Abstract

Enzymatic and immunological properties of Bungarus flaviceps (red-headed krait) venom

Tan NH ^I; Fung SY ^I; Ponnudurai G^II

^IDepartment of Molecular Medicine, School of Medicine, University of Malaya, Kuala Lumpur, Malaysia

IIInternational Medical University, Kuala Lumpur, Malaysia

^{Correspondence to} Correspondence to: Nget Hong Tan Department of Molecular Medicine, School of Medicine, University of Malaya Kuala Lumpur, Malaysia Phone: +60 3 79674912. Fax: +60 3 79674957. Email: tanngethong@yahoo.com.sg

ABSTRACT

Bungarus flaviceps (red-headed krait) venom presents an intravenous LD₅₀ of 0.32 μg/g and exhibits enzymatic activities similar to other Bungarus toxins. ELISA cross-reactions between anti-Bungarus flaviceps and a variety of elapid and viperid venoms were observed in the current study. Double-sandwich ELISA was highly specific, since anti-B. flaviceps serum did not cross-react with any tested venom, indicating that this assay can be used for species diagnosis in B. flaviceps bites. In the indirect ELISA, anti-B. flaviceps serum cross-reacted moderately with three different Bungarus venoms (9-18%) and Notechis scutatus venom, but minimally with other elapid and viperid toxins. The results indicated that B. flaviceps venom shares common epitopes with other Bungarus species as well as with N. scutatus. The lethality of the B. flaviceps venom was neutralized effectively by antiserum prepared against B. candidus and B. flaviceps toxins and a commercial bivalent elapid antivenom prepared against B. multicinctus and Naja naja atra venoms, but was not neutralized by commercial antivenoms prepared against Thai cobra, king cobra and banded krait. These data also suggested that the major lethal toxins of B. flaviceps venom are similar to those found in B. multicinctus and B. candidus venoms.

Key words:Bungarus flaviceps venom, enzymes, ELISA, neutralization.

INTRODUCTION

Bungarus flaviceps (red-headed krait) is a rare snake found in Southeast Asia. It presents a very striking and distinctive coloration – namely a bright red head and tail with a black body that includes a low-lateral narrow bluish white stripe (Figure 1). The snake occurs in Burma, Thailand, Vietnam, Malaysia and Indonesia (1). The major lethal toxin having been isolated and cloned from Bungarus flaviceps is considered a novel isoform of β-bungarotoxin (2, 3). Its venom also contains a novel postsynaptic neurotoxin, termed κ-flavitoxin, which is a potent inhibitor of nicotinic transmission in autonomic ganglia (4, 5). The venoms of several common Bungarus species have been well investigated (6-8). Little, however, is known about the enzymatic properties of the Bungarus flaviceps venom. Chanhome et al. (9) reported that a commercial Bungarus fasciatus antivenom could neutralize the lethal toxicity of B. flaviceps venom. The immunological cross-reactivity of this venom, however, has not been investigated. We report herein a preliminary study on the enzymatic activities of B. flaviceps venom and its immunological cross-reactivities using enzyme-linked immunosorbent assay (ELISA).

Bungarus flaviceps venom was obtained from a single milking of an adult individual (Figure 1) captured in central Malaysia. Venom yield was 50 mg dry weight. Other types of snake venoms used in this study were obtained from the Miami Serpentarium Laboratories (USA), Latoxan (France) and Venom Supplies (Australia). All reagents, enzyme substrates and chemicals were purchased from Sigma Chemical Company (USA) or Bio-Rad Laboratories (USA). Monovalent antivenoms against Ophiophagus hannah (king cobra), Naja kaouthia (Thai cobra) and Bungarus fasciatus (banded krait) were obtained from the Thai Red Cross Society (TRCS), Bangkok, Thailand. Bivalent elapid antivenom (NIPM elapid antivenom, prepared against Bungarus multicinctus and Naja naja atra) was obtained from the National Institute of Preventive Medicine (NIPM), Taipei, Taiwan.

Lethality, procoagulant and anticoagulant, hemorrhagic and enzymatic activities were determined as previously described (7, 10). Monospecific rabbit antisera against Bungarus flaviceps, Bungarus fasciatus, Bungarus candidus (Malayan krait) and Notechis scutatus (Eastern tiger snake) were prepared using a hyperimmunization scheme (11). IgG antibodies and IgG-horseradish peroxidase conjugate were prepared according to Hudson and Hay (12) and Tijssen (13), respectively.

The indirect ELISA and double sandwich procedures were performed according to a previous description (14). The LD₅₀ value was calculated according to Weil (15). To determine the neutralization effect of the antiserum/antivenom, a quantity equivalent to 3.0 LD₅₀ of the venom, in 0.05 mL of saline solution, was mixed with 0.1-0.2 mL of antiserum (with appropriate dilution) and incubated for 30 minutes at 37ºC. After centrifugation, the supernatant was injected intravenously into the tail vein of ICR mice. Mortality after 24 hours was determined (n = 6). The rabbits and mice were supplied by the Central Animal House, School of Medicine, University of Malaya, Kuala Lumpur, Malaysia.

