Purification of gyroxin from a South American rattlesnake (Crotalus durissus terrificus) venom

The PLA2 inhibitor from Crotalus durissus terrificus blood plasma (CNF) inhibits group III-PLA2 from honeybee venom

Crotalus neutralizing factor (CNF) is an endogenous glycoprotein from Crotalus durissus terrificus snake blood that inhibits secretory phospholipases A2 (sPLA2) from the Viperid but not from Elapid venoms (subgroups IA and IIA, respectively). In the present study, we demonstrated that CNF can inhibit group III-PLA2 from bee venom by forming a stable enzyme-inhibitor complex. This finding opens up new possibilities for the potential use of CNF and/or CNF-based derivatives in the therapeutics of bee stings.

South American snake venoms with abundant neurotoxic components. Composition and toxicological properties. A literature review

In South America there are three snake genera with predominantly neurotoxic venoms: Crotalus, Micrurus and Hydrophis, which include nine species/subspecies, 97 species and a single marine species, respectively. Although accidents with neurotoxic venoms are less frequent than those with anticoagulant, cytotoxic or necrotic venoms (e.g. from Bothrops), they are of major public health importance. Venoms from genus Crotalus have been extensively studied, while data on the venoms from the other two genera are very limited, especially for Hydrophis. The venoms of North and South American Crotalus species show biochemical and physiopathological differences. The former species cause bothrops-like envenomation symptoms, while the latter mainly have neurotoxic and myotoxic effects, leading to respiratory paralysis and, occasionally, renal failure by myoglobinuria and death, often with no local lesions. Micrurus and Hydrophis also cause neurotoxic envenomations. Many studies have isolated, identified and characterized new enzymes and toxins, thus expanding the knowledge of snake venom composition. The present review summarizes the currently available information on neurotoxic venoms from South American snakes, with a focus on protein composition and toxicological properties. It also includes some comments concerning potential medical applications of elapid and crotalic toxins.

Crotalus Neutralizing Factor (CNF) inhibits the toxic effects of Crotoxin at mouse neuromuscular preparations

Crotalus Neutralizing Factor (CNF) was the first phospholipase A₂ inhibitor isolated from the plasma of the South American rattlesnake (Crotalus durissus terrificus). Previous biochemical and biophysical studies demonstrate an interaction of CNF with Crotoxin (CTX), the main toxic component in the venom of these snakes. CTX promotes the blockade of neuromuscular transmission by a sum of neurotoxic and myotoxic activities. However, the ability of CNF to inhibit these activities has not been shown until the present study. We performed a myographic study to compare the neuromuscular effects of CTX and the mixture CTX plus CNF in mice phrenic nerve-diaphragm muscle preparations. CTX (5 μg/mL) alone, or pre-incubated with CNF (5, 20 or 50 μg/mL) for 15 min was added to the preparations and maintained throughout the experimentation period. Myotoxicity was assessed by light microscopic analysis of diaphragm muscle after myographic study. CTX (5 μg/mL) blocked both indirectly and directly evoked twitches in neuromuscular preparations. In addition, CTX induced histological alterations in diaphragm muscle. Pre-incubation with CNF (50 μg/mL) abolished both the muscle-paralyzing and muscle-damaging activities of CTX. Therefore, the present study confirms, through functional studies, the antiophidic potential of CNF.

Geographic variation of individual venom profile of Crotalus durissus snakes

Background:

South American rattlesnakes are represented in Brazil by a single species, Crotalus durissus, which has public health importance due to the severity of its envenomation and to its wide geographical distribution. The species is subdivided into several subspecies, but the current classification is controversial. In Brazil, the venoms of C. d. terrificus and C. d. collilineatus are used for hyperimmunization of horses for antivenom production, even though the distinction of these two subspecies are mostly by their geographical distribution. In this context, we described a comparative compositional and functional characterization of individual C. d. collilineatus and C. d. terrificus venoms from three Brazilian states.

