Hainantoxins (HNTX) are neurotoxins from the venom of the Chinese bird spider Cyriopagopus hainanus . Hainantoxins specifically inhibit tetrodotoxin-sensitive Voltage-gated sodium channels, thereby causing blockage of neuromuscular transmission and paralysis. [1] [2] Currently, 13 different hainantoxins are known (HNTX-I – HNTX-XIII), but only HNTX-I, -II, -III, -IV and -V have been investigated in detail. [3]
HNTX-I, HNTX-III, HNTX-IV and HNTX-V are made by the Chinese bird spider Haplopelma hainanum (=Ornithoctonus hainana, Selenocosmia hainana). [1] [2] [4] [5] [6] [7] [8] [9] [10] [11]
Hainantoxins I, III, IV and V show high homology, including the presence of three disulfide bonds that form an inhibitor cysteine knot (ICK) motif.
The main component of the venom of O. hainana is HNTX-I. [12] It has 33 amino acid residues, with a total molecular weight of 3605-3608 Da. HNTX-I contains a short triple-stranded anti-parallel beta-sheet and four beta-turns. [4] The amino acid residues His28 and Asp26 are needed for the bioactivity of HNTX-I. [13]
HNTX-II has a molecular weight of 4253 Da and contains 37 amino acid residues. The complete amino acid sequence of HNTX-II is NH2-LFECSV SCEIEK EGNKD CKKKK CKGGW KCKFN MCVKV-COOH. [14]
The structure of HNTX-III consists of 33-35 amino acid residues, which form a beta-sheet with connections between Asp7 and Cys9, Tyr21 and Ser23, and Lys27 and Val30. [6] [8]
HNTX-IV has 35 amino acid residues with a total molecular weight of 3989 Da. The first strand consists of an antiparallel beta-sheet. [11] The complete amino acid sequence of HNTX-IV is NH2-ECLGFG KGCNPS NDQCCK SSNLVC SRKHRW CKYEI-CONH2. [11] Lys 27, His28, Arg29 and Lys 32 are the neuroactive amino acid residues. [1] [5] [10]
HNTX-V consists of 35 amino acid residues. [2] The whole amino acid residue sequence of HNTX-V is NH2-ECLGFG KGCNPS NDQCCK SANLVC SRKHRW CKYEI-COOH. At the active binding site of HNTX-V, Lys27 and Arg 29 are the most important. [2]
Hainantoxins selectively inhibit tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channels (VGSCs). [1] [5] [6] [9] Voltage-gated Ca2+ channels (VGCCs), tetrodotoxin-resistant (TTX-R) VGSCs and rectifier-delayed potassium channels are not affected. [8] HNTX-III and HNTX-IV are part of the Huwentoxin-I family. [3] [8] Toxins from the Huwentoxin-I family are thought to bind to site 1 on the sodium channels. Other hainantoxins bind at site 3 of the sodium channels. HNTX-I specifically blocks mammalian Nav1.2 and insect para/tipE channels expressed in Xenopus laevis oocytes. HNTX-I is a weak antagonist of the vertebrate TTX-S VGSCs, but is more potent on insect VGSCs. [4] [10]
For the blockage of sodium channels, electrostatic interactions or hydrogen bonds are needed. Important for the electrostatic interaction is the presence of a positively charged region in the toxin, because the receptor site of the sodium channel contains a lot of negatively charged residues. [1] [2] In HNTX-I, the positively charged residues and a vicinal hydrophobic patch have most influence on the binding to the sodium channels. [4] HNTX-IV has a positively charged patch containing the amino acids Arg26, Lys27, His28, Arg29 and Lys32, of which Lys27, Arg29 and Lys32 are the most important for interaction with the TTX-S VGSCs. [10] [15] HNTX-V also shows an interface of positively charged amino acids that are responsible for the binding with the TTX-S VGSCs, where also Lys27 and Arg29 are the most important. Subtle differences in the positively charged patch can result in altered electrostatic properties, causing altered pharmacological effects. [4]
Table 1: IC50 values of four subgroups of hainantoxins
IC50 | |
---|---|
HNTX-I | 68 μM [4] |
HNTX-III | 1.1 nM [8] |
HNTX-IV | 44.6 nM [8] |
HNTX-V | 42.3 nM [2] |
HNTX-I, HNTX-III, HNTX-IV, and HNTX-V are thought to bind to site 1 of voltage-dependent sodium channels, similar to TTX, and thereby block the channel pore. They do not alter activation and inactivation kinetics. [1] [4] Ion selectivity of the VGSCs is not changed by hainantoxin. [8] [9] The mode of action of HNTX-II is unclear, but is unlikely to involve sodium channels. [14]
Hainantoxins can affect both vertebrates and invertebrates. HNTX-I has no significant effect on insects or rats. [2] [12] HNTX-III and HNTX-IV cause spontaneous contractions of the diaphragm muscle and the vas deferens smooth muscle of the rat. [8] [9] HNTX-III and HNTX-IV are able to paralyze cockroaches, and HNTX-IV can even paralyze rats. [15]
Intracerebroventricular injection in mice with HNTX-II shows an LD50 of 1.41 μg/g. The intraperitoneal LD50 value of HNTX-IV in mice is 0.2 mg/kg. [8] [9] HNTX-III is 40 times more potent than HNTX-IV. [8]
HNTX-III and HNTX-IV have an antagonistic effect on the toxin BMK-I, a toxic protein in the venom of the scorpion Buthus martensii . [8]