Naja atra cardiotoxins enhance the protease activity of chymotrypsin

Specific Amino Acid Residues in the Three Loops of Snake Cytotoxins Determine Their Membrane Activity and Provide a Rationale for a New Classification of These Toxins

Cytotoxins (CTs) are three-finger membrane-active toxins present mainly in cobra venom. Our analysis of the available CT amino acid sequences, literature data on their membrane activity, and conformational equilibria in aqueous solution and detergent micelles allowed us to identify specific amino acid residues which interfere with CT incorporation into membranes. They include Pro9, Ser28, and Asn/Asp45 within the N-terminal, central, and C-terminal loops, respectively. There is a hierarchy in the effect of these residues on membrane activity: Pro9 > Ser28 > Asn/Asp45. Taking into account all the possible combinations of special residues, we propose to divide CTs into eight groups. Group 1 includes toxins containing all of the above residues. Their representatives demonstrated the lowest membrane activity. Group 8 combines CTs that lack these residues. For the toxins from this group, the greatest membrane activity was observed. We predict that when solely membrane activity determines the cytotoxic effects, the activity of CTs from a group with a higher number should exceed that of CTs from a group with a lower number. This classification is supported by the available data on the cytotoxicity and membranotropic properties of CTs. We hypothesize that the special amino acid residues within the loops of the CT molecule may indicate their involvement in the interaction with non-lipid targets.

Effects of Cobra Cardiotoxins on Intracellular Calcium and the Contracture of Rat Cardiomyocytes Depend on Their Structural Types

Cardiotoxins (CaTx) of the three-finger toxin family are one of the main components of cobra venoms. Depending on the structure of the N-terminal or the central polypeptide loop, they are classified into either group I and II or P- and S-types, respectively, and toxins of different groups or types interact with lipid membranes variably. While their main target in the organism is the cardiovascular system, there is no data on the effects of CaTxs from different groups or types on cardiomyocytes. To evaluate these effects, a fluorescence measurement of intracellular Ca2+ concentration and an assessment of the rat cardiomyocytes' shape were used. The obtained results showed that CaTxs of group I containing two adjacent proline residues in the N-terminal loop were less toxic to cardiomyocytes than group II toxins and that CaTxs of S-type were less active than P-type ones. The highest activity was observed for Naja oxiana cobra cardiotoxin 2, which is of P-type and belongs to group II. For the first time, the effects of CaTxs of different groups and types on the cardiomyocytes were studied, and the data obtained showed that the CaTx toxicity to cardiomyocytes depends on the structures both of the N-terminal and central polypeptide loops.

Effect of Several Naja atra Antivenom Injection Methods on the Rabbit Model of Naja naja atra Bite Poisoning

Snakebite is a global public health concern, which often occurs in tropical and subtropical underdeveloped areas, but it is often neglected. In the southern China, Naja naja atra (Chinese cobra) is a common venomous snake that causes swelling and necrosis of local tissues, even amputation and death. Currently, the main therapy is the administration of Naja atra antivenom, which greatly reduces mortality. However, the antivenom is not particularly effective in the improvement of local tissue necrosis. Clinically, antivenom is mainly administered intravenously. We speculated that the method of injection influences the efficacy of antivenom. In this study, the rabbit model was used to explore the effects of different antivenom injection methods on systemic and local poisoning symptoms. If topical injection of antivenom contributes to ameliorate tissue necrosis, then we need to reconsider the use of Naja atra antivenom.