Methods:

We compared the compositional patterns of C. d. terrificus and C. d. collilineatus individual venoms by 1-DE and RP-HPLC. For functional analyzes, the enzymatic activities of PLA₂, LAAO, and coagulant activity were evaluated. Finally, the immunorecognition of venom toxins by the crotalic antivenom produced at Butantan Institute was evaluated using Western blotting.

Results:

The protein profile of individual venoms from C. d. collilineatus and C. d. terrificus showed a comparable overall composition, despite some intraspecific variation, especially regarding crotamine and LAAO. Interestingly, HPLC analysis showed a geographic pattern concerning PLA₂. In addition, a remarkable intraspecific variation was also observed in PLA₂, LAAO and coagulant activities. The immunorecognition pattern of individual venoms from C. d. collilineatus and C. d. terrificus by crotalic antivenom produced at Butantan Institute was similar.

Conclusions:

The results highlighted the individual variability among the venoms of C. durissus ssp. specimens. Importantly, our data point to a geographical variation of C. durissus ssp. venom profile, regardless of the subspecies, as evidenced by PLA₂ isoforms complexity, which may explain the increase in venom neurotoxicity from Northeastern through Southern Brazil reported for the species.

Heterologous fibrin sealant derived from snake venom: from bench to bedside - an overview

Hemostatic and adhesive agents date back to World War II, when homologous fibrin sealant came onto scene. Considering that infectious diseases can be transmitted via human blood, a new heterologous fibrin sealant was standardized in the 1990s. Its components were a serine protease (a thrombin-like enzyme) extracted from the venom of Crotalus durissus terrificus snakes and a fibrinogen-rich cryoprecipitate extracted from the blood of Bubalus bubalis buffaloes. This new bioproduct has been used as a coagulant, sealant, adhesive and recently as a candidate scaffold for mesenchymal stem cells and bone and cartilage repair. This review discusses the composition of a new heterologous fibrin sealant, and cites published articles related to its preclinical applications aiming at repairing nervous system traumas and regenerating bone marrow. Finally, we present an innovative safety trial I/II that found the product to be a safe and clinically promising candidate for treating chronic venous ulcers. A multicenter clinical trial, phase II/III, with a larger number of participants will be performed to prove the efficacy of an innovative biopharmaceutical product derived from animal venom.

Rapid purification and procoagulant and platelet aggregating activities of Rhombeobin: a thrombin-like/gyroxin-like enzyme from Lachesis muta rhombeata snake venom

We report a rapid purification method using one-step chromatography of SVSP Rhombeobin (LMR-47) from Lachesis muta rhombeata venom and its procoagulant activities and effects on platelet aggregation. The venom was fractionated by a single chromatographic step in RP-HPLC on a C8 Discovery BIO Wide Pore, showing high degree of molecular homogeneity with molecular mass of 47035.49 Da. Rhombeobin showed amidolytic activity upon BA ρ NA, with a broad optimum pH (7-10) and was stable in solution up to 60°C. The amidolytic activity was inhibited by serine proteinase inhibitors and reducing agents, but not chelating agents. Rhombeobin showed high coagulant activity on mice plasma and bovine fibrinogen. The deduced amino acid sequence of Rhombeobin showed homology with other SVSPs, especially with LM-TL (L. m. muta) and Gyroxin (C. d. terrificus). Rhombeobin acts, in vitro, as a strong procoagulant enzyme on mice citrated plasma, shortening the APTT and PT tests in adose-dependent manner. The protein showed, "ex vivo", a strong defibrinogenating effect with 1 µg/animal. Lower doses activated the intrinsic and extrinsic coagulation pathways and impaired the platelet aggregation induced by ADP. Thus, this is the first report of a venom component that produces a venom-induced consumptive coagulopathy (VICC).

