mr 7762. The amino acid sequence showed considerable homology with postsynaptic long neurotoxins, but there were striking differences. Pseudonajatoxin b displayed relatively high lethality, LD50 15 micrograms/kg in mice.

Synthesis of prodan-phosphatidylcholine, a new fluorescent probe, and its interactions with pancreatic and snake venom phospholipases A2

A new fluorescent probe, prodan-PC, was synthesized by incubating thio-PC, a thiol ester analogue of phosphatidylcholine [1,2-bis(decanoylthio)-1,2-dideoxy-sn-glycero-3-phosphocholine], with acrylodan, a fluorescent thiol-reactive reagent [6-acryloyl-2-(dimethylamino)naphthalene], in the presence of phospholipase A2, which served to generate lysothio-PC in situ. Prodan-PC (PPC) showed maximum absorption in ethanol at 370 nm. The fluorescence emission spectrum showed maximum emission at 530 nm in water and at 498 nm in ethanol. In the presence of a saturating amount of phospholipase A2, the emission maximum shifted to about 470 nm. PPC showed a critical micellar concentration around 5 microM, with evidence of premicellar aggregation above 1 microM. Binding of PPC to Crotalus adamanteus phospholipase A2 was evidenced by an increase in emission at 480 nm and an increase in fluorescence anisotropy. An apparent dissociation constant of 0.323 microM was calculated for this enzyme complex. Binding was dependent on the presence of calcium ion and was abolished by blocking the active site with p-bromophenacyl bromide. Binding was also followed by energy transfer from tryptophan in the enzyme to PPC. Apparent dissociation constants for PPC complexes with phospholipases A2 from Naja naja naja and porcine pancreas and the prophospholipase A2 from porcine pancreas were 0.509, 0.107, and 0.114 microM, respectively. PPC was shown to inhibit the activity of pancreatic phospholipase A2 in thio-PC-sodium cholate mixed micelles. Inhibition studies were complicated because PPC can also serve as an activator of the snake venom enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological study on phospholipases A2 isolated from Naja mossambica mossambica venom

The pharmacological properties of three phospholipases A2 (CM-I, CM-II and CM-III) purified from Naja mossambica mossambica venom were studied. The order of their catalytic and indirect hemolytic potencies was CM-I = CM-II greater than CM-III. Among them, only CM-III had a direct hemolytic action on the guinea-pig RBC, which was greatly inhibited by heparin. In the chick biventer cervicis nerve- muscle preparation, both CM-II and CM-III caused neuromuscular blockade with a gradual contracture and a decreased sensitivity to ACh and KCl, whereas no complete neuromuscular block was observed with CM-I up to 30 micrograms/ml. In the mouse phrenic nerve-diaphragm preparation, these three PLA2s abolished twitches evoked by indirect stimulation earlier than those by direct stimulation. Contracture was also produced by CM-II and CM-III. However only the latter was inhibited by pretreatment with heparin. These PLA2s caused myonecrosis in the hind-leg muscle of the mouse when injected intramuscularly. From these results, it is concluded that all of these PLA2s are both neurotoxic and myotoxic.

Mixed phospholipid-cardiotoxin monomolecular films studied by intrinsic polarized surface fluorescence

Surface fluorescence spectroscopy was used to study mixed phospholipid-cardiotoxin monomolecular films. Using polarized incident light we were able to detect the intrinsic fluorescence emission at a wavelength of 339 nm of the single Trp11 in the cardiotoxin molecule. Its fluorescence intensity increased continuously upon film compression up to a maximal value reached at 42 mN x m-1 in surface pressure. A relative polarization index can be used as an indication of the orientation of the indole ring. A transition at around 25 +/- 5 mN x m-1 in the orientation of the Trp11 relative to the plane of the interface was clearly evidenced during film compression, in agreement with our independent data showing a flipping of the whole toxin molecule (Bougis et al., (1981), Biochemistry 20, 4915-4920).

Skeletal muscle necrosis induced by a phospholipase A2 isolated from the venom of the coral snake Micrurus nigrocinctus nigrocinctus

1. The venom of the coral snake Micrurus nigrocinctus nigrocinctus was fractionated by reverse-phase high performance liquid chromatography and an acidic myotoxic phospholipase A2 was purified to homogeneity. 2. After intramuscular injection, the toxin induced rapid and drastic myonecrosis, as serum creatine kinase levels increased markedly, reaching their highest values by 1.5 hr. 3. Ultrastructural observations indicate that the plasma membrane was the first structure to be affected, with the presence of focal disruptions in its integrity. 4. Myofilaments were hypercontracted and formed dense clumps. Sarcoplasmic reticulum integrity was lost, as evidenced by the presence of many small vesicles in the cellular space. 5. Some mitochondria were swollen, whereas others contained dense intracristal spaces and flocculent densities. Moreover, some had only one membrane. 6. In conclusion, pathogenesis of myonecrosis induced by this phospholipase A2 is similar to that induced by crude Micrurus nigrocinctus nigrocinctus venom.