Potential toxicity of bisphenol A to α-chymotrypsin and the corresponding mechanisms of their binding

Bisphenol A (BPA) is an endocrine disruptor widely existing in plastics and resins, which can accumulate in animals and human bodies, posing a potential threat to the physiological and biochemical reactions of human beings or other organisms. α-Chymotrypsin is a kind of proteolytic enzyme existing in humans and animals, which can cause diseases when its activity is excessive. However, there is a lack of research on the mechanism of endocrine disruptors affecting α-chymotrypsin activity. In this study, the interaction between BPA and α-chymotrypsin was proved via multiple spectroscopic approaches, enzyme activity change, isothermal titration calorimetry and molecular docking. Results showed that α-chymotrypsin's polypeptide chains were unfolded, and protein skeletons were loosened with the exposure to BPA. α-Helix content increased and β-sheet content was decreased. The particle size of the BPA-α-chymotrypsin complex became smaller. Fluorescence sensitization may also be explained by a perturbation of the chromophore Trp 141. The thermodynamic parameters of the binding reaction were measured by isothermal titration calorimetry (ITC), which showed that there was hydrophobic interaction between BPA and α-chymotrypsin, which was consistent with the results of molecular docking. Moreover, BPA may stop near the active center of α-chymotrypsin and interact with the key residues His 57 and Ser 195. The above phenomenon explained the result that the activity of α-chymotrypsin increased to 139% when exposed to high dose BPA (40 μM). Taken together, the effects of BPA on the structure and function of α-chymotrypsin were clarified at the molecular level, which made up the gap in the mechanism of BPA on the proteolytic enzyme, and provided a reliable basis for disease avoidance and prevention.

Naja atra Cardiotoxin 1 Induces the FasL/Fas Death Pathway in Human Leukemia Cells

This study aimed to investigate the mechanistic pathway of Naja atra (Taiwan cobra) cardiotoxin 1 (CTX1)-induced death of leukemia cell lines U937 and HL-60. CTX1 increased cytoplasmic Ca²⁺ and reactive oxygen species (ROS) production, leading to the death of U937 cells. It was found that Ca²⁺-induced NOX4 upregulation promoted ROS-mediated p38 MAPK phosphorylation, which consequently induced c-Jun and ATF-2 phosphorylation. Using siRNA knockdown, activated c-Jun and ATF-2 were demonstrated to regulate the expression of Fas and FasL, respectively. Suppression of Ca²⁺-mediated NOX4 expression or ROS-mediated p38 MAPK activation increased the survival of U937 cells exposed to CTX1. FADD depletion abolished CTX1-induced cell death, caspase-8 activation, and t-Bid production, supporting the correlation between the Fas death pathway and CTX1-mediated cytotoxicity. Among the tested N. atra CTX isotoxins, only CTX1 induced Fas and FasL expression. Chemical modification studies revealed that intact Met residues were essential for the activity of CTX1 to upregulate Fas and FasL expression. Taken together, the data in this study indicate that CTX1 induces c-Jun-mediated Fas and ATF-2-mediated FasL transcription by the Ca²⁺/NOX4/ROS/p38 MAPK axis, thereby activating the Fas death pathway in U937 cells. Furthermore, CTX1 activates Fas/FasL death signaling in the leukemia cell line HL-60.

Blocking of negative charged carboxyl groups converts Naja atra neurotoxin to cardiotoxin-like protein

Naja atra cobrotoxin and cardiotoxin 3 (CTX3) exhibit neurotoxicity and cytotoxicity, respectively. In the present study, we aimed to investigate whether the carboxyl groups of cobrotoxin play a role in structural constraints, thereby preventing cobrotoxin from exhibiting cytotoxic activity. Six of the seven carboxyl groups in cobrotoxin were conjugated with semicarbazide. Measurement of circular dichroism spectra and Trp fluorescence quenching showed that the gross conformation of semicarbazide-modified cobrotoxin (SEM-cobrotoxin) and cobrotoxin differed. In sharp contrast to cobrotoxin, SEM-cobrotoxin demonstrated membrane-damaging activity and cytotoxicity, which are feature more characteristic of CTX3. Furthermore, both SEM-cobrotoxin and CTX3 induced cell death through AMPK activation. Analyses of the interaction between polydiacetylene/lipid vesicles and fluorescence-labeled lipids revealed that SEM-cobrotoxin and cobrotoxin adopted different membrane-bound states. The structural characteristics of SEM-cobrotoxin were similar to those of CTX3, including trifluoroethanol (TFE)-induced structural transformation and membrane binding-induced conformational change. Conversely, cobrotoxin was insensitive to the TFE-induced effect. Collectively, the data of this study indicate that blocking negatively charged residues confers cobrotoxin with membrane-damaging activity and cytotoxicity. The findings also suggest that the structural constraints imposed by carboxyl groups control the functional properties of snake venom α-neurotoxins during the divergent evolution of snake venom neurotoxins and cardiotoxins.