A new fibrin sealant from Crotalus durissus terrificus venom: applications in medicine

Fibrin sealant, a widely available tissue adhesive, has been used since 1940 in a variety of clinical applications. Commercially available fibrin sealant products are synthesized from bovine thrombin and human fibrinogen, which may transmit infectious diseases, and recipients may also develop antibodies against bovine thrombin. Bearing these disadvantages in mind, a new fibrin sealant was developed in 1989 by a group of researchers from the Center for the Study of Venoms and Venomous Animals, in Sao Paulo State, Brazil. The main purpose was to produce an adhesive fibrin without using human blood, to avoid transmitting infectious diseases. The components of this novel sealant were extracted from large animals and a serine proteinase extracted from Crotalus durissus terrificus snake venom. The applicability of this sealant was tested in animals and humans with beneficial results. The new fibrin sealant can be a useful tool clinically due to its flexibility and diversity of applications. This sealant is a biological and biodegradable product that (1) does not produce adverse reactions, (1) contains no human blood, (3) has a good adhesive capacity, (4) gives no transmission of infectious diseases, and (5) may be used as an adjuvant in conventional suture procedures. The effectiveness of this new fibrin sealant is reviewed and its development and employment are described.

The co-purification of a lectin (BJcuL) with phospholipases A2 from Bothrops jararacussu snake venom by immunoaffinity chromatography with antibodies to crotoxin

Antigens of Bothrops jararacussu snake venom cross-reacting with specific antibodies against crotoxin, an Asp49 PLA(2)-containing heterodimeric complex from Crotalus durissus terrificus snake venom, were purified by two steps of immunoaffinity chromatography. The resulting fraction (Bj-F) was shown to be non-toxic (to mice and rabbits) and immunogenic to rabbits. Antibodies raised against Bj-F were able to protect mice against the lethal effect of both B. jararacussu and Crotalus durissus terrificus snake venoms. Then, the procedure developed showed to be useful for the rapid preparation of an antigen able to elicit neutralizing antibodies against the lethal activities of both venoms. Further fractionation of Bj-F revealed the concomitant presence of two major components: BJcuL, a lectin present in B. jararacussu venom, and BthTX-I, a Lys49 PLA(2) homolog, besides other molecules in minor amounts. Our data are discussed and raise the point that the presence of unrelated molecules may be taken into account when immuno-based methods are considered for purification purposes.

Inhibition of crotoxin binding to synaptosomes by a receptor-like protein from Crotalus durissus terrificus (the South American rattlesnake)

Crotoxin (Ctx) is a potent neurotoxin of the venom of Crotalus durissus terrificus (the South American rattlesnake). Ctx is a heterodimer composed of CB, a toxic PLA(2) subunit, and CA, a non-toxic and non-enzymatic subunit, that potentiates the neurotoxicity of CB in vivo. The deleterious action of Ctx upon C. d. terrificus snakes themselves is known to be prevented by a PLA(2) inhibitor (CNF) present in their blood serum. CNF acts by replacing CA in Ctx, thus forming a new stable complex CNF-CB. This complex no longer interacts with the target receptor (TR) to deliver CB to cause its lethal effect. Furthermore, CNF-CB seems to be reminiscent of the interaction Ctx-TR at the pre-synaptic site. In the present work, the binding competition between rat brain synaptosomes (TR) and CNF for Ctx was investigated. Radiolabeled Ctx, made of CA and one isoform of CB (CA-(125)ICB(2)), was used as ligand. The competition by unlabeled Ctx was taken as a reference. The potency of CNF as a competitor was evaluated under different incubation conditions with varying time scale addition of reagents (CA-(125)ICB(2), synaptosomes and CA-CB(2) or CNF). CNF was able to inhibit the binding of the toxin to synaptosomes as well as to partially displace the toxin already bound to its membrane target. The mechanisms of competition involved were discussed and a previous schematic model of interactions between Ctx, TR and CNF was updated.

Snake venom serine proteinases: sequence homology vs. substrate specificity, a paradox to be solved

Snake venom glands synthesize a variety of serine proteinases capable of affecting the haemostatic system. They act on macromolecular substrates of the coagulation, fibrinolytic, and kallikrein-kinin systems, and on platelets to cause an imbalance of the haemostatic system of the prey. In this review we describe their biochemical/biophysical characteristics, biological activities as well as aspects of their evolution and structure-activity relationship.