The intravenous LD₅₀ of the B. flaviceps venom was determined to be 0.32 µg/g in mouse, a value comparable to B. caeruleus venom (0.13-0.23 µg/g) but higher than that of B. candidus and B. multicinctus venoms (0.04-0.13 µg/g) and lower than that of B. fasciatus venom (1.2-1.4 µg/g) (16). The LD₅₀ reported herein is slightly higher than an earlier one reported by Chanhome et al. (9), which could be due to either geographic or individual variation.

B. flaviceps venom exhibited enzymatic properties similar to other Bungarus venoms (Table 1), including proteolytic, phosphodiesterase, alkaline phosphomonosterase, L-amino acid oxidase, acetylcholinesterase, phospholipase A, 5'-nucleotidase and hyaluronidase activities. Particularly noteworthy are its very low protease and high acetylcholinesterase and phospholipase A₂ activities, characteristic of venoms from the genus Bungarus (15). Like the other Bungarus venoms, it did not exhibit hemorrhagic, procoagulant or anticoagulant activity in vitro.

Both the indirect and double-sandwich ELISA procedures for B. flaviceps venom yielded an exponential dose-response curve at venom concentrations from 3 ng/mL to 100 ng/mL (not shown). The ELISA cross-reactions between antibodies against B. flaviceps venom and 28 venoms from snakes of the families Viperidae and Elapidae are shown in Table 2. The double-sandwich ELISA for B. flaviceps venom was highly specific: there were minimum cross-reactions between the IgG anti-B. flaviceps antibodies to all the venoms tested. Thus, double sandwich ELISA can be used for species diagnosis in B. flaviceps bite. In the indirect ELISA procedure, there were also no substantial cross-reactions between the antibodies and the various snake venoms tested, except for some venoms from the same genus, Bungarus, and the genus Notechis. It is interesting to note that venom from the Australian elapid, Notechis scutatus, yielded a very high level of indirect ELISA cross-reactions (45%) with anti-B. flaviceps, while venoms of B. candidus and B. multicinctus yielded only low cross-reaction proportions (5-9%), indicating that B. flaviceps venom shares more common epitopes with N. scutatus venom than with the two venoms from the same Bungarus genus. This is another illustration of how venoms from unrelated snakes may share common antigens (17). A surprising feature is the total lack of indirect ELISA cross-reactions between anti-B. flaviceps and venoms of Naja, Ophiophagus hannah and Enhydrina schistosa under the experimental conditions. Both B. flaviceps venom and these elapid venoms contain high phospholipase A₂ content (8). While prior investigations of the immunological relationships presented by snake venom phospholipase A₂ indicated that elapid phospholipases A₂ were antigenically similar (18), our results suggest that the major B. flaviceps phospholipase A₂ may exhibit unique antigenic characteristics that differ substantially from venom phospholipases A₂ from the other elapids including Naja, O. hannah and E. schistosa.

Laboratory-prepared monospecific rabbit anti-B. flaviceps, anti-B. candidus and the commercial NIPM bivalent elapid antivenom were effective in neutralizing the lethal effect of the venom in mice; the amounts of B. flaviceps venom neutralized by these three antisera were 1600 µg/mL (167 LD₅₀'s per mL), 400 µg/mL (42 LD₅₀'s per mL) and 6667 µg/mL (695 LD₅₀'s per mL), respectively, for anti-B. flaviceps, anti-B. candidus and NIPM bivalent elapid antivenom, which was known to neutralize B. multicinctus venom effectively (19). The other two laboratory-prepared monospecific rabbit antisera (anti-B. fasciatus and anti-N. scutatus) and three commercial Thai Red Cross Society antivenoms (king cobra antivenom, Thai cobra antivenom and banded krait antivenom) all failed to protect the mice even when 200 µL of the antiserum was injected per mouse. Our results, however, are in contrary to that of Chanhome (9), who reported that the Thai Red Cross Society antivenom developed against banded krait (B. fasciatus) could effectively neutralize B. flaviceps venom. The reason for this discrepancy is not clear but it could be due to geographic/individual variation.

It is interesting to note that in the indirect ELISA, both B. fasciatus and N. scutatus venoms cross-reacted strongly (18%) with anti-B. flaviceps, while both B. candidus and B. multicinctus venoms yielded rather low cross-reaction levels (< 9%). This lack of correlation between antigenic similarity and neutralizing capacity of snake venoms has been reported by many other authors (16). These data also suggest that the major lethal toxins of B. flaviceps venom are similar to those found in B. multicinctus and B. candidus venoms, that is, polypeptide neurotoxins and phospholipase A₂ toxins (8, 20). Khow et al. (2) have indeed shown that the major lethal toxin of B. flaviceps venom was similar to β-bungarotoxin from B. multicinctus venom.

ACKNOWLEDGMENTS

This work was supported by research grants, IRPA-3-07-04-097 and RG 088/09HTM from the government of Malaysia.

Received: June 29, 2009

Accepted: October 27, 2009

Abstract published online: November 11, 2009

Conflicts of interest: There is no conflict.

Financial source:Government of Malaysia.

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