Status of Asp29 and Asp40 in the Interaction of Naja atra Cardiotoxins with Lipid Bilayers

It is widely accepted that snake venom cardiotoxins (CTXs) target the plasma membranes of cells. In the present study, we investigated the role of Asp residues in the interaction of Naja atra cardiotoxin 1 (CTX1) and cardiotoxin 3 (CTX3) with phospholipid bilayers using chemical modification. CTX1 contains three Asp residues at positions 29, 40, and 57; CTX3 contains two Asp residues at positions 40 and 57. Compared to Asp29 and Asp40, Asp57 was sparingly modified with semi-carbazide, as revealed by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass and mass/mass analyses. Thus, semi-carbazide-modified CTX1 (SEM-CTX1) mainly contained modified Asp29 and Asp40, while SEM-CTX3 contained modified Asp40. Compared to that of native toxins, trifluoroethanol easily induced structural transition of SEM-CTX1 and SEM-CTX3, suggesting that the structural flexibility of CTXs was constrained by Asp40. Modification of Asp29 and Asp40 markedly promoted the ability of CTX1 to induce permeability of cell membranes and lipid vesicles; CTX3 and SEM-CTX3 showed similar membrane-damaging activity. Modification of Asp residues did not affect the membrane-binding capability of CTXs. Circular dichroism spectra of SEM-CTX3 and CTX3 were similar, while the gross conformation of SEM-CTX1 was distinct from that of CTX1. The interaction of CTX1 with membrane was distinctly changed by Asp modification. Collectively, our data suggest that Asp29 of CTX1 suppresses the optimization of membrane-bound conformation to a fully active state and that the function of Asp40 in the structural constraints of CTX1 and CTX3 is not important for the manifestation of membrane-perturbing activity.

Perspective on chymotrypsin detection

Chymotrypsin is one of the most extensively known proteases participating in the pathogenesis of various diseases, which can be used in drug discovery and clinical diagnosis.

Naja atra Cardiotoxin 3 Elicits Autophagy and Apoptosis in U937 Human Leukemia Cells through the Ca2+/PP2A/AMPK Axis

Cardiotoxins (CTXs) are suggested to exert their cytotoxicity through cell membrane damage. Other studies show that penetration of CTXs into cells elicits mitochondrial fragmentation or lysosome disruption, leading to cell death. Considering the role of AMPK-activated protein kinase (AMPK) in mitochondrial biogenesis and lysosomal biogenesis, we aimed to investigate whether the AMPK-mediated pathway modulated Naja atra (Taiwan cobra) CTX3 cytotoxicity in U937 human leukemia cells. Our results showed that CTX3 induced autophagy and apoptosis in U937 cells, whereas autophagic inhibitors suppressed CTX3-induced apoptosis. CTX3 treatment elicited Ca²⁺-dependent degradation of the protein phosphatase 2A (PP2A) catalytic subunit (PP2Acα) and phosphorylation of AMPKα. Overexpression of PP2Acα mitigated the CTX3-induced AMPKα phosphorylation. CTX3-induced autophagy was via AMPK-mediated suppression of the Akt/mTOR pathway. Removal of Ca²⁺ or suppression of AMPKα phosphorylation inhibited the CTX3-induced cell death. CTX3 was unable to induce autophagy and apoptosis in U937 cells expressing constitutively active Akt. Met-modified CTX3 retained its membrane-perturbing activity, however, it did not induce AMPK activation and death of U937 cells. These results conclusively indicate that CTX3 induces autophagy and apoptosis in U937 cells via the Ca²⁺/PP2A/AMPK axis, and suggest that the membrane-perturbing activity of CTX3 is not crucial for the cell death signaling pathway induction.