Primary structure, behavioral and electroencephalographic effects of an epileptogenic peptide from the sea anemone Bunodosoma cangicum

The primary structure of cangitoxin (CGX), a 4958 Da peptide from the sea anemone Bunodosoma cangicum, was determined: GVACRCDSDGPTVRGNSLSGTLWLTGGCPSGWHNCRGSGPFIGYCCKK. CGX contains all the 11 residues that are conserved and the 5 that are conservatively substituted within or between the type 1 and type 2 sequences of sea anemone peptides with specific action on voltage-sensitive sodium channels. Furthermore, it also has 6 identities (Asp9, Arg14, Asn16, Leu18, Trp33 and Lys48) and 1 homology (Arg36) in the 8 residues of the pharmacophore of the sea anemone ApB which are essential for interaction with mammalian sodium channels. The intrahippocampal injection of CGX induces several sequential behavioral alterations--episodes of akinesia alternating with facial automatisms and head tremor, salivation, rearing, jumping, barrel-rolling, wet dog shakes and forelimb clonic movements--and the electroencephalography analysis shows that they were followed by important seizure periods that gradually evolved to status epilepticus that lasted 8-12 h, similar to that observed in the acute phase of the pilocarpine model of epilepsy. These results suggest that CGX may be an important tool to develop a new experimental model of status epilepticus which may contribute to understanding the etiology of epilepsy and to test the effects of new antiepileptic drugs.

Analysis of fatty acids released by crotoxin in rat brain synaptosomes

Crotoxin, the main toxin of Crotalus durissus terrificus venom, exerts its lethal effect by blocking neurotransmission at the neuromuscular junction level through a triphasic mechanism. This effect seems to depend on its phospholipasic activity, suggesting that the mechanism of neurotransmission blockage may be related to fatty acids release in specific sites of the nervous terminal. In this work, we purified the fatty acids released by crotoxin's activity and this outline was compared with other phospholipases A(2), including CB, a subunit of crotoxin. Our results show a higher release of palmitate and arachidonate by crotoxin when compared to other phospholipases A(2). Since palmitate has a role in protein acylation processes and arachidonate participates in signal transduction events, these mechanisms may be related to the neurotoxic actions of crotoxin.

Gyroxin fails to modify in vitro release of labelled dopamine and acetylcholine from rat and mouse striatal tissue

Gyroxin fails to modify in vitro release of labelled dopamine and acetylcholine from rat and mouse striatal tissue. Gyroxin is a thrombin-like peptide with amidasic, esterasic and fibrinogenolitic activities, found in the venom of snakes like Lachesis muta muta and Crotalus durissus terrificus. Intravenous injections of small doses of gyroxin induce a typical barrel rotation behaviour that has been thought to be a neurotoxic effect. The aim of this study was to determine whether gyroxin-induced barrel rotation behaviour involves changes in neurotransmitter release. Gyroxin was isolated from crude venoms by gel filtration and affinity chromatography. Its properties were determined by assaying esterasic, amidasic and fibrinogenolitic enzymatic activities and tested for barrel rotation behaviour. Neurotransmitter release tests employed rat and mouse superfused brain striatal chopped tissue preloaded with [(3)H]-dopamine, [(3)H]-acetylcholine or in a double labelling procedure. They were stimulated by 20mM K(+) in control conditions or in the presence of several concentrations of toxins. Crotoxin and crotamine were used as positive controls. Gyroxins failed at modifying both basal and stimulated neurotransmitter releases, suggesting a lack of direct neurotoxic effect. We therefore suggest that gyroxin may not be a neurotoxin but rather, induces this behavioural syndrome by other means possibly related to haemodynamic disturbance. The possible role of vasopressin is discussed.