Rate-determining step in phospholipase A2 mechanism. 18O isotope exchange determined by 13C NMR

H2(18)O isotope exchange into specifically 13C-labeled substrate was used to obtain information on the rate-limiting step in the action of the phospholipase A2 from the venom of the Indian cobra (Naja naja naja). Incorporation of 18O was detected by the effect of 18O on 13C chemical shifts in 13C NMR. The enzymatic hydrolysis of a micellar phosphatidylcholine analogue of platelet-activating factor 1-alkyl-2-[1-13C]lauroyl-sn-glycero-3-phosphorylcholine proceeds by an O-acyl cleavage of the sn-2 ester bond. The reaction was examined for simultaneous 18O incorporation into the substrate. No exchange was found, suggesting that the hydrolytic step is not followed by a higher energy transition state and that it or a step before it appears to be rate-limiting. Previous experiments on phosphatidylethanolamine activation indicate that kcat is altered but that the km remains the same upon activation, suggesting that the binding steps occurring before the hydrolytic step are not affected. This strongly suggests that the hydrolytic step is in fact the rate-limiting step under these conditions. The 13C, 18O NMR technique should be generally applicable to mechanistic questions of this type.

Studies in the rat of antibody-coated and N-ethylmaleimide-treated erythrocyte clearance by the spleen. I. Effects of in vivo complement activation

The splenic component of the mononuclear phagocyte system (MPS) was investigated in the rat using N-ethylmaleimide-treated erythrocytes (NEM) and erythrocytes coated with a monoclonal IgG2b antibody (R3/13) directed against the rat RT1Aa major histocompatibility antigen. Both cell suspensions were removed by the spleen, and their clearance times were significantly longer in splenectomized animals. The mean clearance times for the NEM-treated cells in both normal and cobra venom-treated rats were similar (19.1 +/- 1.1 min and 19.0 +/- 1.0 min, respectively) but differences were seen between the clearance of R3/13 antibody-sensitized cells in these two groups (normal rats 38.3 +/- 2.8 min and CVF-treated rats 51.7 +/- 4.2 min, P less than 0.02). Different receptors were also involved in the removal of these cells; in normal animals recognition entailed interaction with complement receptors, whereas in CVF-treated animals this was implemented by Fc receptors. Complement activation prolonged the clearance rates of both R3/13 cells and NEM cells in normal animals, but the effect of complement activation on the clearance of NEM-treated cells was achieved via changes in splenic blood flow. When this was prevented from taking place no effect was seen on the clearance of NEM cells, although the clearance of R3/13 cells was inhibited by the complement fragments generated by complement activation.

Estimation of the phospholipid distribution in the human platelet plasma membrane based on the effect of phospholipase A2 from Naja nigricollis

Human platelets in three physiological states were prepared. These states were the gel-filtered, the thrombin-induced shape-changed, and the thrombin-activated platelets. The phospholipid distributions in these three types of membrane were probed by using the basic phospholipase A2 of Naja nigricollis. This enzyme could penetrate through these membranes to hydrolyze all of their accessible phospholipids and to cause cell lysis. The hydrolytic time-courses displayed three phases. The state of platelet in each lipid hydrolytic phase was examined by: (1) measuring the leakage of lactate dehydrogenase; (2) analyzing the morphology by both scanning and transmission electron microscopy (scanning EM and transmission EM); and (3) estimating the hydrolysis of the [32P]phosphate-labeled platelets. The existence of these three hydrolytic phases may signify that the phospholipase A2 sequentially hydrolyzed its substrates in the membrane outer leaflet, in the inner one, and in the cytosol. The content and the distribution of each phospholipid class in the plasma membranes of the resting and of the shape-changed platelets were similar. These membrane surfaces consisted mainly of phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Phosphatidylserine (PS) was not exposed on the surface of the shape-changed platelet. The content of each lipid class in the activated platelet membrane was 10% more than that in the resting platelet. PS was found on the activated platelet cell surface. This implies that PS is exposed only during platelet secretion.

Proton nuclear magnetic resonance analyses of the molecular conformations of unique long neurotoxins bearing Phe-25: Astrotia stokesii b, Astrotia stokesii c, and Acanthophis antarcticus b

The 270-MHz proton NMR spectra of the unique long neurotoxins bearing Phe-25, Astrotia stokesii b (As b) and Astrotia stokesii c (As c) from Astrotia stokesii, and Acanthophis antarcticus b (Aa b) from Acanthophis antarcticus, have been analyzed. The aromatic proton resonances of Phe-25 in As b and Aa b were assigned on the basis of the nuclear Overhauser effects observed on irradiation of slowly exchanging amide protons. Phe-25 was found to be involved in hydrophobic interactions with Ile/Val-42, Ala-46 and Ile-58 in As b and As c, and with Ala-46 and Val-58 in Aa b. These hydrophobic interactions, instead of the hydrogen bond between Tyr-25 and Glu-42 found in other neurotoxins, appear to be important for maintenance of the biologically active tertiary structure. The pH dependency of the chemical shift and intensity of the Trp-72 N-1 proton resonance of As b indicates that the indole ring is not fully exposed to the solvent and that the extra tail segment of this long neurotoxin interacts with the main part of the molecule.