Behavioral and electroencephalographic analysis of seizures induced by intrahippocampal injection of granulitoxin, a neurotoxic peptide from the sea anemone Bunodosoma granulifera

In this study, the behavioral and electroencephalographic (EEG) analysis of seizures induced by the intrahippocampal injection in rats of granulitoxin, a neurotoxic peptide from the sea anemone Bunodosoma granulifera, was determined. The first alterations occurred during microinjection of granulitoxin (8 microg) into the dorsal hippocampus and consisted of seizure activity that began in the hippocampus and spread rapidly to the occipital cortex. This activity lasted 20-30 s, and during this period the rats presented immobility. During the first 40-50 min after its administration, three to four other similar short EEG seizure periods occurred and the rats presented the following behavioral alterations: akinesia, facial automatisms, head tremor, salivation, rearing, jumping, barrel-rolling, wet dog shakes and forelimb clonic movements. Within 40-50 min, the status epilepticus was established and lasted 8-12 h. These results are similar to those observed in the acute phase of the pilocarpine model of temporal lobe epilepsy and suggest that granulitoxin may be a useful tool not only to study the sodium channels, but also to develop a new experimental model of status epilepticus.

Encapsulation of native crotoxin in liposomes: a safe approach for the production of antivenom and vaccination against Crotalus durissus terrificus venom

Crotoxin, the neurotoxic component of Crotalus durissus terrificus (Cdt) venom that displays phospholipase A2 activity, was successfully encapsulated into dehydration-rehydration vesicles (DRV/crotoxin) and reverse-phase evaporation vesicles (REV/crotoxin) made from sphingomyelin and cholesterol. The encapsulation efficiency of native crotoxin was higher in DRV/crotoxin than in REV/crotoxin. DRV/crotoxin was not toxic when i.v. inoculated in mice at a dose of crotoxin as high as 91 times its L.D50 or when s.c. inoculated at 42 times its LD50. On the other hand, crotoxin released from DRV/crotoxin retained its original toxicity. REV/crotoxin was found to be at least 1.9 times more toxic than DRV/crotoxin. The fact that DRV/crotoxin retained crotoxin more efficiently than REV/crotoxin may account for the difference in acute toxicity between the two preparations. DRV/crotoxin, when s.c. inoculated in mice, induced anti-crotoxin antibodies that protected animals against the lethal effect of Cdt venom. Following immunization with three doses of DRV/crotoxin (3 x 20 micrograms of crotoxin/mouse) and challenge with 8 x LD50 of Cdt venom, 75% of mice were protected. The DRV/crotoxin preparation was compared to crotoxin emulsified in Freund's adjuvant (FCA/crotoxin). DRV/crotoxin was found to be less toxic than FCA/crotoxin, and to induce lower levels of anti-crotoxin antibodies but similar levels of protection when inoculated at high doses (20 or 70 micrograms crotoxin/mouse). When DRV/crotoxin was adsorbed to alum at the time of immunization, it induced antibody and protection levels comparable to those produced by FCA/crotoxin.

Different sensitivity of fast- and slow-twitch muscles to some snake venoms and myotoxins

We examined the effect of some crude snake venoms, isolated toxins and non-specific cytotoxic agents on isolated extensor digitorum longus (EDL) and soleus (SOL) muscles of the mouse. The muscles were continuously perfused with a physiological saline solution. Crude venoms from Crotalus viridis viridis, Agkistrodon contortrix laticinctus and Notechis scutatus scutatus were tested at a concentration of 25-50 micrograms ml-1. The increase in the rate of creatine kinase (CK) release (above basal levels) induced in each muscle by each venom or toxin was measured. Also, the myotoxic effect of these agents was investigated with the light microscope. EDL and SOL had the same range of basal rate of CK release (0.30 +/- 0.06 U g-1 hr-1, N = 26), weight (7-10 mg) and content of CK (717.18 +/- 80.19 U g-1 and 501.00 +/- 62.28 U g-1, N = 8), but they had a different sensitivity to the myotoxic action of the tested venoms. The rate of CK release in EDL muscles was in the range of 24-60 U g-1 hr-1 after 60 min of exposure to 25 micrograms ml-1 of each crude venom, whereas the increase of rate of CK release in the SOL was in the range of 1.5-4.0 U g-1 hr-1. Crotoxin and myotoxin a (10 and 25 micrograms ml-1, respectively) were also more effective in EDL than in SOL muscles. The non-specific cytotoxic agents Triton X-100 (0.01%) and polylysine (100 micrograms ml-1) induced the same increase of rate of CK release in both muscles. The data presented in this article show that isolated murine EDL muscles are more sensitive than SOL to the myotoxic action of some snake venoms and toxins.

VRCTC-310--a novel compound of purified animal toxins separates antitumor efficacy from neurotoxicity

Two purified animal venom toxins, crotoxin and cardiotoxin, have been combined to produce a unique natural product (VRCTC-310) currently under investigation as an antitumor agent by the National Cancer Institute. In vitro, it has demonstrated cytotoxic disease specificity and a unique mechanism of action when submitted to COMPARE analysis. In vivo, tolerance was developed to the neurotoxic properties of crotoxin which allowed comparison of several schedules of fixed and escalating daily i.m. doses to mice bearing s.c. Lewis Lung carcinoma. An 83% inhibition of tumor growth was achieved using an escalating dose schedule starting at 1.8 mg/kg and reaching 6.3 mg/kg/day on day 20. Although some irritation around the sites of i.m. injection was noted, animal weight loss was negligible and there were no other signs of adverse toxicity. This natural product represents a new, membrane interactive anticancer agent which produces a unique spectrum of cytotoxicity in vitro and which has demonstrated interesting in vivo antitumor efficacy.

Protection against the lethal effects of Crotalus durissus terrificus (South American rattlesnake) venom in animals immunized with crotoxin

Mice and rabbits were immunized against crotoxin (the neurotoxic component isolated from Crotalus durissus terrificus venom) using small amounts of antigen in a water-in-oil emulsion. Following boosting (three times at 21-day intervals) a high titre of antibodies against crotoxin was obtained. Crotoxin immunoglobulin G antibody recognizes whole venom antigen at a level comparable with that of crotoxin antigen, using the ELISA method for antibody detection. The antibodies generated by crotoxin were capable of providing 100% protection against challenge with 11 and 50 i.p. LD50 doses of whole venom in mice. When 100 i.p. LD50 doses of whole venom were injected survival was 77.8%.

Preliminary fractionation of tiger rattlesnake (Crotalus tigris) venom

Tiger rattlesnake (Crotalus tigris) venom was fractioned by using fast protein liquid chromatography (FPLC). The crude venom had low protease activity, lacked hemolytic activity and had an i.p. LD50 of 0.070 mg/kg for mice. Lethal fractions obtained by anion and cation exchange were examined for antigenic identity with crotoxin and Mojave toxin. Four toxins were obtained by anion exchange chromatography which showed immunoidentity with these toxins, and one fraction caused rear limb paresis in mice. A lethal toxin (about 10% of total venom protein) purified further with Superose-12 FPLC (molecular sieve) had an i.p. LD50 of 0.050 mg/kg for mice, reacted strongly with anti-crotoxin and anti-Mojave toxin antiserum in ELISA and immunoelectrophoresis. This toxin also showed complete immunoidentity with crotoxin and Mojave toxin in immunodiffusion assays with anti-crotoxin antiserum. The results indicated the presence of crotoxin and/or Mojave toxin isoforms in this venom. Although this species has a low venom yield (average 10 mg per snake), the venom is highly toxic and contains high concentrations of several neurotoxic isotoxins.

A gyroxin analog from the venom of the bushmaster (Lachesis muta muta)

Clinical observations of possible neurotoxic activity in bushmaster (Lachesis muta muta) envenomations, coupled with the accepted ancestral relationship of Lachesis to other crotalids, suggested that Lachesis venom might contain a crotoxin-like molecule. Crude venom and gel-filtration fractions showed modest reactivity in enzyme-linked immunosorbent assays using rabbit polyclonal antibodies raised against the basic subunit of crotoxin, but no reaction was detected with a murine monoclonal antibody raised against the same antigen. Phospholipase assays, LD50 determinations and SDS-polyacrylamide gel electrophoresis indicated the presence of non-toxic phospholipases, but no crotoxin homologs. A higher mol.wt, toxic protein (60,000) with an LD50 of 0.07 micrograms/g in mice was isolated and purified, which induced gyroxin-like, rapid rolling motions in mice. Its amino terminal sequence shows considerable amino acid sequence identity with gyroxin from the venom of Crotalus durissus terrificus and other serine proteases.

Gyroxin, a toxin from the venom of Crotalus durissus terrificus, is a thrombin-like enzyme

We report a simple method for the isolation of gyroxin, a protein from the venom of the South American rattlesnake Crotalus durissus terrificus. The intravenous injection of gyroxin into mice produces temporary episodes characterized by opisthotonos and rotations around the long axis of the animal. We found gyroxin to be a glycoprotein with thrombin-like and esterase activities. Gyroxin loses its ability to produce the gyroxin syndrome, its thrombin-like activity and its esterase activity with heat, dithiothreitol, phenylmethylsulfonyl fluoride or diisopropylfluorophosphate. We also report that three other thrombin-like enzymes, crotalase from the eastern diamondback rattlesnake (Crotalus adamanteus), ancrod from the Malayan pit viper (Agkistrodon rhodostoma) and a thrombin-like enzyme from the Central American rattlesnake (Crotalus durissus durissus), produce the gyroxin syndrome in mice. These enzymes may work by releasing neuroactive peptides from endogenous precursors.

The mechanism of inhibition of phospholipase activity of crotoxin B by crotoxin A

In the crotoxin complex isolated from Crotalus durissus terrificus venom, the component A inhibits the phospholipase A2 activity of crotoxin B only when the substrate is in the aggregated form, preventing the interaction of the enzyme with lecithin--water interfaces. In contrast, with similar rates of hydrolysis of dihexanoyllecithin monomers, the activity of the crotoxin complex is lower than that of crotoxin B when the substrate is aggregated into micelles. Crotoxin B readily hydrolyses dimyristoyllecithin vesicles, the rate being modulated by the physical state of the phospholipid, suggesting that the enzyme is tightly bound to the interface. With the crotoxin complex the rate of vesicle hydrolysis is much slower (about 1/10 that of crotoxin B) and is little affected by the physical state of the lecithin. Direct binding experiments demonstrate that, in contrast to crotoxin B, the crotoxin complex is unable to interact with lecithin--water interfaces. Together with the free accessibility of the enzyme active site in the crotoxin complex, this evidence suggests that a specific area on the enzyme surface, different from the active site and shielded by crotoxin A in the complex, is responsible for the interaction of crotoxin B with lipid--water interfaces.

Accessibility of the active site of crotoxin B in the crotoxin complex

Basic phospholipases A and the crotoxin complex isolated from Crotalus durissus terrificus venom exhibited similar initial reaction rates, time course and degree of hydrolysis of synthetic short chain lecithins in the monomeric state. Although monomeric lecithins seem to promote dissociation of crotoxin up to a certain extent, this cannot explain the high activity observed with the complex. The crotoxin complex is able to bind the non-hydrolyzable analog D-diheptanoyllecithin, as demonstrated by equilibrium gel-filtration, with a dissociation constant of 0.12 mM. This value is similar to the dissociation constant of the crotoxin B-D-diheptanoyllecithin complex (about 0.13 mM), estimated from the protection against enzyme inactivation by p-bromophenacyl bromide, which further supports the free accessibility of the substrate to the enzyme active site in the crotoxin complex. The lack of enzyme inactivation when crotoxin is treated with p-bromophenacyl bromide may be interpreted in terms of the specific requirements of the reagent to react with the enzyme rather than protection of the active site. Crotoxin B inhibition by complex formation with crotoxin A, which is not apparent on monomeric substrates, seems not to involve the active site of the enzyme.